<figure><img src="image_1.png" /></figure>

HAL
open science

# A Novel Acyl-CoA Beta-Transaminase Characterized from a Metagenome

Alain Perret, Christophe Lechaplais, Sabine Tricot, Nadia Perchat, Carine Vergne, Christine Pellé, Karine Bastard, Annett Kreimeyer, David Vallenet, Anne Zaparucha, et al.

► To cite this version:

Alain Perret, Christophe Lechaplais, Sabine Tricot, Nadia Perchat, Carine Vergne, et al. A Novel Acyl-CoA Beta-Transaminase Characterized from a Metagenome. PLoS ONE, 2011, 6 (8), pp.e22918. 10.1371/journal.pone.0022918. hal-04579105

HAL Id: hal-04579105

https://hal.science/hal-04579105

Submitted on 17 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# A Novel Acyl-CoA Beta-Transaminase Characterized from a Metagenome

Alain Perret¹,²,³*, Christophe Lechaplais¹,²,³, Sabine Tricot¹,²,³, Nadia Perchat¹,²,³, Carine Vergne¹,²,³, Christine Pellé¹,²,³, Karine Bastard¹,²,³, Annett Kreimeyer¹,²,³, David Vallenet¹,²,³, Anne Zaparucha¹,²,³, Jean Weissenbach¹,²,³, Marcel Salanoubat¹,²,³

1 Commissariat à l'Energie Atomique et aux Energies Alternatives, Institut de Génomique, Genoscope, Evry, France, 2 CNRS-UMR 8030, Evry, France, 3 UEVE, Université d'Evry, Evry, France

## Abstract

**Background:** Bacteria are key components in all ecosystems. However, our knowledge of bacterial metabolism is based solely on the study of cultivated organisms which represent just a tiny fraction of microbial diversity. To access new enzymatic reactions and new or alternative pathways, we investigated bacterial metabolism through analyses of uncultivated bacterial consortia.

**Methodology/Principal Findings:** We applied the gene context approach to assembled sequences of the metagenome of the anaerobic digester of a municipal wastewater treatment plant, and identified a new gene which may participate in an alternative pathway of lysine fermentation.

**Conclusions:** We characterized a novel, unique aminotransferase that acts exclusively on Coenzyme A (CoA) esters, and proposed a variant route for lysine fermentation. Results suggest that most of the lysine fermenting organisms use this new pathway in the digester. Its presence in organisms representative of two distinct bacterial divisions indicate that it may also be present in other organisms.

**Citation:** Perret A, Lechaplais C, Tricot S, Perchat N, Vergne C, et al. (2011) A Novel Acyl-CoA Beta-Transaminase Characterized from a Metagenome. PLoS ONE 6(8): e22918. doi:10.1371/journal.pone.0022918

**Editor:** Fernando Rodrigues-Lima, University Paris Diderot-Paris 7, France

**Received** December 17, 2010; **Accepted** July 9, 2011; **Published** August 3, 2011

**Copyright:** © 2011 Perret et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

**Funding:** This work was supported by the Commissariat à l'Energie Atomique (CEA). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

**Competing Interests:** The authors have declared that no competing interests exist.

* E-mail: aperret@genoscope.cns.fr

## Introduction

Microorganisms are the most abundant and diverse forms of life and are essential in the functioning of all ecosystems. However, despite their importance and ubiquity, only a tiny fraction of them is well understood due to their failure to grow under standard laboratory culture conditions. With this limitation, less than 1% of the total number of microbial species have been isolated in pure cultures [1,2,3]. Our knowledge of microbial biodiversity is thus severely impaired by relying solely on cultivated microorganisms, leading to a limited appreciation of functional diversity. Recently the development of metagenomic approaches has opened the window on the richness of uncultured biodiversity [4]. These cultivation-independent techniques have shed light on the functioning of microbial communities and led to major surprises, such as the discovery of a new bacteriorhodopsin in a γ-proteobacterium that has since been found widely represented in different taxa, in diverse oceans [5]; the Photosystem I gene cassettes that were shown to be present in marine virus genomes [6]; the nitrite-driven anaerobic methane oxidation by oxygenic bacteria [7]; the expansion of protein families in these newly-studied ecosystems [8]; or the discovery of multi-kingdom Pfam domains that highlight new biological processes conserved through evolution [9].

Here, we have used the wealth of the metagenomic data extracted from the anaerobic digester of a wastewater treatment plant to explore metabolic capabilities of anaerobic bacteria. Previous studies described the archeal and bacterial molecular diversity of this digester, revealing the occurrence of previously undescribed phylogenetic groups and phylotypes [10]. A quantification of the bacterial diversity was conducted using 16S and 23S rRNA-targeted hybridization. Gram-positive bacteria represented the most abundant phyla (22%), followed by the *Chloroflexi* (20%). *Proteobacteria* and *Bacteroidetes* accounted for 14% each. *Planctomycetes* and *Synergistetes* represented less than 2% each while *WWE1*, a novel phylum, accounted for 12% and could have considerable importance in the community. The genome of "*Candidatus Cloacamonas acidaminovorans*", an uncultivated representative of this lineage, has been reconstructed in *silico* [11].

In anaerobic digestion, microorganisms break down organic material in the absence of oxygen. Three main groups of microorganisms are involved: fermenting bacteria, organic acid oxidizing bacteria, and methanogenic archaea. In a first step, hydrolytic and fermenting bacteria digest the input materials in order to break down complex and polymeric compounds (carbohydrates, proteins...) and make them available for acidogenic bacteria which convert these sugars and amino acids into organic acids. Then acetogenic bacteria convert them into acetic

<figure><img src="image_1.png" /></figure>
# A Novel Acyl-CoA Beta-Transaminase Characterized from a Metagenome

Alain Perret¹,²,³*, Christophe Lechaplais¹,²,³, Sabine Tricot¹,²,³, Nadia Perchat¹,²,³, Carine Vergne¹,²,³, Christine Pellé¹,²,³, Karine Bastard¹,²,³, Annett Kreimeyer¹,²,³, David Vallenet¹,²,³, Anne Zaparucha¹,²,³, Jean Weissenbach¹,²,³, Marcel Salanoubat¹,²,³

1 Commissariat à l'Energie Atomique et aux Energies Alternatives, Institut de Génomique, Genoscope, Evry, France, 2 CNRS-UMR 8030, Evry, France, 3 UEVE, Université d'Evry, Evry, France

## Abstract

**Background:** Bacteria are key components in all ecosystems. However, our knowledge of bacterial metabolism is based solely on the study of cultivated organisms which represent just a tiny fraction of microbial diversity. To access new enzymatic reactions and new or alternative pathways, we investigated bacterial metabolism through analyses of uncultivated bacterial consortia.

**Methodology/Principal Findings:** We applied the gene context approach to assembled sequences of the metagenome of the anaerobic digester of a municipal wastewater treatment plant, and identified a new gene which may participate in an alternative pathway of lysine fermentation.

**Conclusions:** We characterized a novel, unique aminotransferase that acts exclusively on Coenzyme A (CoA) esters, and proposed a variant route for lysine fermentation. Results suggest that most of the lysine fermenting organisms use this new pathway in the digester. Its presence in organisms representative of two distinct bacterial divisions indicate that it may also be present in other organisms.

**Citation:** Perret A, Lechaplais C, Tricot S, Perchat N, Vergne C, et al. (2011) A Novel Acyl-CoA Beta-Transaminase Characterized from a Metagenome. PLoS ONE 6(8): e22918. doi:10.1371/journal.pone.0022918

**Editor:** Fernando Rodrigues-Lima, University Paris Diderot-Paris 7, France

**Received** December 17, 2010; **Accepted** July 9, 2011; **Published** August 3, 2011

**Copyright:** © 2011 Perret et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

**Funding:** This work was supported by the Commissariat à l'Energie Atomique (CEA). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

**Competing Interests:** The authors have declared that no competing interests exist.

* E-mail: aperret@genoscope.cns.fr

## Introduction

Microorganisms are the most abundant and diverse forms of life and are essential in the functioning of all ecosystems. However, despite their importance and ubiquity, only a tiny fraction of them is well understood due to their failure to grow under standard laboratory culture conditions. With this limitation, less than 1% of the total number of microbial species have been isolated in pure cultures [1,2,3]. Our knowledge of microbial biodiversity is thus severely impaired by relying solely on cultivated microorganisms, leading to a limited appreciation of functional diversity. Recently the development of metagenomic approaches has opened the window on the richness of uncultured biodiversity [4]. These cultivation-independent techniques have shed light on the functioning of microbial communities and led to major surprises, such as the discovery of a new bacteriorhodopsin in a γ-proteobacterium that has since been found widely represented in different taxa, in diverse oceans [5]; the Photosystem I gene cassettes that were shown to be present in marine virus genomes [6]; the nitrite-driven anaerobic methane oxidation by oxygenic bacteria [7]; the expansion of protein families in these newly-studied ecosystems [8]; or the discovery of multi-kingdom Pfam domains that highlight new biological processes conserved through evolution [9].

Here, we have used the wealth of the metagenomic data extracted from the anaerobic digester of a wastewater treatment plant to explore metabolic capabilities of anaerobic bacteria. Previous studies described the archeal and bacterial molecular diversity of this digester, revealing the occurrence of previously undescribed phylogenetic groups and phylotypes [10]. A quantification of the bacterial diversity was conducted using 16S and 23S rRNA-targeted hybridization. Gram-positive bacteria represented the most abundant phyla (22%), followed by the *Chloroflexi* (20%). *Proteobacteria* and *Bacteroidetes* accounted for 14% each. *Planctomycetes* and *Synergistetes* represented less than 2% each while *WWE1*, a novel phylum, accounted for 12% and could have considerable importance in the community. The genome of "*Candidatus Cloacamonas acidaminovorans*", an uncultivated representative of this lineage, has been reconstructed in *silico* [11].

In anaerobic digestion, microorganisms break down organic material in the absence of oxygen. Three main groups of microorganisms are involved: fermenting bacteria, organic acid oxidizing bacteria, and methanogenic archaea. In a first step, hydrolytic and fermenting bacteria digest the input materials in order to break down complex and polymeric compounds (carbohydrates, proteins...) and make them available for acidogenic bacteria which convert these sugars and amino acids into organic acids. Then acetogenic bacteria convert them into acetic

<figure><img src="image_3.png" /></figure>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** The lysine fermentation pathway. Lysine is converted to β-lysine by lysine-2,3 aminomutase (1) and then to L-erythro-3,5-diaminohexanoate by β-lysine-5,6-aminomutase (2). This compound is deaminated and oxidized by a NAD(P)-dependent L-erythro-3,5-diaminohexanoate dehydrogenase (3) to yield 3-keto-5-aminohexanoate. It is further converted into 3-aminobutyryl-CoA and acetoacetate in the presence of acetyl-CoA by 3-keto-5-aminohexanoate cleavage enzyme (4). 3-aminobutyryl-CoA is deaminated to crotonyl-CoA by an ammonia lyase (5). Crotonyl-CoA is reduced to butyryl-CoA by butyryl-CoA dehydrogenase (6), which reacts with acetoacetate to form butyrate and acetoacetyl-CoA through the action of acetoacetate:butyrate CoA transferase (7). The latter compound is converted to acetate via acetyl-CoA and acetyl phosphate by acetoacetyl-CoA thiolase (8), phosphate acetyltransferase (9), and acetate kinase (10), respectively.
doi:10.1371/journal.pone.0022918.g001</figcaption></figure>

acid. Finally, methanogens convert these products to methane [12]. Carbon dioxide, hydrogen and ammonia are produced during all the steps of this process. "Candidatus Cloacamonas acidaminovorans" is considered as a fermentative bacterium, and is suggested to be a hydrogen-producing syntroph [11].

Our previous research on the lysine fermentation pathway at the genetic level [13] and on the predicted metabolism of "Candidatus Cloacamonas acidaminovorans" which is supposed to be a lysine fermenting organism [11], led us to re-analyze this metabolic route. We have experimentally identified a novel enzymatic reaction which is involved in a hitherto unobserved variant route for lysine fermentation in this organism.

## Results and Discussion

### Identification of a candidate gene involved in an alternative lysine fermentation pathway

It was demonstrated since the early 1950s that in *Clostridium sticklandii*, lysine was degraded to acetate, butyrate, and ammonia. This catabolic pathway contains ten distinctive reactions (see figure 1). All these genes have been identified [13]. The genome annotation of "Candidatus Cloacamonas acidaminovorans" suggested that this bacterium ferments lysine. However, *kal*, responsible for ammonia elimination from 3-aminobutyryl-CoA

to produce crotonyl-CoA was absent. One hypothesis was that "Candidatus Cloacamonas acidaminovorans" ferments lysine through a variant pathway. We then searched for the genes involved in this alternative route by studying the genomic neighbourhood of the genes involved in lysine fermentation in this bacterium. However, this approach was unfruitful. In this organism, a low percentage of genes are found in groups of co-localized orthologs shared with other species. This led us to study the genomic context of the genes involved in the lysine fermentation pathway in other assembled genomic regions from the anaerobic digester. In particular, a large contig of 50 kbp containing the clustered genes of interest was studied. This contig was later integrated into an *in silico* reconstructed genome of 4.58 Mbp that has been shown to be affiliated with the Bacteroidetes phylum (Fonknechten N. and Le Paslier D., manuscript in preparation). Surprisingly, while the genes involved in lysine fermentation were clustered, *kal* was also missing as observed in "Candidatus Cloacamonas acidaminovorans" (figure 2). In these assembled sequences, a gene annotated as "putative glutamate-1-semialdehyde-2,1-aminomutase" (*hemL*) was found at the *kal* locus. A gene also coding for a protein showing strong homology (>65% of sequence identity over the total length of the proteins) to this HemL-like protein was found in "Candidatus Cloacamonas acidaminovorans" (CLOAM0809) co-localized with

<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 1. The lysine fermentation pathway.** Lysine is converted to β-lysine by lysine-2,3 aminomutase (1) and then to L-erythro-3,5-diaminohexanoate by β-lysine-5,6-aminomutase (2). This compound is deaminated and oxidized by a NAD(P)-dependent L-erythro-3,5-diaminohexanoate dehydrogenase (3) to yield 3-keto-5-aminohexanoate. It is further converted into 3-aminobutyryl-CoA and acetoacetate in the presence of acetyl-CoA by 3-keto-5-aminohexanoate cleavage enzyme (4). 3-aminobutyryl-CoA is deaminated to crotonyl-CoA by an ammonia lyase (5). Crotonyl-CoA is reduced to butyryl-CoA by butyryl-CoA dehydrogenase (6), which reacts with acetoacetate to form butyrate and acetoacetyl-CoA through the action of acetoacetate:butyrate CoA transferase (7). The latter compound is converted to acetate via acetyl-CoA and acetyl phosphate by acetoacetyl-CoA thiolase (8), phosphate acetyltransferase (9), and acetate kinase (10), respectively.</figcaption></figure>

doi:10.1371/journal.pone.0022918.g001

acid. Finally, methanogens convert these products to methane [12]. Carbon dioxide, hydrogen and ammonia are produced during all the steps of this process. "Candidatus Cloacamonas acidaminovorans" is considered as a fermentative bacterium, and is suggested to be a hydrogen-producing syntroph [11].

Our previous research on the lysine fermentation pathway at the genetic level [13] and on the predicted metabolism of "Candidatus Cloacamonas acidaminovorans" which is supposed to be a lysine fermenting organism [11], led us to re-analyze this metabolic route. We have experimentally identified a novel enzymatic reaction which is involved in a hitherto unobserved variant route for lysine fermentation in this organism.

## Results and Discussion

### Identification of a candidate gene involved in an alternative lysine fermentation pathway

It was demonstrated since the early 1950s that in *Clostridium sticklandii*, lysine was degraded to acetate, butyrate, and ammonia. This catabolic pathway contains ten distinctive reactions (see figure 1). All these genes have been identified [13]. The genome annotation of "Candidatus Cloacamonas acidaminovorans" suggested that this bacterium ferments lysine. However, *kal*, responsible for ammonia elimination from 3-aminobutyryl-CoA

to produce crotonyl-CoA was absent. One hypothesis was that "Candidatus Cloacamonas acidaminovorans" ferments lysine through a variant pathway. We then searched for the genes involved in this alternative route by studying the genomic neighbourhood of the genes involved in lysine fermentation in this bacterium. However, this approach was unfruitful. In this organism, a low percentage of genes are found in groups of co-localized orthologs shared with other species. This led us to study the genomic context of the genes involved in the lysine fermentation pathway in other assembled genomic regions from the anaerobic digester. In particular, a large contig of 50 kbp containing the clustered genes of interest was studied. This contig was later integrated into an *in silico* reconstructed genome of 4.58 Mbp that has been shown to be affiliated with the Bacteroidetes phylum (Fonknechten N. and Le Paslier D., manuscript in preparation). Surprisingly, while the genes involved in lysine fermentation were clustered, *kal* was also missing as observed in "Candidatus Cloacamonas acidaminovorans" (figure 2). In these assembled sequences, a gene annotated as "putative glutamate-1-semialdehyde-2,1-aminomutase" (*hemL*) was found at the *kal* locus. A gene also coding for a protein showing strong homology (>65% of sequence identity over the total length of the proteins) to this HemL-like protein was found in "Candidatus Cloacamonas acidaminovorans" (CLOAM0809) co-localized with

<figure><img src="image_5.png" /></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Models of the gene cluster organization for lysine fermentation. (A): *Fusobacterium nucleatum* ATCC 25586; (B): a contig sequence affiliated with the *Bacteroidetes* phylum originating from the metagenome of the anaerobic digester of a wastewater treatment plant; (C): the reconstructed genome of "*Candidatus* Cloacamonas acidaminovorans", an uncultivated bacterium found in this digester (only the ID number of the CLOAM genes are reported). The symbol "/" means an interruption of >5.5 kb in the cluster. HP indicates a gene encoding a Hypothetical Protein; *mutS* a gene encoding a MutS family protein.
doi:10.1371/journal.pone.0022918.g002</figcaption></figure>

the gene encoding acetoacetyl-CoA thiolase (EC 2.3.1.9; CLOAM0810) (figure 2) that is known to participate to lysine degradation [13].

HemL, which converts glutamate-1-semialdehyde to 5-amino-
levulinate (EC 5.4.3.8) is an aminomutase/aminotransferase
involved in the biosynthesis of porphyrins [14,15]. However, the
*in silico* genome analysis of "*Candidatus* Cloacamonas acidamino-
vorans" indicated that it does not possess any other gene involved
in porphyrin biosynthesis [11]. Together, these data suggested that
*hemL*-like was misannotated and was linked in some way to the
degradation of lysine, making it a promising candidate for the
experimental elucidation of a variant lysine fermentation pathway.
Due to the substitution of *kal* in "*Candidatus* Cloacamonas
acidaminovorans" and in the reconstructed Bacteroidetes genome,
we suspected 3-aminobutyryl-CoA to be metabolized in an
unreported manner. We thus focused on HemL-like from
"*Candidatus* Cloacamonas acidaminovorans" to analyze how this
organism could ferment lysine in the absence of *kat*. Considering
the annotated function of HemL-like as an aminomutase/
aminotransferase, we hypothesized that it may transfer the amine
of 3-aminobutyryl-CoA to an α-keto acid, to directly form
acetoacetyl-CoA (Figure 3).

Purification of the enzymes involved in the alternative
lysine fermentation pathway

The recombinant HemL-like, AtoA, and AtoD (the two
components of the butyrate-acetoacetate CoA transferase com-
plex) from "Candidatus cloacamonas acidaminovorans" were produced
in E. coli and purified. According to gel filtration experiments,
AtoA and AtoD were both recovered as monomers (not shown).
Since butyrate-acetoacetate CoA transferase is functional as an
α₂β₂ heterotetramer [16,17], an equimolar amount of AtoA and
AtoD were mixed and applied onto the gel filtration column to
purify the functional complex. The mobility of purified AtoA/
AtoD complex on the Superdex 200 gel filtration column (figure
S1) indicated that it is a tetramer with an apparent molecular mass
of ~93 kDa (theoretical mass : 101.5 kDa). A sample of purified
HemL-like gel filtered under the same conditions yielded an
apparent molecular mass of 81 kDa (figure S1), consistent with a
dimeric structure (theoretical mass: 94.4 kDa). Since the sequence
of the HemL-like possesses an aminotransferase class-III pyridox-
al-phosphate (PLP) attachment site (PROSITE motif PS00600),
the purified protein was incubated with excess PLP, subjected
again to gel filtration, and its spectrum recorded between 250 and

600 nm. The presence of an absorption maximum near 410 nm
confirmed that HemL-like is a PLP-dependent protein (figure S2).

Biochemical and enzymatic characterization of HemL-like

The putative activity of HemL-like was evaluated experimen-
tally by spectrophotometric monitoring at 310 nm of the
formation of acetoacetyl-CoA. 3-aminobutyryl-CoA was incubat-
ed with HemL-like in the presence of various α-ketoacids (α-
ketoglutarate, pyruvate, oxaloacetate, and glyoxylate). Under these
conditions, rapid formation of acetoacetyl-CoA was observed in
the presence of α-ketoglutarate, and a slower reaction occured
with pyruvate. Acetoacetyl-CoA formation was observed in the
presence of 3-aminobutyryl-CoA produced both enzymatically
and chemically. The enzymatic reaction could also be observed
through LC/MS analysis. In the presence of HemL-like,
acetoacetyl-CoA was detected in the positive ionization mode at
m/z 852.1442 (accuracy: 0.7 ppm), and was correlated with a
decrease in 3-aminobutyryl-CoA concentration (detected at m/z
853.1722 with an accuracy of 3.6 ppm). Its formation was also
observed in the presence of both α-ketoglutarate and pyruvate.
The kinetic constants of the enzyme were determined spectro-
photometrically using (3S)-3-aminobutyryl-CoA (Table 1). There-
fore, hemL-like codes for a 3-aminobutyryl-CoA aminotransferase
that we have named kat. This novel β-aminotransferase is, to our
knowledge, the first that acts on a CoA ester.

Its activity in the presence of saturating substrate concentrations
was high only in a narrow pH range (7.5–8.5), with an optimum
around pH 8.0. The enzyme was inactive below pH 7.0 and
exhibited only 20% activity at pH 10 (figure S3a). Kat's activity
could not be quantified in HEPES/NaOH nor in Na/K
phosphate buffers. At optimal pH, the activity of Kat increased
with temperature, up to a maximum at ~30°C, and sharply
decreased with higher temperatures to be undetectable above

<figure><img src="image_5.png" /><figcaption>**Figure 3.** The hypothetical alternative metabolism of 3-aminobutyryl-CoA.
doi:10.1371/journal.pone.0022918.g003</figcaption></figure>

<figure><img src="image_6.png" /></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Models of the gene cluster organization for lysine fermentation. (A): *Fusobacterium nucleatum* ATCC 25586; (B): a contig sequence affiliated with the *Bacteroidetes* phylum originating from the metagenome of the anaerobic digester of a wastewater treatment plant; (C): the reconstructed genome of "*Candidatus* Cloacamonas acidaminovorans", an uncultivated bacterium found in this digester (only the ID number of the CLOAM genes are reported). The symbol "/" means an interruption of >5.5 kb in the cluster. HP indicates a gene encoding a Hypothetical Protein; *mutS* a gene encoding a MutS family protein.
doi:10.1371/journal.pone.0022918.g002</figcaption></figure>

the gene encoding acetoacetyl-CoA thiolase (EC 2.3.1.9; CLOAM0810) (figure 2) that is known to participate to lysine degradation [13].

HemL, which converts glutamate-1-semialdehyde to 5-amino-
levulinate (EC 5.4.3.8) is an aminomutase/aminotransferase
involved in the biosynthesis of porphyrins [14,15]. However, the
*in silico* genome analysis of “*Candidatus* Cloacamonas acidamin-
vorans” indicated that it does not possess any other gene involved
in porphyrin biosynthesis [11]. Together, these data suggested that
*hemL*-like was misannotated and was linked in some way to the
degradation of lysine, making it a promising candidate for the
experimental elucidation of a variant lysine fermentation pathway.
Due to the substitution of *kal* in “*Candidatus* Cloacamonas
acidaminovorans” and in the reconstructed Bacteroidetes genome,
we suspected 3-aminobutyryl-CoA to be metabolized in an
unreported manner. We thus focused on HemL-like from
“*Candidatus* Cloacamonas acidaminovorans” to analyze how this
organism could ferment lysine in the absence of *kal*. Considering
the annotated function of HemL-like as an aminomutase/
aminotransferase, we hypothesized that it may transfer the amine
of 3-aminobutyryl-CoA to an α-keto acid, to directly form
acetoacetyl-CoA (Figure 3).

Purification of the enzymes involved in the alternative
lysine fermentation pathway

The recombinant HemL-like, AtoA, and AtoD (the two
components of the butyrate-acetoacetate CoA transferase com-
plex) from “*Candidatus cloacamonas acidaminovorans”* were produced
in *E. coli* and purified. According to gel filtration experiments,
AtoA and AtoD were both recovered as monomers (not shown).
Since butyrate-acetoacetate CoA transferase is functional as an
α₂β₂ heterotetramer [16,17], an equimolar amount of AtoA and
AtoD were mixed and applied onto the gel filtration column to
purify the functional complex. The mobility of purified AtoA/
AtoD complex on the Superdex 200 gel filtration column (figure
S1) indicated that it is a tetramer with an apparent molecular mass
of ~93 kDa (theoretical mass : 101.5 kDa). A sample of purified
HemL-like gel filtered under the same conditions yielded an
apparent molecular mass of 81 kDa (figure S1), consistent with a
dimeric structure (theoretical mass: 94.4 kDa). Since the sequence
of the HemL-like possesses an aminotransferase class-III pyridox-
al-phosphate (PLP) attachment site (PROSITE motif PS00600),
the purified protein was incubated with excess PLP, subjected
again to gel filtration, and its spectrum recorded between 250 and

600 nm. The presence of an absorption maximum near 410 nm
confirmed that HemL-like is a PLP-dependent protein (figure S2).

Biochemical and enzymatic characterization of HemL-like

The putative activity of HemL-like was evaluated experimen-
tally by spectrophotometric monitoring at 310 nm of the
formation of acetoacetyl-CoA. 3-aminobutyryl-CoA was incubat-
ed with HemL-like in the presence of various α-ketoacids (α-
ketoglutarate, pyruvate, oxaloacetate, and glyoxylate). Under these
conditions, rapid formation of acetoacetyl-CoA was observed in
the presence of α-ketoglutarate, and a slower reaction occured
with pyruvate. Acetoacetyl-CoA formation was observed in the
presence of 3-aminobutyryl-CoA produced both enzymatically
and chemically. The enzymatic reaction could also be observed
through LC/MS analysis. In the presence of HemL-like,
acetoacetyl-CoA was detected in the positive ionization mode at
m/z 852.1442 (accuracy: 0.7 ppm), and was correlated with a
decrease in 3-aminobutyryl-CoA concentration (detected at m/z
853.1722 with an accuracy of 3.6 ppm). Its formation was also
observed in the presence of both α-ketoglutarate and pyruvate.
The kinetic constants of the enzyme were determined spectro-
photometrically using (3S)-3-aminobutyryl-CoA (Table 1). There-
fore, hemL-like codes for a 3-aminobutyryl-CoA aminotransferase
that we have named kat. This novel β-aminotransferase is, to our
knowledge, the first that acts on a CoA ester.

Its activity in the presence of saturating substrate concentrations
was high only in a narrow pH range (7.5–8.5), with an optimum
around pH 8.0. The enzyme was inactive below pH 7.0 and
exhibited only 20% activity at pH 10 (figure S3a). Kat's activity
could not be quantified in HEPES/NaOH nor in Na/K
phosphate buffers. At optimal pH, the activity of Kat increased
with temperature, up to a maximum at ~30°C, and sharply
decreased with higher temperatures to be undetectable above

<figure><img src="image_7.png" /><figcaption>**Figure 3.** The hypothetical alternative metabolism of 3-aminobutyryl-CoA.
doi:10.1371/journal.pone.0022918.g003</figcaption></figure>

<figure><img src="image_8.png" /></figure>
<table><caption>**Table 1.** Kinetic parameters of 3-aminobutyryl-CoA transamination.</caption><thead><tr><th>Substrate</th><th>K<sub>m</sub> (M)</th><th>k<sub>cat</sub> (s<sup>-1</sup>)</th><th>k<sub>cat</sub>/k<sub>m</sub> (s<sup>-1</sup> M<sup>-1</sup>)</th></tr></thead><tbody><tr><td>3-aminobutyryl-CoA<sup>1</sup></td><td>4.3&plusmn;1.5 10<sup>-6</sup></td><td>1.80&plusmn;0.20</td><td>4.2 10<sup>5</sup></td></tr><tr><td>&alpha;-ketoglutarate<sup>2</sup></td><td>2.9&plusmn;0.1 10<sup>-3</sup></td><td>1.80&plusmn;0.10</td><td>620</td></tr><tr><td>pyruvate<sup>2</sup></td><td>20.1&plusmn;1.3 10<sup>-3</sup></td><td>0.40&plusmn;0.01</td><td>20</td></tr></tbody></table>

Values correspond to the average of two replicates.

33°C (figure S3b). This optimum temperature is similar to that of the digester [18].

Kim *et al.* had recently characterized a novel β-aminotransferase from *Mesorhizobium* sp [19], which shows striking properties: its amino acid sequence shows the highest similarity with a glutamate-1-semialdehyde 2,1-aminomutase (53% identity with the enzyme from *Polaromonas* sp. strain JS666, which does not have β-aminocarboxylic transaminase activity). Furthermore, this *Mesorhizobium* β-transaminase shows its highest activity in the presence of α-ketoglutarate as the amino acceptor as well. It has strong activity with β-aminocarboxylic acids, in particular 3-aminobutyrate, which is structurally close to 3-aminobutyryl-CoA. While the enzyme from *Mesorhizobium* sp has only 24% identity with Kat, its properties prompted us to check whether Kat could have a similar activity. Kat was thus assayed for activity in the presence of the best amino donors of the β-transaminase from *Mesorhizobium* sp: 3-aminobutyrate, β-homoleucine and β-phenylalanine. Since none of these compounds could be transformed by Kat, we hypothesized that the CoA moiety is essential for substrate recognition. Previous work has demonstrated that N-acetylcysteine (NAC) thioesters can serve as activated acyl-CoA mimics that

are used by the cell [20]. Because the acyl-CoA compounds are difficult to synthesize and to handle due to their high sensitivity to hydrolysis reaction, we chose to use the NAC thioesters. More particularly, the CoA side chain of acetyl-CoA could be substituted by NAC in the reaction catalyzed by the 3-keto-5-aminohexanoate cleavage enzyme (Kce) in the lysine fermentation pathway: in the presence of 3-keto-5-aminohexanoate and acetyl-NAC, the enzyme yielded acetoacetate and 3-aminobutyryl-S-NAC (unpublished results). NAC thioesters of 3-aminobutyrate, β-homoleucine, and β-phenylalanine were thus synthesized and tested as amino donors for Kat. Their structure, along with the corresponding products that would be formed in the presence of α-ketoglutarate and the enzyme are presented in figure 4. Kat was incubated in 10 mM Tris/HCl pH 8.1 in the presence of 20 mM α-ketoglutarate and 50 µM S-NAC-thioester for 2 hours before LC/MS analysis (positive ionization mode). Under these conditions acetoacetyl-S-NAC was detected at *m/z* 204.0682 (Table 2), illustrating that NAC can serve as a CoA mimic for Kat. More interestingly, β-homoleucine-NAC and β-phenylalanine-NAC were also converted to their corresponding oxo-compounds, because 5-methyl-3-oxohexanoate-NAC and 3-oxo-3-phenylpropanoate-NAC were detected at *m/z* 204.0682 and 266.0838, respectively (Table 2). The formation of glutamate was also detected in these three reactions. Additional MS/MS experiments conducted on these three reaction products yielded a common fragment at *m/z* 119.98. This mass is consistent with the protonated form of NAC. Together, these results further confirm the identity of each oxo-metabolite. In conclusion, it appears that the thioester moiety (CoA or its NAC mimic) is actually necessary for substrate recognition, but most of all, Kat is able to transform other β-amino compounds besides 3-aminobutyryl-CoA, including aromatic ones.

### Determination of a variant lysine fermentation pathway

We have demonstrated that *kat* codes for a 3-aminobutyryl-CoA aminotransferase. The concomitant absence of *kal* and presence of

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Structure of the different NAC-thioesters and their corresponding products after β-transamination catalyzed by Kat in the presence of α-ketoglutarate.</figcaption></figure>

<figure><img src="image_8.png" /></figure>

<footnote n="1">¹α-ketoglutarate concentration was 50 mM.</footnote>

<footnote n="2">²3-aminobutyryl-CoA concentration was 25 µM.</footnote>

<footnote>doi:10.1371/journal.pone.0022918.t001</footnote>

<footnote>doi:10.1371/journal.pone.0022918.g004</footnote>
<table><caption>**Table 1.** Kinetic parameters of 3-aminobutyryl-CoA transamination.</caption><thead><tr><th>Substrate</th><th>K<sub>m</sub> (M)</th><th>k<sub>cat</sub> (s<sup>-1</sup>)</th><th>k<sub>cat</sub>/k<sub>m</sub> (s<sup>-1</sup> M<sup>-1</sup>)</th></tr></thead><tbody><tr><td>3-aminobutyryl-CoA<sup>1</sup></td><td>4.3±1.5 10<sup>-6</sup></td><td>1.80±0.20</td><td>4.2 10<sup>5</sup></td></tr><tr><td>α-ketoglutarate<sup>2</sup></td><td>2.9±0.1 10<sup>-3</sup></td><td>1.80±0.10</td><td>620</td></tr><tr><td>pyruvate<sup>2</sup></td><td>20.1±1.3 10<sup>-3</sup></td><td>0.40±0.01</td><td>20</td></tr></tbody></table>

Values correspond to the average of two replicates.

33°C (figure S3b). This optimum temperature is similar to that of the digester [18].

Kim *et al.* had recently characterized a novel β-aminotransferase from *Mesorhizobium* sp [19], which shows striking properties: its amino acid sequence shows the highest similarity with a glutamate-1-semialdehyde 2,1-aminomutase (53% identity with the enzyme from *Polaromonas* sp. strain JS666, which does not have β-aminocarboxylic transaminase activity). Furthermore, this *Mesorhizobium* β-transaminase shows its highest activity in the presence of α-ketoglutarate as the amino acceptor as well. It has strong activity with β-aminocarboxylic acids, in particular 3-aminobutyrate, which is structurally close to 3-aminobutyryl-CoA. While the enzyme from *Mesorhizobium* sp has only 24% identity with Kat, its properties prompted us to check whether Kat could have a similar activity. Kat was thus assayed for activity in the presence of the best amino donors of the β-transaminase from *Mesorhizobium* sp: 3-aminobutyrate, β-homoleucine and β-phenylalanine. Since none of these compounds could be transformed by Kat, we hypothesized that the CoA moiety is essential for substrate recognition. Previous work has demonstrated that N-acetylcysteine (NAC) thioesters can serve as activated acyl-CoA mimics that

are used by the cell [20]. Because the acyl-CoA compounds are difficult to synthesize and to handle due to their high sensitivity to hydrolysis reaction, we chose to use the NAC thioesters. More particularly, the CoA side chain of acetyl-CoA could be substituted by NAC in the reaction catalyzed by the 3-keto-5-aminohexanoate cleavage enzyme (Kce) in the lysine fermentation pathway: in the presence of 3-keto-5-aminohexanoate and acetyl-NAC, the enzyme yielded acetoacetate and 3-aminobutyryl-S-NAC (unpublished results). NAC thioesters of 3-aminobutyrate, β-homoleucine, and β-phenylalanine were thus synthesized and tested as amino donors for Kat. Their structure, along with the corresponding products that would be formed in the presence of α-ketoglutarate and the enzyme are presented in figure 4. Kat was incubated in 10 mM Tris/HCl pH 8.1 in the presence of 20 mM α-ketoglutarate and 50 µM S-NAC-thioester for 2 hours before LC/MS analysis (positive ionization mode). Under these conditions acetoacetyl-S-NAC was detected at *m/z* 204.0682 (Table 2), illustrating that NAC can serve as a CoA mimic for Kat. More interestingly, β-homoleucine-NAC and β-phenylalanine-NAC were also converted to their corresponding oxo-compounds, because 5-methyl-3-oxohexanoate-NAC and 3-oxo-3-phenylpropanoate-NAC were detected at *m/z* 204.0682 and 266.0838, respectively (Table 2). The formation of glutamate was also detected in these three reactions. Additional MS/MS experiments conducted on these three reaction products yielded a common fragment at *m/z* 119.98. This mass is consistent with the protonated form of NAC. Together, these results further confirm the identity of each oxo-metabolite. In conclusion, it appears that the thioester moiety (CoA or its NAC mimic) is actually necessary for substrate recognition, but most of all, Kat is able to transform other β-amino compounds besides 3-aminobutyryl-CoA, including aromatic ones.

## Determination of a variant lysine fermentation pathway

We have demonstrated that *kat* codes for a 3-aminobutyryl-CoA aminotransferase. The concomitant absence of *kal* and presence of

<figure><img src="image_9.png" /><figcaption>**Figure 4.** Structure of the different NAC-thioesters and their corresponding products after β-transamination catalyzed by Kat in the presence of α-ketoglutarate.</figcaption></figure>

<figure><img src="image_10.png" /></figure>

<footnote n="1">¹α-ketoglutarate concentration was 50 mM.</footnote>

<footnote n="2">²3-aminobutyryl-CoA concentration was 25 µM.</footnote>

<footnote>doi:10.1371/journal.pone.0022918.t001</footnote>

<footnote>doi:10.1371/journal.pone.0022918.g004</footnote>
<figure><img src="image_9.png" /><figcaption>**Figure 5.** The alternative lysine fermentation pathway. Enzymes involved are L-lysine-2,3-aminomutase (1); β-L-lysine-5,6-aminomutase (2); 3,5-diaminohexanoate dehydrogenase (3); 3-keto-5-aminohexanoate cleavage enzyme (4); 3-aminobutyryl-CoA aminotransferase (Kat; 5); butyrate-acetoacetate CoA transferase (6); glutamate dehydrogenase (7); acetoacetyl-CoA thiolase (8); phosphate acetyltransferase (9); acetate kinase (10); butyryl-CoA dehydrogenase (11); butyrate-acetoacetate CoA transferase (12). doi:10.1371/journal.pone.0022918.g005</figcaption></figure>

kat suggests that lysine is metabolized in a different way. In the well-known route, the conversion of 3-aminobutyryl-CoA to acetoacetyl-CoA occurs through three distinct enzymatic steps: 1) ammonia elimination of 3-aminobutyryl-CoA to crotonyl-CoA (EC 4.3.1.14), 2) reduction of crotonyl-CoA to butyryl-CoA (EC 1.3.99.2) and 3) CoA-SH transfer from butyryl-CoA to acetoacetate (EC 2.8.3.9) [13]. In the proposed alternative route, Kat generates acetoacetyl-CoA, and the AtoA/AtoD complex may yield a second molecule of acetoacetyl-CoA from acetoacetate and acetyl-CoA. Following this scheme, the exergonic reduction of crotonyl-CoA to butyryl-CoA would be suppressed, which could be detrimental to the cell, as this reaction can be coupled to energy

conservation *via* electron bifurcation [21,22]. The genome of “Candidatus Cloacomonas acidaminovorans” contains good candidate genes (CLOAM1274, 1482, and 0104) coding for a butyryl-CoA dehydrogenase/electron-transferring-flavoprotein complex (Bcd/EtfAB). However, the formation of crotonyl-CoA from acetoacetyl-CoA or other metabolites remains enigmatic, because no genes coding for 3-hydroxybutyryl-CoA dehydrogenase (EC 1.1.1.157), crotonase (EC 4.2.1.55) or glutaryl-CoA dehydrogenase (EC 1.3.99.7) could be detected. Syntrophic bacteria are able to catalyse the energetically unfavourable formation of crotonyl-CoA from butyryl-CoA using a reverse electron transfer *via* the Rnf system [23]. Since *C. acidaminovorans* is predicted to be a

<table><caption>**Table 2.** Identification of NAC-thioesters formed by Kat.</caption><thead><tr><th rowspan="2">NAC-thioester</th><th rowspan="2">Formula</th><th colspan="2">[M+H]<sup>+</sup></th><th rowspan="2">Accuracy (ppm)</th><th rowspan="2">RT (min)*</th></tr><tr><th>Theoretical</th><th>Observed</th></tr></thead><tbody><tr><td>acetoacetyl-</td><td>C<sub>8</sub>H<sub>13</sub>NO<sub>3</sub>S</td><td>204.0689</td><td>204.0682</td><td>-3.4</td><td>6.14</td></tr><tr><td>5-methyl-3-oxohexanoate-</td><td>C<sub>11</sub>H<sub>19</sub>NO<sub>3</sub>S</td><td>246.1158</td><td>246.1152</td><td>-2.4</td><td>8.06</td></tr><tr><td>3-oxo-3-phenylpropanoate-</td><td>C<sub>11</sub>H<sub>19</sub>NO<sub>3</sub>S</td><td>266.0845</td><td>266.0838</td><td>-2.6</td><td>8.02</td></tr></tbody></table>

<figure><img src="image_10.png" /></figure>

<footnote>*RT indicates the retention time of the compound on the chromatography column; [M+H]<sup>+</sup>, protonated molecular ion (positive ionization mode). doi:10.1371/journal.pone.0022918.t002</footnote>
<figure><img src="image_11.png" /><figcaption>**Figure 5.** The alternative lysine fermentation pathway. Enzymes involved are L-lysine-2,3-aminomutase (1); β-L-lysine-5,6-aminomutase (2); 3,5-diaminohexanoate dehydrogenase (3); 3-keto-5-aminohexanoate cleavage enzyme (4); 3-aminobutyryl-CoA aminotransferase (Kat; 5); butyrate-acetoacetate CoA transferase (6); glutamate dehydrogenase (7); acetoacetyl-CoA thiolase (8); phosphate acetyltransferase (9); acetate kinase (10); butyryl-CoA dehydrogenase (11); butyrate-acetoacetate CoA transferase (12). doi:10.1371/journal.pone.0022918.g005</figcaption></figure>

kat suggests that lysine is metabolized in a different way. In the well-known route, the conversion of 3-aminobutyryl-CoA to acetoacetyl-CoA occurs through three distinct enzymatic steps: 1) ammonia elimination of 3-aminobutyryl-CoA to crotonyl-CoA (EC 4.3.1.14), 2) reduction of crotonyl-CoA to butyryl-CoA (EC 1.3.99.2) and 3) CoA-SH transfer from butyryl-CoA to acetoacetate (EC 2.8.3.9) [13]. In the proposed alternative route, Kat generates acetoacetyl-CoA, and the AtoA/AtoD complex may yield a second molecule of acetoacetyl-CoA from acetoacetate and acetyl-CoA. Following this scheme, the exergonic reduction of crotonyl-CoA to butyryl-CoA would be suppressed, which could be detrimental to the cell, as this reaction can be coupled to energy

conservation *via* electron bifurcation [21,22]. The genome of “Candidatus Cloacomonas acidaminovorans” contains good candidate genes (CLOAM1274, 1482, and 0104) coding for a butyryl-CoA dehydrogenase/electron-transferring-flavoprotein complex (Bcd/EtfAB). However, the formation of crotonyl-CoA from acetoacetyl-CoA or other metabolites remains enigmatic, because no genes coding for 3-hydroxybutyryl-CoA dehydrogenase (EC 1.1.1.157), crotonase (EC 4.2.1.55) or glutaryl-CoA dehydrogenase (EC 1.3.99.7) could be detected. Syntrophic bacteria are able to catalyse the energetically unfavourable formation of crotonyl-CoA from butyryl-CoA using a reverse electron transfer *via* the Rnf system [23]. Since *C. acidaminovorans* is predicted to be a

<table><caption>**Table 2.** Identification of NAC-thioesters formed by Kat.</caption><thead><tr><th rowspan="2">NAC-thioester</th><th rowspan="2">Formula</th><th colspan="2">[M+H<sup>+</sup>]<sup>+</sup></th><th rowspan="2">Accuracy (ppm)</th><th rowspan="2">RT (min)*</th></tr><tr><th>Theoretical</th><th>Observed</th></tr></thead><tbody><tr><td>acetoacetyl-</td><td>C<sub>8</sub>H<sub>13</sub>NO<sub>3</sub>S</td><td>204.0689</td><td>204.0682</td><td>-3.4</td><td>6.14</td></tr><tr><td>5-methyl-3-oxohexanoate-</td><td>C<sub>11</sub>H<sub>19</sub>NO<sub>3</sub>S</td><td>246.1158</td><td>246.1152</td><td>-2.4</td><td>8.06</td></tr><tr><td>3-oxo-3-phenylpropanoate-</td><td>C<sub>11</sub>H<sub>19</sub>NO<sub>3</sub>S</td><td>266.0845</td><td>266.0838</td><td>-2.6</td><td>8.02</td></tr></tbody></table>

<figure><img src="image_12.png" /></figure>

<footnote>*RT indicates the retention time of the compound on the chromatography column; [M+H<sup>+</sup>]<sup>+</sup>, protonated molecular ion (positive ionization mode). doi:10.1371/journal.pone.0022918.t002</footnote>
syntroph, one may consider that in this organism the Bcd/Etf complex catalyses the formation of crotonyl-CoA from butyryl-CoA, a reaction coupled to the formation of hydrogen via a formate hydrogen lyase. However sequences of proteins catalysing the oxidative degradation of crotonyl-CoA to acetoacetyl-CoA are not detected in the genome, making this reverse electron transfer rather unlikely. Thus, we can not exclude that the two enzymatic reactions catalysing the formation of crotonyl-CoA from acetoacetyl-CoA are carried out by genes with no homology to those known so far, or that the formation of crotonyl-CoA could be performed through a novel enzymatic sequence.

On this basis, a variant lysine fermentation pathway is proposed in figure 5. The divergence with the classical pathway begins in step 5, where acetoacetyl-CoA and glutamate are generated from 3-aminobutyryl-CoA and α-ketoglutarate. The AtoA/AtoD complex can then catalyse the formation of acetoacetyl-CoA and acetate from acetoacetate and acetyl-CoA, yielding a second molecule of acetoacetyl-CoA (step 6). Recycling of α-ketoglutarate may be performed by a glutamate dehydrogenase (step 7). The two acetoacetyl-CoA molecules could be metabolized in two different ways. First, acetoacetyl-CoA can generate ATP and acetate, as described in the classical pathway (Figure 1, steps 8–10). Besides, acetoacetyl-CoA may also be converted to crotonyl-CoA, via unidentified enzymatic steps. Afterwards, the butyryl-CoA dehydrogenase complex may carry out the concomitant reduction of crotonyl-CoA and ferredoxin (step 11) [21]. Butyryl-CoA may finally be converted to butyrate and acetyl-CoA in the presence of acetate (step 12). Experimental evidence strengthens our model : the previous characterisation of *kdd* and *kce* in “*Candidatus* Cloacomonas acidaminovorans” [13] validates that L-*erythro*-3,5-diaminohexanoate is actually metabolized to acetoacetate and 3-aminobutyryl-CoA, in this organism. Furthermore in addition to the established function of Kat, the purified AtoA/AtoD complex has been shown to catalyze the formation *in vitro* of acetoacetyl-CoA and acetate from acetoacetate and acetyl-CoA (Table S1).

Thus, cultivable organisms which possess the same gene cluster organization as *F. nucleatum* and “*Candidatus* Cloacomonas acidaminovorans” may ferment lysine to identical products, but through different pathways. This is not totally surprising, as it has been demonstrated that the fermentation of glutamate in *Clostridium tetanomorphum* and *Peptococcus aerogenes* leads to identical products, but proceeds *via* entirely different metabolic pathways [24,25]. Furthermore, previous studies using tracer experiments have indicated that at least three more or less distinct pathways participate in the process of lysine fermentation in *Clostridia* [26,27]. The first one corresponds to that described in *F. nucleatum* [13], but another pathway involves a different carbon cleavage in lysine to form butyrate and acetate. Finally, a third pathway involves the formation of butyrate from two acetate or acetyl moieties. Only the first pathway has been established with confidence because there is little data available for the last two.

## Occurrence of kat in genomes and metagenomes

To determine if in the digester lysine is fermented through the canonical or the alternative pathway we searched first in the metagenomic sequences those related to the five genes present in both pathways (i.e *kdd* (coding for the 3,5 diaminohexanoate dehydrogenase), *kce* (coding for the 3-keto-5-aminohexanoate cleavage enzyme, *kamD* (coding for the lysine 5,6-aminomutase α-subunit), and *kamE* (coding for the lysine 5,6 aminomutase β-subunit, *kamA* (coding for the lysine 2,3 aminomutase)) The results are reported in Table 3. *kdd*, *kce*, *kamD* and *kamE* are present between 100 and 200 times. The overabundance of *kamA* could be explained by the similarity between its encoded protein and other

<table><caption>**Table 3.** Occurrences in the metagenome from the anaerobic digester of a wastewater treatment plant of the genes known to be involved in lysine fermentation.</caption><thead><tr><td>Gene name</td><td>Number of occurrences</td></tr></thead><tbody><tr><td>kdd</td><td>124</td></tr><tr><td>kce</td><td>125</td></tr><tr><td>kal</td><td>7</td></tr><tr><td>kamA</td><td>285</td></tr><tr><td>kamD</td><td>184</td></tr><tr><td>kamE</td><td>89</td></tr><tr><td>kat</td><td>159</td></tr></tbody></table>

doi:10.1371/journal.pone.0022918.t003

aminomutases found in organisms which probably do not ferment lysine. Then we searched for sequences similar to either Kal or Kat. The scarcity of *kal* suggests that a very few organisms present in the digester contained this gene and ferment lysine using the canonical pathway. On the other hand, since *kat* is found with a similar frequency to *kdd*, *kce*, *kamD*, and *kamE*, the alternate pathway may be present in most if not all organisms fermenting lysine in the digester. Kat sequences can be divided into two subgroups composed of almost identical sequences that share ~65% identity. The first one contains sequences related to WWE1 organisms while in the other one, sequences were affiliated with Bacteroidetes organisms. This indicates that the variant lysine fermentation pathway proceeds in at least two different phyla. A gene (DIG1_60031) from the reconstructed Bacteroidetes genome was cloned and expressed in *E. coli* for protein purification and biochemical characterization. Its enzymatic activity, tested in the presence of 25 µM 3-aminobutyryl-CoA and 30 mM α-ketoglutarate at 28°C in Activity Buffer, showed a similar rate (84%) to the one found for Kat under the same conditions. These data confirm that the variant route can be actually found in distinct phyla.

When Kat is compared to the millions of protein sequences accessible through (1) the Integrated Microbial Genomes interface (http://img.jgi.doe.gov; database : all img_w_v310.suser), (2) the CAMERA web site [28] (3) large scale gut microbiome survey [29], no clear Kat homolog could be detected. However, when Kat is compared to the NCBI RefSeq collection (http://www.ncbi.nlm.nih.gov/projects/RefSeq/), a protein with 60% identity over the whole length of the sequence was identified (NCBI Reference Sequence: ZP_07200410.1). This protein, annotated as a Class III aminotransferase belongs to the delta proteobacterium NaphS2 [30]. Despite this homology, this cultivable anaerobic marine sulphate-reducing bacterium does not possess the lysine fermentation genes, indicating that the function of its aminotransferase probably differs from that of Kat.

On the basis of sequence similarity, aminotransferases can be grouped into subfamilies (class I to V). Kat is proposed as a class III aminotransferase since it contains a Pfam-A domain (PF00202). As the amino acid similarity between all the aminotransferases is low (~30% identity), homology modelling appears to be a method of choice to compare protein sequences [31]. We have decided to build an aminotransferase tree including Kat from “*Candidatus* Cloacomonas acidaminovorans” based on a classification of active site homology models using the Active Sites Modelling and Clustering (ASMC) method [32]. ASMC is a phylogeny independent approach which permits to divide a family into

<figure><img src="image_11.png" /></figure>
syntroph, one may consider that in this organism the Bcd/Etf complex catalyses the formation of crotonyl-CoA from butyryl-CoA, a reaction coupled to the formation of hydrogen via a formate hydrogen lyase. However sequences of proteins catalysing the oxidative degradation of crotonyl-CoA to acetoacetyl-CoA are not detected in the genome, making this reverse electron transfer rather unlikely. Thus, we can not exclude that the two enzymatic reactions catalysing the formation of crotonyl-CoA from acetoacetyl-CoA are carried out by genes with no homology to those known so far, or that the formation of crotonyl-CoA could be performed through a novel enzymatic sequence.

On this basis, a variant lysine fermentation pathway is proposed in figure 5. The divergence with the classical pathway begins in step 5, where acetoacetyl-CoA and glutamate are generated from 3-aminobutyryl-CoA and α-ketoglutarate. The AtoA/AtoD complex can then catalyse the formation of acetoacetyl-CoA and acetate from acetoacetate and acetyl-CoA, yielding a second molecule of acetoacetyl-CoA (step 6). Recycling of α-ketoglutarate may be performed by a glutamate dehydrogenase (step 7). The two acetoacetyl-CoA molecules could be metabolized in two different ways. First, acetoacetyl-CoA can generate ATP and acetate, as described in the classical pathway (Figure 1, steps 8–10). Besides, acetoacetyl-CoA may also be converted to crotonyl-CoA, via unidentified enzymatic steps. Afterwards, the butyryl-CoA dehydrogenase complex may carry out the concomitant reduction of crotonyl-CoA and ferredoxin (step 11) [21]. Butyryl-CoA may finally be converted to butyrate and acetyl-CoA in the presence of acetate (step 12). Experimental evidence strengthens our model : the previous characterisation of *kdd* and *kce* in “*Candidatus* Cloacomonas acidaminovorans” [13] validates that L-*erythro*-3,5-diaminohexanoate is actually metabolized to acetoacetate and 3-aminobutyryl-CoA, in this organism. Furthermore in addition to the established function of Kat, the purified AtoA/AtoD complex has been shown to catalyze the formation *in vitro* of acetoacetyl-CoA and acetate from acetoacetate and acetyl-CoA (Table S1).

Thus, cultivable organisms which possess the same gene cluster organization as *F. nucleatum* and “*Candidatus* Cloacomonas acidaminovorans” may ferment lysine to identical products, but through different pathways. This is not totally surprising, as it has been demonstrated that the fermentation of glutamate in *Clostridium tetanomorphum* and *Peptococcus aerogenes* leads to identical products, but proceeds *via* entirely different metabolic pathways [24,25]. Furthermore, previous studies using tracer experiments have indicated that at least three more or less distinct pathways participate in the process of lysine fermentation in *Clostridia* [26,27]. The first one corresponds to that described in *F. nucleatum* [13], but another pathway involves a different carbon cleavage in lysine to form butyrate and acetate. Finally, a third pathway involves the formation of butyrate from two acetate or acetyl moieties. Only the first pathway has been established with confidence because there is little data available for the last two.

## Occurrence of kat in genomes and metagenomes

To determine if in the digester lysine is fermented through the canonical or the alternative pathway we searched first in the metagenomic sequences those related to the five genes present in both pathways (i.e *kdd* (coding for the 3,5 diaminohexanoate dehydrogenase), *kce* (coding for the 3-keto-5-aminohexanoate cleavage enzyme, *kamD* (coding for the lysine 5,6-aminomutase α-subunit), and *kamE* (coding for the lysine 5,6 aminomutase β-subunit, *kamA* (coding for the lysine 2,3 aminomutase)) The results are reported in Table 3. *kdd*, *kce*, *kamD* and *kamE* are present between 100 and 200 times. The overabundance of *kamA* could be explained by the similarity between its encoded protein and other

<table><caption>**Table 3.** Occurrences in the metagenome from the anaerobic digester of a wastewater treatment plant of the genes known to be involved in lysine fermentation.</caption><thead><tr><td>Gene name</td><td>Number of occurrences</td></tr></thead><tbody><tr><td>kdd</td><td>124</td></tr><tr><td>kce</td><td>125</td></tr><tr><td>kal</td><td>7</td></tr><tr><td>kamA</td><td>285</td></tr><tr><td>kamD</td><td>184</td></tr><tr><td>kamE</td><td>89</td></tr><tr><td>kat</td><td>159</td></tr></tbody></table>

doi:10.1371/journal.pone.0022918.t003

aminomutases found in organisms which probably do not ferment lysine. Then we searched for sequences similar to either Kal or Kat. The scarcity of *kal* suggests that a very few organisms present in the digester contained this gene and ferment lysine using the canonical pathway. On the other hand, since *kat* is found with a similar frequency to *kdd*, *kce*, *kamD*, and *kamE*, the alternate pathway may be present in most if not all organisms fermenting lysine in the digester. Kat sequences can be divided into two subgroups composed of almost identical sequences that share ~65% identity. The first one contains sequences related to WWE1 organisms while in the other one, sequences were affiliated with Bacteroidetes organisms. This indicates that the variant lysine fermentation pathway proceeds in at least two different phyla. A gene (DIG1_60031) from the reconstructed Bacteroidetes genome was cloned and expressed in *E. coli* for protein purification and biochemical characterization. Its enzymatic activity, tested in the presence of 25 µM 3-aminobutyryl-CoA and 30 mM α-ketoglutarate at 28°C in Activity Buffer, showed a similar rate (84%) to the one found for Kat under the same conditions. These data confirm that the variant route can be actually found in distinct phyla.

When Kat is compared to the millions of protein sequences accessible through (1) the Integrated Microbial Genomes interface (http://img.jgi.doe.gov; database : all img_w_v310.suser), (2) the CAMERA web site [28] (3) large scale gut microbiome survey [29], no clear Kat homolog could be detected. However, when Kat is compared to the NCBI RefSeq collection (http://www.ncbi.nlm.nih.gov/projects/RefSeq/), a protein with 60% identity over the whole length of the sequence was identified (NCBI Reference Sequence: ZP_07200410.1). This protein, annotated as a Class III aminotransferase belongs to the delta proteobacterium NaphS2 [30]. Despite this homology, this cultivable anaerobic marine sulphate-reducing bacterium does not possess the lysine fermentation genes, indicating that the function of its aminotransferase probably differs from that of Kat.

On the basis of sequence similarity, aminotransferases can be grouped into subfamilies (class I to V). Kat is proposed as a class III aminotransferase since it contains a Pfam-A domain (PF00202). As the amino acid similarity between all the aminotransferases is low (~30% identity), homology modelling appears to be a method of choice to compare protein sequences [31]. We have decided to build an aminotransferase tree including Kat from “*Candidatus* Cloacomonas acidaminovorans” based on a classification of active site homology models using the Active Sites Modelling and Clustering (ASMC) method [32]. ASMC is a phylogeny independent approach which permits to divide a family into

<figure><img src="image_13.png" /></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 6.** ASMC tree based on sequence and structural similarities of modelled active sites for 1343 enzymes belonging to the aminotransferases class III family (see Material and Methods). Known enzymatic activities are reported. Green: Adenosylmethionine-8-amino-7-oxononanoate transaminase. Red: 3-aminobutyryl-CoA transaminase. Orange: glutamate-1-semialdehyde-2, 1-aminomutase. Yellow: diaminobutyrate-2-oxoglutarate transaminase. Pink: taurine-pyruvate aminotransferase. Blue: ornithine, acetylornithine, pyruvate, 4-aminobutyrate, and diaminobutyrate-2-oxoglutarate transaminases.
doi:10.1371/journal.pone.0022918.g006</figcaption></figure>

sub-families depending on the composition of the active sites of its members. Since Kat does not show sequence homology with enzymes belonging to the other classes, the analysis was restricted to the class III aminotranferase family. A reduced set of 1343 sequences corresponding to a redundancy of 50% (UniRef50) was selected. Sequences from the digester that are homologous to Kat and the aminotransferases from NaphS2 and *Mesorhizobium* sp were added to this set. Results show that the sequence from "Candidatus Cloacamonas acidaminovorans" and the one from the reconstructed Bacteroidetes genome are clustered, beside other Kat homologous sequences from the anaerobic digester (figure 6). The aminotransferase from the delta proteobacterium NaphS2 belongs also to the same cluster. Ten additional enzymes have been classified in this cluster, but surprisingly, they are poorly related to Kat (~30% identity) and belong to organisms not expected to ferment lysine (UniProtKB entry: A8ZXI1, D6ZIK6, A0YBF7, Q2GBS7, Q39NX5, D8F1V2, B5JCU4, C1A9A5, Q2J7L8, and C4YXY7). This suggests that structural features of the active site of Kat are shared by other members of the aminotransferases class III family. It is worthy to note that the enzyme from *Mesorhizobium* sp that metabolizes the same substrate as Kat (minus the CoA moiety) is located in a distinct cluster. One may hypothesize that in the Kat cluster, the enzymes are involved in acyl-CoA transamination.

## Conclusion

Taking advantage of the enormous metabolic potential of uncultivated microbial diversity, we have identified and kinetically characterized Kat, a novel β-transaminase involved in an alternative lysine fermentation pathway. The enzyme catalyzes the transamination of both aliphatic and aromatic substrates. β-aminoacids are chiral synthetic targets found in free forms, i.e. β-alanine or β-homoleucine, or as chiral moieties in molecules of pharmaceutical interest. Since β-transaminases catalyze the reductive amination of the prochiral keto group of β-ketoacids to produce optically pure β-amino acids, they could become useful tools for the organic chemist. Although several β-transaminases have been isolated to date, this enzyme is unique as it shows activity exclusively on thioesters (CoA and CoA mimics). This substrate specificity could be seen as an advantage because β-ketoacid substrates are prone to decarboxylation and are therefore difficult to manipulate. Coupling the enzymatic reductive amination catalyzed by this new β-transaminase with a thioester cleavage enzyme, would produce the β-aminoacids of interest.

The gene encoding this enzyme was detected using gene context methods on assembled sequences of a metagenome from the anaerobic digester of a wastewater treatment plant. To date, this gene seems to be found exclusively in this metagenome (with the exception of a gene from the delta proteobacterium NaphS2,

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /><figcaption>**Figure 6.** ASMC tree based on sequence and structural similarities of modelled active sites for 1343 enzymes belonging to the aminotransferases class III family (see Material and Methods). Known enzymatic activities are reported. Green: Adenosylmethionine-8-amino-7-oxononanoate transaminase. Red: 3-aminobutyryl-CoA transaminase. Orange: glutamate-1-semialdehyde-2, 1-aminomutase. Yellow: diaminobutyrate-2-oxoglutarate transaminase. Pink: taurine-pyruvate aminotransferase. Blue: ornithine, acetylornithine, pyruvate, 4-aminobutyrate, and diaminobutyrate-2-oxoglutarate transaminases.
doi:10.1371/journal.pone.0022918.g006</figcaption></figure>

sub-families depending on the composition of the active sites of its members. Since Kat does not show sequence homology with enzymes belonging to the other classes, the analysis was restricted to the class III aminotranferase family. A reduced set of 1343 sequences corresponding to a redundancy of 50% (UniRef50) was selected. Sequences from the digester that are homologous to Kat and the aminotransferases from NaphS2 and *Mesorhizobium* sp were added to this set. Results show that the sequence from "Candidatus Cloacamonas acidaminovorans" and the one from the reconstructed Bacteroidetes genome are clustered, beside other Kat homologous sequences from the anaerobic digester (figure 6). The aminotransferase from the delta proteobacterium NaphS2 belongs also to the same cluster. Ten additional enzymes have been classified in this cluster, but surprisingly, they are poorly related to Kat (~30% identity) and belong to organisms not expected to ferment lysine (UniProtKB entry: A8ZXI1, D6ZIK6, A0YBF7, Q2GBS7, Q39NX5, D8F1V2, B5JCU4, C1A9A5, Q2J7L8, and C4YXY7). This suggests that structural features of the active site of Kat are shared by other members of the aminotransferases class III family. It is worthy to note that the enzyme from *Mesorhizobium* sp that metabolizes the same substrate as Kat (minus the CoA moiety) is located in a distinct cluster. One may hypothesize that in the Kat cluster, the enzymes are involved in acyl-CoA transamination.

## Conclusion

Taking advantage of the enormous metabolic potential of uncultivated microbial diversity, we have identified and kinetically characterized Kat, a novel β-transaminase involved in an alternative lysine fermentation pathway. The enzyme catalyzes the transamination of both aliphatic and aromatic substrates. β-aminoacids are chiral synthetic targets found in free forms, i.e. β-alanine or β-homoleucine, or as chiral moieties in molecules of pharmaceutical interest. Since β-transaminases catalyze the reductive amination of the prochiral keto group of β-ketoacids to produce optically pure β-amino acids, they could become useful tools for the organic chemist. Although several β-transaminases have been isolated to date, this enzyme is unique as it shows activity exclusively on thioesters (CoA and CoA mimics). This substrate specificity could be seen as an advantage because β-ketoacid substrates are prone to decarboxylation and are therefore difficult to manipulate. Coupling the enzymatic reductive amination catalyzed by this new β-transaminase with a thioester cleavage enzyme, would produce the β-aminoacids of interest.

The gene encoding this enzyme was detected using gene context methods on assembled sequences of a metagenome from the anaerobic digester of a wastewater treatment plant. To date, this gene seems to be found exclusively in this metagenome (with the exception of a gene from the delta proteobacterium NaphS2,

<figure><img src="image_15.png" /></figure>
which obviously does not ferment lysine). Since *kat* is over
represented as compared to *kal* (a distinctive gene of the classical
pathway), one may suggest that a higher number of organisms
ferment lysine according to the alternative pathway. It should be
noticed that this pathway was found in organisms representative of
two abundant phyla in the digester. This suggests that it should
also be present in other organisms yet to be described. This raises
the question of whether the classical pathway observed so far in
cultivable organisms is actually representative. A complementary
study of the occurrence of *kat* and *kal* in metagenomes from other
digesters of wastewater treatment plants should address this issue,
as representative sequences of the WWE1 lineage have been
identified in at least two other anaerobic digesters [33]. More
generally, this study raises a series of questions that will need to be
answered, such as: is this kind of situation an exception or a
common feature? And what is the biological relevance of the
variant pathway?

**Materials and Methods**

Chemicals

All chemicals and enzymes were purchased from Sigma-
Aldrich. Reagents for molecular biology were from Invitrogen.
Oligonucleotides were from Sigma Genosys. Proteinase inhibitor
Pefabloc SC was purchased from Roche Applied Science. The
Lysonase™ Bioprocessing Reagent was from Novagen. (3S)-3-
aminobutyryl-CoA was prepared by enzymatic conversion of 3,5-
diaminohexanoate [13]. The compound was produced using
70 µg of 3,5-diaminohexanoate dehydrogenase, 140 µg of 3-keto-
5-aminohexanoate cleavage enzyme, 30 mM 3,5-diaminohexano-
ate, 6 mM NAD+, and 4 mM acetyl-CoA in 500 µl 50 mM Tris/
HCl pH 9.1. Once equilibrium was reached (as indicated by
NAD+ reduction kinetics), the reaction was stopped by 1% 13 M
trifluoroacetic acid. 3-aminobutyryl-CoA concentration was esti-
mated by NADH formation. (R,S)-3-aminobutyryl-CoA was
synthesized by Alpha Chimica from (R,S)-3-aminobutyrate. S-
NAC thioesters were synthesized using conventional routes [34]
(see Data S1).

**Construction of the expression vectors**

The coding sequences of *hemL*-like (CLOAM0809) and *atoA*/
*atoD* from “Candidatus *Cloacamonas acidaminovorans***” were ampli-
fied by PCR with the following primers:

CLOAM0809-fwd-5'-GCAGAAAAATTGAAATTAGCC-3' and
CLOAM0809 -rev-5'-TTATAACATCTTTTTCACTTCG-3';

*atoA*-fwd-5'-GCATTAGATAAACGAGCTATG-3' and

*atoA*-rev-5'-TCAGAAGGTATTAACTCTTTTC-3';

*atoD*-fwd-5'-GTACAAATTATTAGTGCCGC-3' and

*atoD*-rev-5'-TTAATTCTCCTTTGCTAAAAC-3'

The amplified sequences were inserted into the Invitrogen
pEXP-5-NT/TOPO vector according to the manufacturer's
protocol. The coding sequence of hemL-like (DIG1_60031) from
the reconstructed genome affiliated with the Bacteroidetes phylum
was amplified with the following primers:

DIG1_60031 -fwd-5'-AAAGAAGGAGATAGGATCATGCA
TCATCACCATCACCATTCAAAGAAACTAAAACTGGACG
-3' and

DIG1_60031 -rev-5'-GTGTAATGGATAGTGATCTTAAA-
TATATTTTTTCAGTTCGCC-3'

The PCR product was inserted into the modified Novagen
pET22b(+) vector (kindly provided by V. Döring) by directional
cloning according to the ligation independent cloning method
[35]. The sequence of the resulting plasmids, named pEXP5-
HEML-LIKE, pEXP5-ATOA, pEXP5-ATOD, and pET22-
HEML-LIKE, respectively, were verified.

**Expression and purification of the recombinant proteins**

Cell culture, cell extracts, and protein purifications were
conducted as previously reported [13].

**Analytical methods**

Acetoacetyl-CoA formation was monitored at 310 nm, in the
presence of Mg²⁺ ions, using a molar extinction coefficient of
11900 M⁻¹·cm⁻¹ at pH 8.1, as described by Stern *et al.* [36].

**Enzyme assays**

Butyrate-acetoacetate CoA transferase activity was assayed in the
presence of various substrates by monitoring the formation of
acetoacetyl-CoA (acyl-CoA+acetoacetate↔acid+acetoacetyl-CoA).
Reactions were performed in 100 µl of Activity Buffer (50 mM
Tris/HCl pH 8.1) containing 10 mM acetoacetate, 15 µg of
reconstituted AtoA/AtoD complex, and 4 mM MgCl₂. The
reaction was initiated by the addition of the acetyl-CoA. 3-
aminobutyryl-CoA aminotransferase activity was assayed following
acetoacetyl-CoA formation. Reactions were performed in 100 µl of
Activity Buffer containing 5 mM MgCl₂ in the presence of various
concentrations of 3-aminobutyryl-CoA, α-ketoglutarate and pyru-
vate. The kinetic parameters were determined by varying one
substrate concentration while keeping the other one at a fixed
concentration. Kinetic constants were obtained from duplicate
experiments by non-linear analysis of initial rates using SigmaPlot
9.0 (Systat Software, Inc.). Activity-pH and activity-temperature
relationships were determined by incubating the enzyme at different
pH values and constant temperature (28°C) and at constant pH (8.1)
and different temperatures. As the apparent molecular extinction
coefficient of acetoacetyl-CoA increases continuously with pH [37],
its value was experimentally determined for each pH tested for
enzyme activity. All enzymatic reactions were performed in a Safas
UV mc² or a Lambda 650 Perkin Elmer double beam
spectrophotometer.

**LC/MS analyses**

LC/MS analyses were carried out using a LTQ/Orbitrap mass
spectrometer coupled to an Accela LC system (Thermo-Fisher).
Chromatographic separation was conducted using an Acquity
BeH C18 column (150×2 mm×1.7 µm; Waters) thermostated at
30°C. A mobile phase gradient was used with a flow rate of
0.4 ml/min in which mobile phase A consisted of 10 mM
ammonium acetate adjusted to pH 4.0 with 0.1% (vol/vol) formic
acid and mobile phase B consisted of methanol. The gradient
started at 100% A for 1 min, followed by a linear gradient at
100% B for 7 min, and finally 5 min at 100% B. The entire eluant
was sprayed into the mass spectrometer using a heated electro-
spray ionization source (175°C) at +4.5 kV with sheath, auxiliary
and sweep gases set at 60, 45 and 8 arbitrary units, respectively.
Desolvation of the droplets was further aided by setting the heated
capillary temperature at 250°C. All metabolites were detected in
the positive mode by full scan mass analysis from *m/z* 50–1000 at a
resolving power of 30,000 at *m/z* = 400. Data dependant scanning
was performed without use of a parent ion list. The resulting ion
fragments were recorded in the LTQ linear trap.

<figure><img src="image_14.png" /></figure>
which obviously does not ferment lysine). Since *kat* is over
represented as compared to *kal* (a distinctive gene of the classical
pathway), one may suggest that a higher number of organisms
ferment lysine according to the alternative pathway. It should be
noticed that this pathway was found in organisms representative of
two abundant phyla in the digester. This suggests that it should
also be present in other organisms yet to be described. This raises
the question of whether the classical pathway observed so far in
cultivable organisms is actually representative. A complementary
study of the occurrence of *kat* and *kal* in metagenomes from other
digesters of wastewater treatment plants should address this issue,
as representative sequences of the WWE1 lineage have been
identified in at least two other anaerobic digesters [33]. More
generally, this study raises a series of questions that will need to be
answered, such as: is this kind of situation an exception or a
common feature? And what is the biological relevance of the
variant pathway?

**Materials and Methods**

Chemicals

All chemicals and enzymes were purchased from Sigma-
Aldrich. Reagents for molecular biology were from Invitrogen.
Oligonucleotides were from Sigma Genosys. Proteinase inhibitor
Pefabloc SC was purchased from Roche Applied Science. The
Lysonase™ Bioprocessing Reagent was from Novagen. (3S)-3-
aminobutyryl-CoA was prepared by enzymatic conversion of 3,5-
diaminohexanoate [13]. The compound was produced using
70 µg of 3,5-diaminohexanoate dehydrogenase, 140 µg of 3-keto-
5-aminohexanoate cleavage enzyme, 30 mM 3,5-diaminohexano-
ate, 6 mM NAD⁺, and 4 mM acetyl-CoA in 500 µl 50 mM Tris/
HCl pH 9.1. Once equilibrium was reached (as indicated by
NAD⁺ reduction kinetics), the reaction was stopped by 1% 13 M
trifluoroacetic acid. 3-aminobutyryl-CoA concentration was esti-
mated by NADH formation. (R,S)-3-aminobutyryl-CoA was
synthesized by Alpha Chimica from (R,S)-3-aminobutyrate. S-
NAC thioesters were synthesized using conventional routes [34]
(see Data S1).

**Construction of the expression vectors**

The coding sequences of *hemL*-like (CLOAM0809) and *atoA*/
*atoD* from “Candidatus *Cloacamonas acidaminovorans***” were ampli-
fied by PCR with the following primers:

CLOAM0809-fwd-5'-GCAGAAAAATTGAAATTAGCC-3'
and
CLOAM0809 -rev-5'-TTATAACATCTTTTTCACTTCG-
3';

*atoA*-fwd-5'-GCATTAGATAAACGAGCTATG-3' and

*atoA*-rev-5'-TCAGAAGGTATTAACTCTTTTC-3';

*atoD*-fwd-5'-GTACAAATTATTAGTGCCGC-3' and

*atoD*-rev-5'-TTAATTCTCCTTTGCTAAAAC-3'

The amplified sequences were inserted into the Invitrogen
pEXP-5-NT/TOPO vector according to the manufacturer's
protocol. The coding sequence of hemL-like (DIG1_60031) from
the reconstructed genome affiliated with the Bacteroidetes phylum
was amplified with the following primers:

DIG1_60031 -fwd-5'-AAAGAAGGAGATAGGATCATGCA
TCATCACCATCACCATTCAAAGAAACTAAAACTGGACG
-3' and

DIG1_60031 -rev-5'-GTGTAATGGATAGTGATCTTAAA-
TATATTTTTTCAGTTCGCC-3'

The PCR product was inserted into the modified Novagen
pET22b(+) vector (kindly provided by V. Döring) by directional
cloning according to the ligation independent cloning method
[35]. The sequence of the resulting plasmids, named pEXP5-
HEML-LIKE, pEXP5-ATOA, pEXP5-ATOD, and pET22-
HEML-LIKE, respectively, were verified.

**Expression and purification of the recombinant proteins**

Cell culture, cell extracts, and protein purifications were
conducted as previously reported [13].

**Analytical methods**

Acetoacetyl-CoA formation was monitored at 310 nm, in the
presence of Mg²⁺ ions, using a molar extinction coefficient of
11900 M⁻¹·cm⁻¹ at pH 8.1, as described by Stern *et al.* [36].

**Enzyme assays**

Butyrate-acetoacetate CoA transferase activity was assayed in the
presence of various substrates by monitoring the formation of
acetoacetyl-CoA (acyl-CoA+acetoacetate↔acid+acetoacetyl-CoA).
Reactions were performed in 100 µl of Activity Buffer (50 mM
Tris/HCl pH 8.1) containing 10 mM acetoacetate, 15 µg of
reconstituted AtoA/AtoD complex, and 4 mM MgCl₂. The
reaction was initiated by the addition of the acetyl-CoA. 3-
aminobutyryl-CoA aminotransferase activity was assayed following
acetoacetyl-CoA formation. Reactions were performed in 100 µl of
Activity Buffer containing 5 mM MgCl₂ in the presence of various
concentrations of 3-aminobutyryl-CoA, α-ketoglutarate and pyru-
vate. The kinetic parameters were determined by varying one
substrate concentration while keeping the other one at a fixed
concentration. Kinetic constants were obtained from duplicate
experiments by non-linear analysis of initial rates using SigmaPlot
9.0 (Systat Software, Inc.). Activity-pH and activity-temperature
relationships were determined by incubating the enzyme at different
pH values and constant temperature (28°C) and at constant pH (8.1)
and different temperatures. As the apparent molecular extinction
coefficient of acetoacetyl-CoA increases continuously with pH [37],
its value was experimentally determined for each pH tested for
enzyme activity. All enzymatic reactions were performed in a Safas
UV mc² or a Lambda 650 Perkin Elmer double beam
spectrophotometer.

**LC/MS analyses**

LC/MS analyses were carried out using a LTQ/Orbitrap mass
spectrometer coupled to an Accela LC system (Thermo-Fisher).
Chromatographic separation was conducted using an Acquity
BeH C18 column (150×2 mm×1.7 µm; Waters) thermostated at
30°C. A mobile phase gradient was used with a flow rate of
0.4 ml/min in which mobile phase A consisted of 10 mM
ammonium acetate adjusted to pH 4.0 with 0.1% (vol/vol) formic
acid and mobile phase B consisted of methanol. The gradient
started at 100% A for 1 min, followed by a linear gradient at
100% B for 7 min, and finally 5 min at 100% B. The entire eluant
was sprayed into the mass spectrometer using a heated electro-
spray ionization source (175°C) at +4.5 kV with sheath, auxiliary
and sweep gases set at 60, 45 and 8 arbitrary units, respectively.
Desolvation of the droplets was further aided by setting the heated
capillary temperature at 250°C. All metabolites were detected in
the positive mode by full scan mass analysis from *m/z* 50–1000 at a
resolving power of 30,000 at *m/z* = 400. Data dependant scanning
was performed without use of a parent ion list. The resulting ion
fragments were recorded in the LTQ linear trap.

<figure><img src="image_16.png" /></figure>
## Sequence analysis

The genome sequence of "Candidatus Cloacomonas acidaminovorans" and the corresponding annotations were extracted from EMBL databank under accession number CU466930. The sequence of genes involved in the lysine fermentation pathway (FN1869, FN1868, FN1867, FN1866, FN1863, FN1862, *Fusobacterium nucleatum* ATCC 25586, AE009951) were compared to the "Candidatus Cloacomonas acidaminovorans" genome and to assembled genomic regions from the anaerobic digester using the BLAST2 algorithm [38]. These genes were also compared to "All Metagenomic Sequence Reads (N) » and All ORF Peptides available through the CAMERA web site (v1.3.2.29) [28] using the BLAST wizard algorithm. Putative orthologous relations between two genomes were defined as gene pairs satisfying the Bidirectional Best Hit criterion and an alignment threshold of 35% sequence identity over 80% of the length of the smallest protein. Synteny groups, i.e. conservation of the chromosomal co-localisation between pairs of orthologous genes from different genomes, were computed as previously described [39]. The data (i.e. syntactic and functional annotations, and results of comparative analysis) were stored in a relational database and explored by using the graphical interface of our microbial genome annotation system, MaGe. This database is publicly available at http://www.genoscope.cns.fr/agc/microscope/cloacamonascope [40]. Following the steps described in de Melo-Minardi et al. [32], homology models were built for each sequence and then aligned. Residues of catalytic pockets were extracted, and then submitted to a hierarchical clustering. The set of sequences for modelling was extracted from Uniprot (Pfam family PF00202; Aminotransferase class III). A reduced set of 1343 sequences corresponding to a redundancy to 50% (UniRef50) has been selected (http://www.uniprot.org/uniref/?query=uniprot%3a(PF00202)+identity:0.5). Thirteen template structures were used for modelling the following enzymes: n-acetylornithine aminotransferase (PDB code: 2PB2, Chain: A), adenosylmethionine-8-amino-7-oxonanoate aminotransferase (1DTY, A), 7,8-diaminopelargonic acid synthase (1QJ5, A), 2,2-dialkylglycine decarboxylase (1D7R, A), gamma-aminobutyrate aminotransferase (1SFF, A), glutamate-1-semialdehyde aminotransferase (3K28, A), lysine aminotransferase (2CIN, A), omega-amino acid:pyruvate aminotransferase (3A8U, X), ornithine aminotransferase (1GBN, A), aminotransferase prk07036 (3I5T, A), d-phenylglycine aminotransferase (2CY8, A), alpha-amino-epsilon-caprolactam racemase (2ZUK, A), and aminotransferase class iii (3I4J, A). Half of these structures have been crystallized with a bound ligand in their active site.

## References

1. Amann RI, Ludwig W, Schleifer KH (1995) Phylogenetic identification and in situ detection of individual microbial cells without cultivation. *Microbiol Rev* 59: 143-169.

2. Cowan DA (2000) Microbial genomes—the untapped resource. *Trends Biotechnol* 18: 14–16.

3. Pace NR (1997) A molecular view of microbial diversity and the biosphere. *Science* 276: 734–740.

4. Hugenholtz P, Tyson GW (2008) Microbiology: metagenomics. *Nature* 455: 481–483.

5. DeLong EF, Beja O (2010) The light-driven proton pump proteorhodopsin enhances bacterial survival during tough times. *PLoS Biol* 8: e1000359.

6. Sharon I, Alperovitch A, Rohwer F, Haynes M, Glaser F, et al. (2009) Photosystem I gene cassettes are present in marine virus genomes. *Nature* 461: 258–262.

7. Ettwig KF, Butler MK, Le Paslier D, Pelletier E, Mangenot S, et al. (2010) Nitrite-driven anaerobic methane oxidation by oxygenic bacteria. *Nature* 464: 543–548.

8. Ellrott K, Jaroszewski L, Li W, Wooley JC, Godzik A (2010) Expansion of the protein repertoire in newly explored environments: human gut microbiome specific protein families. *PLoS Comput Biol* 6: e1000798.

## Supporting Information

**Figure S1** Determination of the molecular weight of the AtoA/AtoD complex and HemL-like by Sephadex G-200 gel filtration. (●) molecular weight standards ovalbumin (mol. wt = 43,000), conalbumin (mol. weight = 75,000), aldolase (mol. wt = 158,000), ferritin (mol. weight = 440,000), and thyroglobulin (mol. weight = 669,000). (△) Peak of eluted AtoA/AtoD complex. (○) Peak of eluted HemL-like. (EPS)

**Figure S2** Absorption spectrum of HemL-like recorded in Activity Buffer (50 mM Tris/HCl pH 8.1) after incubation with excess PLP and submitted to gel filtration. (EPS)

**Figure S3** pH- and temperature-dependent activity profiles of Kat from "Candidatus Cloacomonas acidaminovorans". (A) pH-dependence of activity of Kat. Activities were measured at 28°C in the presence of 25 µM 3-aminobutyryl-CoA, 30 mM α-ketoglutarate, and 4 mM MgCl₂. The buffers used (50 mM) were Tris/HCl (●), and glycine/NaOH (○). (B) Temperature-dependence of activity of Kat. Activities were determined in 50 mM Tris/HCl pH 8.1 with 25 µM 3-aminobutyryl-CoA and 30 mM α-ketoglutarate. Values correspond to the average of two replicates. (EPS)

**Table S1** Substrate specificity of AtoA/AtoD. Reactions were performed in 100 µl of Activity Buffer containing 4 mM MgCl2, 10 mM acetoacetate, 15 µg of reconstituted AtoA/AtoD complex, and 100 µM acyl-CoA. 100% activity corresponds to 1.7 µmole of product/min/mg. Values correspond to the average of two replicates. ND: non detected. (DOC)

**Data S1** S-NAC thioesters synthesis. (DOC)

## Acknowledgments

We are very grateful to Nuria Fonknechten, Maxime Durot, and Georges Cohen for their helpful discussions. We also thank Aurélie Fossey for excellent technical assistance and Susan Cure for reading the manuscript.

## Author Contributions

Conceived and designed the experiments: AP MS. Performed the experiments: CL ST NP CP. Analyzed the data: AP MS CL AK ST NP CP. Contributed reagents/materials/analysis tools: DV AZ CV KB. Wrote the paper: AP MS JW.

9. Yooseph S, Sutton G, Rusch DB, Halpern AL, Williamson SJ, et al. (2007) The Sorcerer II Global Ocean Sampling expedition: expanding the universe of protein families. *PLoS Biol* 5: e16.

10. Chouari R, Le Paslier D, Daegelen P, Ginestet P, Weissenbach J, et al. (2005) Novel predominant archaeal and bacterial groups revealed by molecular analysis of an anaerobic sludge digester. *Environ Microbiol* 7: 1104–1115.

11. Pelletier E, Kreimeyer A, Bocs S, Rouy Z, Gyapay G, et al. (2008) "Candidatus Cloacomonas acidaminovorans": genome sequence reconstruction provides a first glimpse of a new bacterial division. *J Bacteriol* 190: 2572–2579.

12. Angelidaki I, Karakashev D, Batstone DJ, Plugge CM, Stams AJ (2011) Biomethanation and its potential. *Methods Enzymol* 494: 327–351.

13. Kreimeyer A, Perret A, Lechaplais C, Vallenet D, Medigue C, et al. (2007) Identification of the last unknown genes in the fermentation pathway of lysine. *J Biol Chem* 282: 7191–7197.

14. Ilag LL, Jahn D (1992) Activity and spectroscopic properties of the Escherichia coli glutamate 1-semialdehyde aminotransferase and the putative active site mutant K265R. *Biochemistry* 31: 7143–7151.

15. Luer C, Schauer S, Mobius K, Schulze J, Schubert WD, et al. (2005) Complex formation between glutamyl-tRNA reductase and glutamate-1-semialdehyde

<figure><img src="image_15.png" /></figure>
## Sequence analysis

The genome sequence of "Candidatus Cloacomonas acidaminovorans" and the corresponding annotations were extracted from EMBL databank under accession number CU466930. The sequence of genes involved in the lysine fermentation pathway (FN1869, FN1868, FN1867, FN1866, FN1863, FN1862, *Fusobacterium nucleatum* ATCC 25586, AE009951) were compared to the "Candidatus Cloacomonas acidaminovorans" genome and to assembled genomic regions from the anaerobic digester using the BLAST2 algorithm [38]. These genes were also compared to "All Metagenomic Sequence Reads (N) » and All ORF Peptides available through the CAMERA web site (v1.3.2.29) [28] using the BLAST wizard algorithm. Putative orthologous relations between two genomes were defined as gene pairs satisfying the Bidirectional Best Hit criterion and an alignment threshold of 35% sequence identity over 80% of the length of the smallest protein. Synteny groups, i.e. conservation of the chromosomal co-localisation between pairs of orthologous genes from different genomes, were computed as previously described [39]. The data (i.e. syntactic and functional annotations, and results of comparative analysis) were stored in a relational database and explored by using the graphical interface of our microbial genome annotation system, MaGe. This database is publicly available at http://www.genoscope.cns.fr/agc/microscope/cloacamonascope [40]. Following the steps described in de Melo-Minardi et al. [32], homology models were built for each sequence and then aligned. Residues of catalytic pockets were extracted, and then submitted to a hierarchical clustering. The set of sequences for modelling was extracted from Uniprot (Pfam family PF00202; Aminotransferase class III). A reduced set of 1343 sequences corresponding to a redundancy to 50% (UniRef50) has been selected (http://www.uniprot.org/uniref/?query=uniprot%3a(PF00202)+identity:0.5). Thirteen template structures were used for modelling the following enzymes: n-acetylornithine aminotransferase (PDB code: 2PB2, Chain: A), adenosylmethionine-8-amino-7-oxonanoate aminotransferase (1DTY, A), 7,8-diaminopelargonic acid synthase (1QJ5, A), 2,2-dialkylglycine decarboxylase (1D7R, A), gamma-aminobutyrate aminotransferase (1SFF, A), glutamate-1-semialdehyde aminotransferase (3K28, A), lysine aminotransferase (2CIN, A), omega-amino acid:pyruvate aminotransferase (3A8U, X), ornithine aminotransferase (1GBN, A), aminotransferase prk07036 (3I5T, A), d-phenylglycine aminotransferase (2CY8, A), alpha-amino-epsilon-caprolactam racemase (2ZUK, A), and aminotransferase class iii (3I4J, A). Half of these structures have been crystallized with a bound ligand in their active site.

## References

1. Amann RI, Ludwig W, Schleifer KH (1995) Phylogenetic identification and in situ detection of individual microbial cells without cultivation. Microbiol Rev 59: 143-169.

2. Cowan DA (2000) Microbial genomes—the untapped resource. Trends Biotechnol 18: 14–16.

3. Pace NR (1997) A molecular view of microbial diversity and the biosphere. Science 276: 734–740.

4. Hugenholtz P, Tyson GW (2008) Microbiology: metagenomics. Nature 455: 481–483.

5. DeLong EF, Beja O (2010) The light-driven proton pump proteorhodopsin enhances bacterial survival during tough times. PLoS Biol 8: e1000359.

6. Sharon I, Alperovitch A, Rohwer F, Haynes M, Glaser F, et al. (2009) Photosystem I gene cassettes are present in marine virus genomes. Nature 461: 258–262.

7. Ettwig KF, Butler MK, Le Paslier D, Pelletier E, Mangenot S, et al. (2010) Nitrite-driven anaerobic methane oxidation by oxygenic bacteria. Nature 464: 543–548.

8. Ellrott K, Jaroszewski L, Li W, Wooley JC, Godzik A (2010) Expansion of the protein repertoire in newly explored environments: human gut microbiome specific protein families. PLoS Comput Biol 6: e1000798.

## Supporting Information

**Figure S1** Determination of the molecular weight of the AtoA/AtoD complex and HemL-like by Sephadex G-200 gel filtration. (●) molecular weight standards ovalbumin (mol. wt = 43,000), conalbumin (mol. weight = 75,000), aldolase (mol. wt = 158,000), ferritin (mol. weight = 440,000), and thyroglobulin (mol. weight = 669,000). (△) Peak of eluted AtoA/AtoD complex. (○) Peak of eluted HemL-like. (EPS)

**Figure S2** Absorption spectrum of HemL-like recorded in Activity Buffer (50 mM Tris/HCl pH 8.1) after incubation with excess PLP and submitted to gel filtration. (EPS)

**Figure S3** pH- and temperature-dependent activity profiles of Kat from "Candidatus Cloacomonas acidaminovorans". (A) pH-dependence of activity of Kat. Activities were measured at 28°C in the presence of 25 µM 3-aminobutyryl-CoA, 30 mM α-ketoglutarate, and 4 mM MgCl₂. The buffers used (50 mM) were Tris/HCl (●), and glycine/NaOH (○). (B) Temperature-dependence of activity of Kat. Activities were determined in 50 mM Tris/HCl pH 8.1 with 25 µM 3-aminobutyryl-CoA and 30 mM α-ketoglutarate. Values correspond to the average of two replicates. (EPS)

**Table S1** Substrate specificity of AtoA/AtoD. Reactions were performed in 100 µl of Activity Buffer containing 4 mM MgCl2, 10 mM acetoacetate, 15 µg of reconstituted AtoA/AtoD complex, and 100 µM acyl-CoA. 100% activity corresponds to 1.7 µmole of product/min/mg. Values correspond to the average of two replicates. ND: non detected. (DOC)

**Data S1** S-NAC thioesters synthesis. (DOC)

## Acknowledgments

We are very grateful to Nuria Fonknechten, Maxime Durot, and Georges Cohen for their helpful discussions. We also thank Aurélie Fossey for excellent technical assistance and Susan Cure for reading the manuscript.

## Author Contributions

Conceived and designed the experiments: AP MS. Performed the experiments: CL ST NP CP. Analyzed the data: AP MS CL AK ST NP CP. Contributed reagents/materials/analysis tools: DV AZ CV KB. Wrote the paper: AP MS JW.

9. Yooseph S, Sutton G, Rusch DB, Halpern AL, Williamson SJ, et al. (2007) The Sorcerer II Global Ocean Sampling expedition: expanding the universe of protein families. PLoS Biol 5: e16.

10. Chouari R, Le Paslier D, Daegelen P, Ginestet P, Weissenbach J, et al. (2005) Novel predominant archaeal and bacterial groups revealed by molecular analysis of an anaerobic sludge digester. Environ Microbiol 7: 1104–1115.

11. Pelletier E, Kreimeyer A, Bocs S, Rouy Z, Gyapay G, et al. (2008) "Candidatus Cloacomonas acidaminovorans": genome sequence reconstruction provides a first glimpse of a new bacterial division. J Bacteriol 190: 2572–2579.

12. Angelidaki I, Karakashev D, Batstone DJ, Plugge CM, Stams AJ (2011) Biomethanation and its potential. Methods Enzymol 494: 327–351.

13. Kreimeyer A, Perret A, Lechaplais C, Vallenet D, Medigue C, et al. (2007) Identification of the last unknown genes in the fermentation pathway of lysine. J Biol Chem 282: 7191–7197.

14. Ilag LL, Jahn D (1992) Activity and spectroscopic properties of the Escherichia coli glutamate 1-semialdehyde aminotransferase and the putative active site mutant K265R. Biochemistry 31: 7143–7151.

15. Luer C, Schauer S, Mobius K, Schulze J, Schubert WD, et al. (2005) Complex formation between glutamyl-tRNA reductase and glutamate-1-semialdehyde

<figure><img src="image_17.png" /></figure>
2,1-aminomutase in Escherichia coli during the initial reactions of porphyrin biosynthesis. J Biol Chem 280: 18568-18572.

16. Barker HA, Jeng IM, Neff N, Robertson JM, Tam FK, et al. (1978) Butyryl-CoA:acetoacetate CoA-transferase from a lysine-fermenting Clostridium. J Biol Chem 253: 1219-1225.

17. Cary JW, Petersen DJ, Papoutsakis ET, Bennett GN (1990) Cloning and expression of Clostridium acetobutylicum ATCC 824 acetoacetyl-coenzyme A:acetate/butyrate:coenzyme A-transferase in Escherichia coli. Appl Environ Microbiol 56: 1576-1583.

18. Riviere D, Desvignes V, Pelletier E, Chaussonnerie S, Guermazi S, et al. (2009) Towards the definition of a core of microorganisms involved in anaerobic digestion of sludge. Isme J 3: 700-714.

19. Kim J, Kyung D, Yun H, Cho BK, Seo JH, et al. (2007) Cloning and characterization of a novel beta-transaminase from Mesorhizobium sp. strain LUK: a new biocatalyst for the synthesis of enantiomerically pure beta-amino acids. Appl Environ Microbiol 73: 1772-1782.

20. Yue S, Duncan JS, Yoshikuni Y, Hutchinson CR (1987) Macrolide biosynthesis. Tylactone formation involves the processive addition of three carbon units. J Am Chem Soc 109: 1253-1255.

21. Herrmann G, Jayamani E, Mai G, Buckel W (2008) Energy conservation via electron-transferring flavoprotein in anaerobic bacteria. J Bacteriol 190: 784-791.

22. Li F, Hinderberger J, Seedorf H, Zhang J, Buckel W, et al. (2008) Coupled ferredoxin and crotonyl coenzyme A (CoA) reduction with NADH catalyzed by the butyryl-CoA dehydrogenase/Etf complex from Clostridium kluyveri. J Bacteriol 190: 843-850.

23. McInerney MJ, Sieber JR, Gunsalus RP (2009) Syntrophy in anaerobic global carbon cycles. Curr Opin Biotechnol 20: 623-632.

24. Buckel W, Barker HA (1974) Two pathways of glutamate fermentation by anaerobic bacteria. J Bacteriol 117: 1248-1260.

25. Lerud RF, Whiteley HR (1971) Purification and properties of alpha-ketoglutarate reductase from Micrococcus aerogenes. J Bacteriol 106: 571-577.

26. Barker HA, Kahn JM, Hedrick L (1982) Pathway of lysine degradation in Fusobacterium nucleatum. J Bacteriol 152: 201-207.

27. Stadtman TC (1973) Lysine metabolism by Clostridia. Advances in Enzymology - and Related Areas of Molecular Biology 38: 413-448.

28. Seshadri R, Kravitz SA, Smarr L, Gilha P, Frazier M (2007) CAMERA: a community resource for metagenomics. PLoS Biol 5: e75.

29. Qin J, Li R, Raes J, Arumugam M, Burgdorf KS, et al. (2010) A human gut microbial gene catalogue established by metagenomic sequencing. Nature 464: 59-65.

30. Galushko A, Minz D, Schink B, Widdel F (1999) Anaerobic degradation of naphthalene by a pure culture of a novel type of marine sulphate-reducing bacterium. Environ Microbiol 1: 415-420.

31. Battey JN, Kopp J, Bordoli L, Read RJ, Clarke ND, et al. (2007) Automated server predictions in CASP7. Proteins 69 Suppl 8: 68-82.

32. de Melo-Minardi RC, Bastard K, Artiguenave F (2010) Identification of subfamily-specific sites based on active sites modeling and clustering. Bioinformatics 26: 3075-3082.

33. Chouari R, Le Paslier D, Dauga C, Daegelen P, Weissenbach J, et al. (2005) Novel major bacterial candidate division within a municipal anaerobic sludge digester. Appl Environ Microbiol 71: 2145-2153.

34. Brobst SW, Townsend CA (1994) The Potential Role of Fatty-Acid Initiation in the Biosynthesis of the Fungal Aromatic Polyketide Aflatoxin B-1. Canadian Journal of Chemistry-Revue Canadienne De Chimie 72: 200-207.

35. de Jong RN, Daniels MA, Kaptein R, Folkers GE (2006) Enzyme free cloning for high throughput gene cloning and expression. J Struct Funct Genomics 7: 109-118.

36. Stern JR, Coon MJ, Delcampillo A, Schneider MC (1956) Enzymes of Fatty Acid Metabolism. 4. Preparation and Properties of Coenzyme A Transferase. Journal of Biological Chemistry 221: 15-31.

37. Stern JR (1956) Optical properties of aceto-acetyl-S-coenzyme A and its metal chelates. J Biol Chem 221: 33-44.

38. Altschul SF, Madden TL, Schaffer AA, Zhang J, Zhang Z, et al. (1997) Gapped BLAST and PSI-BLAST: a new generation of protein database search programs. Nucleic Acids Res 25: 3389-3402.

39. Vallenet D, Labarre L, Rouy Z, Barbe V, Bocs S, et al. (2006) MaGe: a microbial genome annotation system supported by synten results. Nucleic Acids Res 34: 53-65.

40. Vallenet D, Engelen S, Mornico D, Cruveiller S, Fleury L, et al. (2009) MicroScope: a platform for microbial genome annotation and comparative genomics. Database (Oxford) 2009: bap021.

<figure><img src="image_16.png" /></figure>
2,1-aminomutase in Escherichia coli during the initial reactions of porphyrin biosynthesis. J Biol Chem 280: 18568-18572.

16. Barker HA, Jeng IM, Neff N, Robertson JM, Tam FK, et al. (1978) Butyryl-CoA:acetoacetate CoA-transferase from a lysine-fermenting Clostridium. J Biol Chem 253: 1219-1225.

17. Cary JW, Petersen DJ, Papoutsakis ET, Bennett GN (1990) Cloning and expression of Clostridium acetobutylicum ATCC 824 acetoacetyl-coenzyme A:acetate/butyrate:coenzyme A-transferase in Escherichia coli. Appl Environ Microbiol 56: 1576-1583.

18. Riviere D, Desvignes V, Pelletier E, Chaussonnerie S, Guermazi S, et al. (2009) Towards the definition of a core of microorganisms involved in anaerobic digestion of sludge. Isme J 3: 700-714.

19. Kim J, Kyung D, Yun H, Cho BK, Seo JH, et al. (2007) Cloning and characterization of a novel beta-transaminase from Mesorhizobium sp. strain LUK: a new biocatalyst for the synthesis of enantiomerically pure beta-amino acids. Appl Environ Microbiol 73: 1772-1782.

20. Yue S, Duncan JS, Yoshikuni Y, Hutchinson CR (1987) Macrolide biosynthesis. Tylactone formation involves the processive addition of three carbon units. J Am Chem Soc 109: 1253-1255.

21. Herrmann G, Jayamani E, Mai G, Buckel W (2008) Energy conservation via electron-transferring flavoprotein in anaerobic bacteria. J Bacteriol 190: 784-791.

22. Li F, Hinderberger J, Seedorf H, Zhang J, Buckel W, et al. (2008) Coupled ferredoxin and crotonyl coenzyme A (CoA) reduction with NADH catalyzed by the butyryl-CoA dehydrogenase/Etf complex from Clostridium kluyveri. J Bacteriol 190: 843-850.

23. McInerney MJ, Sieber JR, Gunsalus RP (2009) Syntrophy in anaerobic global carbon cycles. Curr Opin Biotechnol 20: 623-632.

24. Buckel W, Barker HA (1974) Two pathways of glutamate fermentation by anaerobic bacteria. J Bacteriol 117: 1248-1260.

25. Lerud RF, Whiteley HR (1971) Purification and properties of alpha-ketoglutarate reductase from Micrococcus aerogenes. J Bacteriol 106: 571-577.

26. Barker HA, Kahn JM, Hedrick L (1982) Pathway of lysine degradation in Fusobacterium nucleatum. J Bacteriol 152: 201-207.

27. Stadtman TC (1973) Lysine metabolism by Clostridia. Advances in Enzymology - and Related Areas of Molecular Biology 38: 413-448.

28. Seshadri R, Kravitz SA, Smarr L, Gilha P, Frazier M (2007) CAMERA: a community resource for metagenomics. PLoS Biol 5: e75.

29. Qin J, Li R, Raes J, Arumugam M, Burgdorf KS, et al. (2010) A human gut microbial gene catalogue established by metagenomic sequencing. Nature 464: 59-65.

30. Galushko A, Minz D, Schink B, Widdel F (1999) Anaerobic degradation of naphthalene by a pure culture of a novel type of marine sulphate-reducing bacterium. Environ Microbiol 1: 415-420.

31. Battey JN, Kopp J, Bordoli L, Read RJ, Clarke ND, et al. (2007) Automated server predictions in CASP7. Proteins 69 Suppl 8: 68-82.

32. de Melo-Minardi RC, Bastard K, Artiguenave F (2010) Identification of subfamily-specific sites based on active sites modeling and clustering. Bioinformatics 26: 3075-3082.

33. Chouari R, Le Paslier D, Dauga C, Daegelen P, Weissenbach J, et al. (2005) Novel major bacterial candidate division within a municipal anaerobic sludge digester. Appl Environ Microbiol 71: 2145-2153.

34. Brobst SW, Townsend CA (1994) The Potential Role of Fatty-Acid Initiation in the Biosynthesis of the Fungal Aromatic Polyketide Aflatoxin B-1. Canadian Journal of Chemistry-Revue Canadienne De Chimie 72: 200-207.

35. de Jong RN, Daniels MA, Kaptein R, Folkers GE (2006) Enzyme free cloning for high throughput gene cloning and expression. J Struct Funct Genomics 7: 109-118.

36. Stern JR, Coon MJ, Delcampillo A, Schneider MC (1956) Enzymes of Fatty Acid Metabolism. 4. Preparation and Properties of Coenzyme A Transferase. Journal of Biological Chemistry 221: 15-31.

37. Stern JR (1956) Optical properties of aceto-acetyl-S-coenzyme A and its metal chelates. J Biol Chem 221: 33-44.

38. Altschul SF, Madden TL, Schaffer AA, Zhang J, Zhang Z, et al. (1997) Gapped BLAST and PSI-BLAST: a new generation of protein database search programs. Nucleic Acids Res 25: 3389-3402.

39. Vallenet D, Labarre L, Rouy Z, Barbe V, Bocs S, et al. (2006) MaGe: a microbial genome annotation system supported by synten results. Nucleic Acids Res 34: 53-65.

40. Vallenet D, Engelen S, Mornico D, Cruveiller S, Fleury L, et al. (2009) MicroScope: a platform for microbial genome annotation and comparative genomics. Database (Oxford) 2009: bap021.

<figure><img src="image_18.png" /></figure>