Tetrahydrodipicolinate N-succinyltransferase, transferase hexapeptide repeat family (DapD_Trfase_Hexpep_rpt_fam)
Type: Family

Bacteria, plants and fungi metabolise aspartic acid to produce four amino acids -lysine, threonine, methionine and isoleucine -in a series of reactions known as the aspartate pathway. Additionally, several important metabolic intermediates are produced by these reactions, such as diaminopimelic acid, an essential component of bacterial cell wall biosynthesis, and dipicolinic acid, which is involved in sporulation in Gram-positive bacteria. Members of the animal kingdom do not posses this pathway and must therefore acquire these essential amino acids through their diet. Research into improving the metabolic flux through this pathway has the potential to increase the yield of the essential amino acids in important crops, thus improving their nutritional value. Additionally, since the enzymes are not present in animals, inhibitors of them are promising targets for the development of novel antibiotics and herbicides. For more information see .

Two lysine biosynthesis pathways evolved separately in organisms, the diaminopimelic acid (DAP) and aminoadipic acid (AAA) pathways. The DAP pathway synthesizes L-lysine from aspartate and pyruvate, and diaminopimelic acid is an intermediate. This pathway is utilised by most bacteria, some archaea, some fungi, some algae, and plants. The AAA pathway synthesizes L-lysine from alpha-ketoglutarate and acetyl coenzyme A (acetyl-CoA), and alpha-aminoadipic acid is an intermediate. This pathway is utilised by most fungi, some algae, the bacterium Thermus thermophilus, and probably some archaea, such as Sulfolobus, Thermoproteus, and Pyrococcus. No organism is known to possess both pathways .

There four known variations of the DAP pathway in bacteria: the succinylase, acetylase, aminotransferase, and dehydrogenase pathways. These pathways share the steps converting L-aspartate to L-2,3,4,5- tetrahydrodipicolinate (THDPA), but the subsequent steps leading to the production of meso-diaminopimelate, the immediate precursor of L-lysine, are different .

The succinylase pathway acylates THDPA with succinyl-CoA to generate N-succinyl-LL-2-amino-6-ketopimelate and forms meso-DAP by subsequent transamination, desuccinylation, and epimerization. This pathway is utilised by proteobacteria and many firmicutes and actinobacteria.

The acetylase pathway is analogous to the succinylase pathway but uses N-acetyl intermediates. This pathway is limited to certain Bacillus species, in which the corresponding genes have not been identified.

The aminotransferase pathway converts THDPA directly to LL-DAP by diaminopimelate aminotransferase (DapL) without acylation. This pathway is shared by cyanobacteria, Chlamydia, the archaeon Methanothermobacter thermautotrophicus, and the plant Arabidopsis thaliana.

The dehydrogenase pathway forms meso-DAP directly from THDPA, NADPH, and NH4 _ by using diaminopimelate dehydrogenase (Ddh). This pathway is utilised by some Bacillus and Brevibacterium species and Corynebacterium glutamicum.

Most bacteria use only one of the four variants, although certain bacteria, such as C. glutamicum and Bacillus macerans, possess both the succinylase and dehydrogenase pathways.

2,3,4,5-tetrahydropyridine-2,6-dicarboxylate N-succinyltransferase (also known as tetrahydrodipicolinate N-succinyltransferase or DapD) is part of the succinyl route of of lysine/DAP biosynthesis. The DapD protein is a homotrimer is a trimeric enzyme with each monomer composed of three domain: an N-terminal helical domain, a distinctive left-handed parallel β-helix (LBH) domain, and a predominantly β C-terminal domain . The LBH structure is encoded by an imperfect tandem-repeated hexapeptide sequence. Each trimer contains three independent active sites, always occuring at the boundary of two subunits, and formed by residues from one N-terminal domain, one C-terminal domain and two adjacent LBH domains.