Enzymes Required for Maltodextrin Catabolism in Enterococcus faecalis Exhibit Novel Activities
Authors: Joyet Philippe , Mokhtari Abdelhamid , Riboulet-Bisson Eliette , Blancato Víctor S. , Espariz Martin , Magni Christian , Hartke Axel , Deutscher Josef , Sauvageot Nicolas 
Published: April 2017

Maltose and maltodextrins are formed during the degradation of starch or glycogen. Maltodextrins are composed of a mixture of maltooligosaccharides formed by α-1,4- but also some α-1,6-linked glucosyl residues. The α-1,6-linked glucosyl residues are derived from branching points in the polysaccharides. In
Enterococcus faecalis
, maltotriose is mainly transported and phosphorylated by a phosphoenolpyruvate:carbohydrate phosphotransferase system. The formed maltotriose-6″-phosphate is intracellularly dephosphorylated by a specific phosphatase, MapP. In contrast, maltotetraose and longer maltooligosaccharides up to maltoheptaose are taken up without phosphorylation via the ATP binding cassette transporter MdxEFG-MsmX. We show that the maltose-producing maltodextrin hydrolase MmdH (GenBank accession no.
EFT41964
) in strain JH2-2 catalyzes the first catabolic step of α-1,4-linked maltooligosaccharides. The purified enzyme converts even-numbered α-1,4-linked maltooligosaccharides (maltotetraose, etc.) into maltose and odd-numbered (maltotriose, etc.) into maltose and glucose. Inactivation of
mmdH
therefore prevents the growth of
E. faecalis
on maltooligosaccharides ranging from maltotriose to maltoheptaose. Surprisingly, MmdH also functions as a maltogenic α-1,6-glucosidase, because it converts the maltotriose isomer isopanose into maltose and glucose. In addition,
E. faecalis
contains a glucose-producing α-1,6-specific maltodextrin hydrolase (GenBank accession no.
EFT41963
, renamed GmdH). This enzyme converts panose, another maltotriose isomer, into glucose and maltose. A
gmdH
mutant had therefore lost the capacity to grow on panose. The genes
mmdH
and
gmdH
are organized in an operon together with GenBank accession no.

EFT41962

(renamed
mmgT
). Purified MmgT transfers glucosyl residues from one α-1,4-linked maltooligosaccharide molecule to another. For example, it catalyzes the disproportionation of maltotriose by transferring a glucosyl residue to another maltotriose molecule, thereby forming maltotetraose and maltose together with a small amount of maltopentaose.

IMPORTANCE
The utilization of maltodextrins by
Enterococcus faecalis
has been shown to increase the virulence of this nosocomial pathogen. However, little is known about how this organism catabolizes maltodextrins. We identified two enzymes involved in the metabolism of various α-1,4- and α-1,6-linked maltooligosaccharides. We found that one of them functions as a maltose-producing α-glucosidase with relaxed linkage specificity (α-1,4 and α-1,6) and exo- and endoglucosidase activities. A third enzyme, which resembles amylomaltase, exclusively transfers glucosyl residues from one maltooligosaccharide molecule to another. Similar enzymes are present in numerous other
Firmicutes
, such as streptococci and lactobacilli, suggesting that these organisms follow the same maltose degradation pathway as
E. faecalis
.