Lactobacillus helveticus glycosyltransferases: from genes to carbohydrate synthesis

Bioactive carbohydrates are crucial in mediating essential biological processes, and their biosynthesis is an essential aspect to develop for a global view of their biological functions. Lactic acid bacteria display an array of diverse and complex carbohydrates and, therefore, are of particular interest. Here we present the identification of a novel exocellular polysaccharide structure and the corresponding gene cluster from Lactobacillus helveticus NCC2745. The development of a glycosyltransferase-specific enzymatic assay allowed the assignment of sugar specificities, which as a general approach will for the future permit a faster and more direct characterization of glycosyltransferase specificities. A model of the biosynthesis of the repeating unit is proposed. EpsE is a phosphoglucosyltransferase initiating the repeating unit biosynthesis by linking a glucose residue to a membrane-associated lipophilic acceptor. EpsF elongates the carbohydrate chain by forming an α(1,3)-Glcp linkage onto the first Glcp, whereas EpsG adds a backbone α(1,6)-Galp onto α-Glcp and EpsH attaches a α(1,6).Glcp branch onto the first glucose residue. Finally, EpsI would add a β(1,6).Galp linkage onto α-Glcp terminating the sidechain and EpsJ would terminate the synthesis of the polysaccharides' repeating unit by forming a β(1,3)-Galp linkage onto α-Galp.