Glucansucrases: mechanism of action and structure-function relationships

Glucansucrases are produced principally by Leuconostoc mesenteroides and or al Streptococcus species, but also by the lactic acid bacteria (Lactococci, Lactobacilli). They catalyse the synthesis of high molecular weight D-gluc ose polymers, named glucans, from sucrose. In the presence of efficient acc epters, they catalyse the synthesis of low molecular weight oligosaccharide s. Glucosidic bond synthesis occurs without the mediation of nucleotide act ivated sugars and cofactors are not necessary. Glucansucrases have an indus trial value because of the production of dextrans and oligosaccharides and a biological importance by their key role in the cariogenic process. They w ere identified more than 50 years ago. The first glucansucrase encoding gen e was cloned more than 10 years ago. But the mechanism of their action rema ins incompletely understood. However, in order to synthesise oligosaccharid es of biological interest or to develop vaccines against dental caries, elu cidation of the factors determining the regiospecificity and the regioselec tivity of glucansucrases is necessary. The cloning of glucansucrase encodin g genes in addition to structure-function relationship studies have allowed the identification of important amino acid residues and have shown that gl ucansucrases are composed of two functional domains: a core region (ca. 100 0 amino acids) involved in sucrose binding and splitting and a C-terminal d omain (ca. 500 amino acids) composed of a series of tandem repeats involved in glucan binding. Enzymology studies have enabled different models for th eir action mechanism to be proposed. The use of secondary structure predict ion has led to a dearer knowledge of structure-function relationships of gl ucansucrases. However, mainly due to the large size of these enzymes, data on the three-dimensional structure of glucansucrases (given by crystallogra phy and modelling) remain necessary to clearly identify those features whic h determine function. (C) 1999 Federation of European Microbiological Socie ties. Published by Elsevier Science B.V. All rights reserved.