Glucansucrase

Glucansucrase (also known as glucosyltransferase) is an enzyme in the glycoside hydrolase family GH70 used by lactic acid bacteria to split sucrose and use resulting glucose molecules to build long, sticky biofilm chains. These extracellular homopolysaccharides are called α-glucan polymers. Glucansucrase enzymes can synthesize a variety of glucans with differing solubilities, rheology, and other properties by altering the type of glycosidic linkage, degree of branching, length, mass, and conformation of the polymers. Glucansucrases are classified according to the glycosidic linkage they catalyze. They can be mutansucrases, dextransucrases, alternansucrases, or reuteransucrases. This versatility has made glucansucrase useful for industrial applications. Glucansucrase's role in cariogenesis is a major point of interest. Glucan polymers stick to teeth in the human mouth and cause tooth decay. Glucansucrases are large, extracellular proteins with average molecular masses around 160,000 Daltons. Therefore crystallography studies have only been carried out for fragments of the enzymes, not complete structures. However, glucansucrase is very similar to α-amylase, another sugar-cutting enzyme. Glucansucrase thus has many of the same structural features. For example, both enzymes have three domains in their catalytic core and a (β/α)8 barrel. Glucansucrase has five major domains: A, B, C, IV, and V. The domains in glucansucrase, however, have a different arrangement than those in α-amylase. The folding characteristics of α-amylase and glucansucrase are still very similar, but their domains are permuted. Domains A, B, IV, and V are built from two discontiguous parts of the polypeptide chain, causing the chain to follow a U-shape. From the N- to C-terminus, the polypeptide chain goes in the following order: V, IV, B, A, C, A, B, IV, V (see figure at top right). The C domain is the only one made up of a continuous polypeptide sequence. Domain A contains the (β/α)8 barrel and the catalytic site.