Glycosidic bond specificity of glucansucrases: on the role of acceptor substrate binding residues

Many lactic acid bacteria produce extracellular alpha-glucan polysaccharides using a glucansucrase and sucrose as glucose donor. The structure and the physicochemical properties of the alpha-glucans produced are determined by the nature of the glucansucrase. Typically, the alpha-glucans contain two types of alpha-glycosidic linkages, for example, (alpha 1-2), (alpha 1-3), (alpha 1-4) or (alpha 1-6), which may be randomly or regularly distributed. Usually, the alpha-glucan chains are also branched, which gives rise to an additional level of complexity. Even though the first crystal structure was reported in 2010, our current understanding of the structure-function relationships of glucansucrases is not advanced enough to predict the alpha-glucan specificity from the sequence alone. Nevertheless, based on sequence alignments and site-directed mutagenesis, a few amino acid residues have been identified as being important for the glycosidic bond specificity of glucansucrases. A new development in GH70 research was the identification of a cluster of alpha-glucan disproportionating enzymes. Here, we discuss the current insights into the structure-function relationships of GH70 enzymes in the light of the recently determined crystal structure of glucansucrases.