The enzymatic synthesis of glycosidic bonds: "Glycosynthases" and glycosyltransferases

The last 10 years has witnessed enormous advances in the structural enzymology of enzymatic glycosyl transfer. Developments in molecular biology, coupled with the possibility for "rapid-throughput" protein crystallography have led to a plethora of 3-D structures for glycoside hydrolases. Synergistic application of synthetic chemistry to structural biology has led to the development of numerous oligosaccharide mimics and mechanistic probes. Underpinning much of this work is a classification of carbohydrate-active enzymes ("CAZymes") which has now been extended to include glycoside hydrolases, carbohydrate esterases, polysaccharide lyases and glycosyltransferases, together with their associated non-catalytic modules. Recently, there have been exciting developments in our understanding of the structures and catalytic mechanisms involved in the synthesis of glycosidic bonds both by Nature's own catalytic apparatus and by mutant glycoside hydrolases termed "glycosynthases". Here we review those developments including the structure of the Bacillus subtilis SpsA, a "family GT-2" glycosyltransferase, and discuss its relevance to the synthesis of biopolymers such as cellulose, chitin, hyaluronan and the Nod factors.