Toward Efficient Enzymatic Glycan Synthesis: Directed Evolution and Enzyme Engineering

Enzymatic synthesis of oligosaccharides has traditionally employed glycosyltransferases, or glycosidases run in transglycosylation mode. Glycosynthases, mutant glycosidases in which the catalytic nucleophile has been removed, function as efficient transferases with glycosyl fluoride donors, frequently giving stoicheometric yields. Glycoligases, in which the acid/base catalyst has been mutated, synthesise sulfur-linked oligosaccharides when an activated donor is used in conjunction with a thiosugar acceptor, or in some cases O-glycosides. Recent results in the engineering and evolution of these two classes of mutant enzymes along with ``classical{''} glycosyltransferases using a variety of screening methodologies, including robot-assisted ELISA assays and FACS cell sorting, are discussed.