Glucan phosphorylase-catalyzed enzymatic synthesis of unnatural oligosaccharides and polysaccharides using nonnative substrates

Oligosaccharides and polysaccharides are comprised of complicated chemical structures owing to the structural variation of monosaccharide repeating units and the differences in the regio- and stereo-arrangements of the glycosidic linkages in their saccharide chains. Glucan phosphorylase (GP, EC 2.4.1.1) catalyzes consecutive enzymatic glycosylation in a manner similar to enzymatic polymerization employing α-d-glucose 1-phosphate (Glc-1-P) and maltooligosaccharide as a glycosyl donor and acceptor (or monomer and primer), respectively, to produce a well-defined α(1→4)-glucan polymer, that is, amylose, while liberating inorganic phosphate (Pi). After understanding the principal reaction mechanism and specificity of GP catalysis, the present review focuses on the enzymatic synthesis of unnatural oligosaccharides and polysaccharides linked through strictly controlled α(1→4)-glycosidic linkages by GP catalysis. Due to the weak specificity of the recognition of substrates by GP, unnatural oligosaccharides having different monosaccharide units at the nonreducing end have been precisely obtained by GP-catalyzed glycosylation using analog substrates of Glc-1-P, i.e., nonnative monosaccharide 1-phosphates. Highly branched α(1→4)-glucans have been employed as polymeric glycosyl acceptors and primers for GP-catalyzed enzymatic glycosylation and polymerization to obtain unnatural amphoteric and hydrogel materials. Thermostable GP catalyzes consecutive enzymatic glycosylation using α-d-glucosamine and α-d-mannose 1-phosphates as glycosyl donors. By removing Pi, consecutive reactions were accelerated, and enzymatic polymerization occurred, resulting in the synthesis of several unnatural α(1→4)-linked polysaccharides with well-defined structures.