Mechanism of action of exo-acting alpha-1,4-glucan lyase: a glycoside hydrolase family 31 enzyme

alpha-1,4-Glucan lyase (GLase) performs a beta-elimination reaction on alpha-1,4-glucans. However, GLase is different in many aspects from the polysaccharide lyase that also performs a beta-elimination reaction on uronic acid containing sugars. While polysaccharide lyase mechanistically takes an anionic reaction pathway involving enolate intermediates/transition states utilizing an activated proton acidified by a uronic acid moiety, GLase does not have assisting groups and thus should overcome an enormous energy barrier to carry out catalysis. GLase achieves this goal by exploiting a similar mechanism to that of retaining glycosidases. This mechanistic aspect also can be deduced by the substantial amino acid sequence similarity of GLase to enzymes of glycoside hydrolase family 31, by which GLase is classified to glycoside hydrolase family 31. Thus, the mechanism involves the formation of a covalent glycosyl-enzyme intermediate, followed by a syn-elimination. Through the course of the reaction, highly cationic transition states are involved, as seen in the mechanism of glycosidases. GLase combines nucleophilic catalysis and general base catalysis as a smart strategy to achieve an otherwise energetically unfavorable reaction.