Substrate-induced activation of maltose phosphorylase: Interaction with the anomeric hydroxyl group of α-maltose and α-d-glucose controls the enzyme's glucosyltransferase activity

Maltose phosphorylase, long considered strictly specific for β-d-glucopyranosyl phosphate (β-d-glucose 1-P), was found to catalyze the reaction β-d-glucosyl fluoride + α-d-glucose → α-maltose + HF, at a rapid rate, V = 11.2 ± 1.2 μmol/(min·mg), and K = 13.1 ± 4.4 mM with α-d-glucose saturating, at 0 °C. This reaction is analogous to the synthesis of maltose from β-d-glucose 1-P + d-glucose (the reverse of maltose phosphorolysis). In acting upon β-d-glucosyl fluoride, maltose phosphorylase was found to use α-d-glucose as a cosubstrate but not β-d-glucose or other close analogs (e.g., α-d-glucosyl fluoride) lacking an axial 1-OH group. Similarly, the enzyme was shown to use α-maltose as a substrate but not β-maltose or close analogs (e.g., α-maltosyl fluoride) lacking an axial 1-OH group. These results indicate that interaction of the axial 1-OH group of the disaccharide donor or sugar acceptor with a particular protein group near the reaction center is required for effective catalysis. This interaction appears to be the means that leads maltose phosphorylase to promote a narrowly defined set of glucosyl transfer reactions with little hydrolysis, in contrast to other glycosylases that catalyze both hydrolytic and nonhydrolytic reactions.