Substrate-induced activation of maltose phosphorylase: interaction with the anomeric hydroxyl group of alpha-maltose and alpha-D-glucose controls the enzyme's glucosyltransferase activity.
Maltose phosphorylase, long considered strictly specific for beta-D-glucopyranosyl phosphate (beta-D-glucose 1-P), was found to catalyze the reaction beta-D-glucosyl fluoride + alpha-D-glucose----alpha-maltose + HF, at a rapid rate, V = 11.2 +/- 1.2 mumol/(min.mg), and K = 13.1 +/- 4.4 mM with alpha-D-glucose saturating, at 0 degrees C. This reaction is analogous to the synthesis of maltose from beta-D-glucose 1-P + D-glucose (the reverse of maltose phosphorolysis). In acting upon beta-D-glucosyl fluoride, maltose phosphorylase was found to use alpha-D-glucose as a cosubstrate but not beta-D-glucose or other close analogs (e.g., alpha-D-glucosyl fluoride) lacking an axial 1-OH group. Similarly, the enzyme was shown to use alpha-maltose as a substrate but not beta-maltose or close analogs (e.g., alpha-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.
Tsumuraya
Y
Y
Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, New York 10461.
Brewer
C F
CF
Hehre
E J
EJ
D001665:Binding Sites; D004789:Enzyme Activation / Q000187:drug effects; D005947:Glucose / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D005964:Glucosyltransferases / Q000378:metabolism*; D007778:Lactobacillus / Q000201:enzymology*; D008320:Maltose / Q000378:metabolism* / Q000494:pharmacology; D013237:Stereoisomerism; D013379:Substrate Specificity
C030364:glucosyl fluoride; D008320:Maltose; D005964:Glucosyltransferases; C021535:maltose phosphorylase; D005947:Glucose