Role of non-covalent enzyme-substrate interactions in the reaction catalysed by cellobiose phosphorylase from Cellulomonas uda.

Steady-state kinetic studies of the enzymic glucosyl transfer to and from phosphate catalysed by cellobiose phosphorylase from Cellulomonas uda have shown that this enzyme operates by a ternary-complex kinetic mechanism in which beta-cellobiose binds before phosphate, and beta-D-glucose and alpha-D-glucopyranosyl phosphate are released in that order. alpha-D-Glucopyranosyl fluoride (but not beta-D-glucopyranosyl fluoride) serves as alternative glucosyl donor for beta-cellobiose synthesis with a specificity constant that is one-ninth that of the corresponding enzymic reaction with alpha-D-glucopyranosyl phosphate (approximately 20000 M(-1).s(-1) at 30 degrees C). The kinetic parameters for a complete series of deoxy and deoxyfluoro analogues of D-glucose have been determined and the data yield estimates of the net strengths of hydrogen-bonding interactions with the non-reacting hydroxy groups of D-glucose at the transition state (0.8-4.0 kcal/mol, where 1 cal identical with 4.184 J) and enable the prediction of the polarities of these hydrogen bonds. Each hydroxy group functions as donor of a hydrogen for bonding to probably a charged (at 3-OH) or neutral (at 2-OH and 6-OH) acceptor group on the enzyme. The equatorial 1-OH is essential for enzyme activity. Derivatives of D-glucose in which the 1-OH or the reacting 4-OH were replaced by hydrogen or fluorine have been tested as inhibitors to measure their affinities for the sugar-binding subsite +1 (numbered from the bond-cleaving/forming site). The data show that hydrogen-bonding interactions between the 1-OH and 4-OH and charged groups on the enzyme stabilize the ground-state ternary complex of the enzymic synthesis of beta-cellobiose by 2.3 and 0.4 kcal/mol, respectively, and assist the precise positioning of beta-D-glucose for catalysis.