Allosteric properties, substrate specificity, and subsite affinities of honeybee alpha-glucosidase I.

The substrate specificity of honeybee alpha-glucosidase I, a monomeric enzyme was kinetically investigated. Unusual kinetic features were observed in the cleavage reactions of sucrose, maltose, p-nitrophenyl alpha-glucoside, phenyl alpha-glucoside, turanose, and maltodextrin (DP = 13). At relatively high substrate concentrations, the velocities of liberation of fructose from sucrose, glucose from maltose, p-nitrophenol from p-nitrophenyl alpha-glucoside, and phenol from phenyl alpha-glucoside were accelerated, and so the Lineweaver-Burk plots were convex, indicating negative kinetic cooperativity: the Hill coefficients were calculated to be 0.50, 0.64, 0.50, and 0.67 for sucrose, maltose, p-nitrophenyl alpha-glucoside, and phenyl alpha-glucoside, respectively. For the degradation of turanose and maltodextrin, the enzyme showed a sigmoidal curve in v versus s plots and thus catalyzed the reaction with positive kinetic cooperativity. The Lineweaver-Burk plots were concave and the Hill coefficients were 1.2 and 1.5 for turanose and maltodextrin, respectively. These unique properties cannot be interpreted by the reaction mechanism that Huber and Thompson proposed: (1973) Biochemistry 12, 4011-4020. The rate parameters for the hydrolysis of sucrose, maltose, p-nitrophenyl alpha-glucoside and phenyl alpha-glucoside were estimated by extrapolating the linear part of the Lineweaver-Burk plots at low substrate concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)