SUBSTRATE-SPECIFICITY AND SUBSITE AFFINITIES OF BUCKWHEAT ALPHA-GLUCOSIDASE

Buckwheat α-glucosidase showed especially high activity toward maltooli go saccharides, phenyl-α-maltoside and nigerose. Its activities toward isomaltose and phenyl-α-glucoside were very weak. The ratios of the maximum velocities for maltose, nigerose, kojibiose, isomaltose, phenyl-α-glucoside and phenyl-α-maltoside were estimated to be 100: 101: 17: 2: 4: 106 in this order; those for maltose, malto-triose, -tetraose, -pentaose, -hexaose, -heptaose, -octaose and maltodextrin (DPn = 13), 100: 110: 106: 121: 131: 109: 111: 86. The Michaelis constants Km, the molecular activities kQ and the ratios k0/Km for a series of maltooligosaccharides were determined. From the experimental data, the subsite affinity of each subsite in the enzyme was evaluated according to the subsite theory. The difference in the substrate specificities between glucoamylase and a-glucosidase was interpreted by the relative value of subsite affinities of the first and the third subsites counting from the terminal (the first subsite) to which the nonreducing end of substrate is bound. © 1979 Taylor & Francis Group, LLC.