Studies on cellulases of a phytopathogenic fungus, Pyricularia oryzae Cavara. IV. Kinetic studies on beta-glucosidases.

Kinetic studies of the two beta-glucosidase isozymes, GB-1 and GB-2, which were from the culture filtrate of a phytopathogenic fungus Pyricularia oryzae, revaled that the latter isozyme was an allosteric protein with two substrate binding sites. The homotropic effects of o- and m-nitrophenyl-beta-D-glucosides on GB-2 showed positive cooperativity, whereas that of cell cellooligosaccharide showed negative cooperatively. The affinity of GB-2 for cellooligosaccharide tended to increase with decreasing chain length, in contrast to that of GB-1. Glucono-delta-lactone and glucose acted as competitive inhibitors of GB-1 and GB-2. As regards the control of the level of glucose formed by the cellulose system, it appears that the rate of formation of glucose by beta-glucosidase is reduced by the presence of the substrate, cellooligosaccharide, as well as by the product, glucose.