Different Binding Behavior to Chitin of Multiple Forms of Glucoamylases from Aspergillus saitoi and Rhizopus sp.

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Multiple forms of glucoamylases [EC 3.2.1.3] from Aspergillus saitoi and Rhizopus sp. were studied for their chitin binding. Two forms of Aspergillus enzyme, Gluc M_1 and Glu M_2,were bindable to chitin, whereas three forms of Rhizopus enzyme, Gluc_1,Gluc_2 and Gluc_3,exhibited no significant binding; of these enzyme forms, only Gluc M_1 and Gluc_1 are bindable to raw starch. Both Gluc M_1 and Gluc M_2,lacking the C-terminal portion of Gluc M_1,bound to chitin most favorably at pH 6.5 but with considerable different strength. The binding constants K of Gluc M_1 and Gluc M_2 to chitin at pH 6.5 and 4℃ were 1.8×10^6 and 0.33×10^6M^<-1>, respectively. Soluble starch necessitated a high concentration of 1.2% for 50% inhibition of chitin binding of Gluc M_1,as compared with 0.069% for 50% inhibition of raw starch binding (J. Biochem. (Tokyo), 98,663 (1985)). Chitin-bound Gluc M_1 retained almost the same soluble starch-hydrolyzing activity as and about 3 times higher maltose-hydrolyzing and 3 times lower raw starch-digesting activities than free Gluc M_1. Thus, a chitin-binding site is not always identical with a raw starch-binding site, and in Gluc M_1 the former is located farther from the active site than the latter.
社団法人日本薬学会の論文
1991-09-25