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

抄録

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 M₁ and Glu M₂, were bindable to chitin, whereas three forms of Rhizopus enzyme, Gluc₁, Gluc₂ and Gluc₃, exhibited no significant binding; of these enzyme forms, only Gluc M₁ and Gluc₁ are bindable to raw starch. Both Gluc M₁ and Gluc M₂, lacking the C-terminal portion of Gluc M₁, bound to chitin most favorably at pH 6.5 but with considerable different strength. The binding constants K of Gluc M₁ and Gluc M₂ to chitin at pH 6.5 and 4°C were 1.8×10⁶ and 0.33×10⁶M^-1, respectively. Soluble starch necessitated a high concentration of 1.2% for 50% inhibition of chitin binding of Gluc M₁, as compared with 0.069% for 50% inhibition of raw starch binding (J. Biochem. (Tokyo), 98, 663 (1985)). Chitin-bound Gluc M₁ 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 M₁. Thus, a chitin-binding site is not always identical with a raw starch-binding site, and in Gluc M₁ the former is located farther from the active site than the latter.