The Number of Subsites in the Active Site of Saccharifying α-Amylase from Bacillus subtilis

Abstract

Rate parameters (Michaelis constant Km and molecular activity k0) for the hydrolyses of various maltodextrins with degrees of polymerization ranging from 3 to 117 catalyzed by saccharifying α-amylase from Bacillus subtilts [EC 3. 2.1.1]* were determined at pH 5.4 and 25°C. The values of Km (expressed in molar substrate concentration units) decreased with increasing degree of polymerization (n) of the substrate. A sharp increase was observed in the value of k0 between n=3 and 4, and it became almost constant for n greater than 4. On the basis of an assumption that the breakdown rate constant of productive ES complexes is identical irrespective of substrate n and of productive binding modes (1), the n-dependence of Km and k0 leads to the conclusion that the active site of this enzyme is composed of four subsites