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

Abstract

1. Rate parameters (Michaelis constant K_m 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 subtilis [EC 3. 2. 1. 1]_* were determined at pH 5.4 and 25°C.
2. The values of K_m (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.
3. 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 K_m and k₀ leads to the conclusion that the active site of this enzyme is composed of four subsites.