Inhibitory Effect of the Conformation of Amylose as a Function of I2 Concentration on Glucoamylase Activity

In order to gain a better understanding of the adaptability of glucoamylase to the conformational change in the substrate related with the specificity of the enzyme action, the effect of the solution conformation of amylose (the substrate) as a function of the I2 concentration on enzyme activity was studied in the initial state by using amyloses with a wide range of average degree of polymerization (DP) in the presence of excess KI. The enzyme activity for the amylose oligomers in the DP range of 20 to 100 decreased monotonically with increasing I2 concentration. This inhibition of the enzyme activity is ascribed to the effect of the conformational change in amylose of the random coil or worm-like chain to a helix that is induced by the binding of I3 (-) ions responsible for inclusion complexation in combination with direct effect of free I3 (-) ions. In a rapidly mixed complex system for amylose with a large DP of 1,000, however, a significant two-step form of the inhibition of enzyme activity appeared with increasing I2 concentration, corresponding to the two-step conformational change of amylose in a random coil or worm-like chain to a helix and then to a rod-like compact structure.