Inhibitory effect of the conformation of amylose as a function of it 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 I-2 concentration on enzyme activity was studied in the initial state by using amyloses with a wide range of average degree of polymerization (<(DP)over bar>) in the presence of excess KI. The enzyme activity for the amylose oligomers in the <(DP)over bar> range of 20 to 100 decreased monotonically with increasing I-2 concentration. This inhibition of the enzyme activity is described to the effect of the conformational change in amylose of the random coil or wormlike chain to a helix that is induced by the binding of I-3(-) ions responsible for inclusion complexation in combination with direct effect of free I-3(-) ions. In a rapidly mixed complex system for amylose with a large <(DP)over bar> of 1,000, however, a significant two-step form of the inhibition of enzyme activity appeared with increasing I-2 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.