Effect of chloride ions on the catalytic properties of human pancreatic a-amylase isozyme produced in Pichia pastoris

摘要

The AMY2B gene, encoding human pancreatic a-amylase isozyme (HPA II), was expressed in Pichia pastoris, and the effects of chloride ions on HPA II activity toward starch substrates were investigated. As seen with chloride ion-dependent a-amylases-including HPA I, the isozyme of HPA II-chloride ions increased enzyme activity and shifted the optimal pH to an alkaline pH. The activity enhancement by chloride was more significant at pH 8 than that at pH 6, suggesting that the protonation state of the general acid/base catalyst of HPA II was important for the hydrolysis of starches at an alkaline pH because of the increase in its pKa by chloride ions. The turnover values for cereal starches as the substrates markedly increased in the presence of chlorideby up to 7.2-fold, whereas that for soluble starch increased by only 1.7-fold. Chloride inhibited substrate hydrolysis at high substrate concentrations, with K- values ranging from 6 to 15 mg/mL.