Proteolysis of α-amylase from Saccharomycopsis fibuligera: characterization of digestion products

α-Amylase from Saccharomycopsis fibuligera R-64 was successfully purified by butyl Toyopearl hydrophobic interaction chromatography, followed by Sephadex G-25 size exclusion and DEAE Toyopearl anion exchange chromatography. The enzyme has a molecular mass of 54 kDa, as judged by SDS PAGE analysis. Upon tryptic digestion, two major fragments with relative molecular masses of 39 kDa and 10 kDa, which resemble the A/B and C-terminal domains in the homologous Taka-amylase, were obtained and were successfully separated with the Sephadex G-50 size exclusion column. The 39-kDa fragment demonstrated a similar amylolytic activity to that of the undigested enzyme. However, it was found that the Km value of the 39-kDa fragment was about two-times higher than that of the undigested enzyme. Moreover, thermostability studies showed a lower half-life time for the 39-kDa fragment. These findings suggest that the 39-kDa fragment is the catalytic domain, while the 10-kDa fragment is the C-terminal one, which plays a role in thermostability and starch binding. Although the undigested enzyme is able to act on raw starches at room temperature, with maize starches as the best substrate, neither the undigested enzyme nor the fragments adsorb the tested raw starches. These results propose Saccharomycopsis fibuligera α-amylase as a raw starch-digesting but not adsorbing amylase, with a similar domain organization to that of Taka-amylase A.