Maltal binding mechanism and a role of the mobile loop of soybean beta-amylase

The inhibition of hydration of maltal (alpha-D-glucopyranosyl-(1-->4)-2-deoxy-D-glucal) catalyzed by soybean beta-amylase with 4-O-alpha-D-glucopyranosyl-(1-->4)-1-deoxynojirimycin (GDN) was investigated at 25 degrees C and at pH 5.4. As the concentrations of GDN used were comparable to that of the enzyme, Henderson's treatment was applied to this system. It was found that two maltal molecules bind to the enzyme according to a random mechanism and GDN inhibits the hydration of maltal competitively at subsites 1 and 2, and noncompetitively at the other site. On the basis of this result, it was inferred that the role of the mobile loop of this enzyme is to create a convenient catalytic environment for the hydration, and the closing of the active site by the mobile loop is induced by the binding of maltal.