Inhibition and binding modes of low-molecular-weight inhibitors of porcine pancreatic alpha-amylase.

Abstract

Inhibition of porcine pancreatic alpha-amylase (1,4-alpha-D-glucan glucanohydrase) [EC 3.2.1.1] with maltotriitol (G3OH) and 4-phenylimidazole was investigated by using maltohexaitol (G6OH) and p-nitrophenyl-alpha-D-maltoside (G2PNP) as substrates. When G6OH was the substrate, both G3OH and 4-phenylimidazole behaved as competitive inhibitors. On the other hand, when G2PNP was the substrate, G3OH behaved as a competitive inhibitor, whereas 4-phenylimidazole behaved as a non-competitive inhibitor. Further inhibition study in the presence of both G3OH and 4-phenylimidazole, with G6OH as the substrate, showed that the two inhibitors compete with each other for the active site of the enzyme. Based on a consideration of the productive (reactive) binding modes of G2PNP and G6OH, and a nonproductive (nonreactive) binding mode of G2PNP, it is suggested that the binding sites of the two inhibitors may be partially overlapping around the catalytic site of the enzyme and that the rest of the binding site of each inhibitor lies along the substrate binding cleft of the enzyme.