Crystal structures of human pancreatic alpha-amylase in complex with carbohydrate and proteinaceous inhibitors

Crystal structures of human pancreatic alpha-amylase (HPA) in complex with naturally occurring inhibitors have been solved. The tetrasaccharide acarbo se and a pseudo-pentasaccharide of the trestatin family produced identical continuous electron densities corresponding to a pentasaccharide species, s panning the - 3 to + 2 subsites of the enzyme, presumably resulting from tr ansglycosylation. Binding of the acarviosine core linked to a glucose resid ue at subsites -1 to +2 appears to be a critical part of the interaction pr ocess between alpha-amylases and trestatinderived inhibitors. Two crystal f orms, obtained at different values of pH, for the complex of HPA with the p rotein inhibitor from Phaseolus vulgaris (alpha-amylase inhibitor) have bee n solved. The flexible loop typical of the mammalian alpha-amylases was sho wn to exist in two different conformations, suggesting that loop closure is pH-sensitive. Structural information is provided for the important inhibit or residue, Arg-74, which has not been observed previously in structural an alyses.