The Conserved Tryptophan-Arginine-Tyrosine Motif of .ALPHA. Proteinaceous .ALPHA.-Amylase Inhibitor T-76 from Streptomyces nitrosporeus Is Important for Inhibition of Animal .ALPHA.-Amylases but not for an .ALPHA.-Amylase from Bacillus sp. No. 195.

Abstract

Site-directed mutagenesis of Trp-16, Arg-17, and Tyr-18, which were thought to form a putative active site in proteinaceous α-amylase inhibitor T-76 from Streptomyces nitrosporeus for inhibition, was performed. The mutation at the site (W16A, R17A, and Y18A) resulted in a marked decrease in inhibitory activity against all animal α-amylases tested. Only the α-amylase from Bacillus sp. no. 195 (BAA) remained sensitive to all the constructed mutant inhibitors. A competition between T-76 mutants and the wild-type for porcine pancreatic α-amylase (PPA) suggest that the loss of inhibitory activity against PPA in mutant inhibitors was due to the decrease in their binding ability for PPA. T-76 formed a complex with BAA as well as PPA at the stoichiometric ratio of 1:1. A competition between BAA and the PPA/T-76 complex suggests that PPA and BAA might bind to the same region or regions close to each other on the T-76 molecule. These results indicate that the conserved Trp-Arg-Tyr motif of T-76 is involved in the interaction between T-76 and PPA while other amino acid residues seem to be important for the T-76/BAA interaction. Since the BAA-type α-amylase is the actual target of the inhibitors from microbes in comparison with animal α-amylases, BAA might be a better material than PPA to elucidate the “true” function of proteinaceous α-amylase inhibitors.