Chemical modification of liquefying α-amylase: Role of tyrosine residues at its active center

Abstract

Liquefying α-amylase from Bacillus amyloliquefaciens was inactivated by treatment with tetranitromethane and N-acetylimidazole. The loss of activity occurred with modification of five tyrosine residues. Preincubation of the enzyme with either the substrate or the competitive inhibitor at saturating levels provided complete protection against inactivation. However, the presence of substrate/inhibitor in the reaction mixture protected only two of the five modifiable tyrosine residues, suggesting the involvement of only two tyrosine residues at the active center. This was confirmed when hydroxylamine treatment of the acetylated enzyme fully restored the enzymatic activity. Both nitration and acetylation increased the apparent K m of the enzyme for soluble starch, which indicated that the tyrosine residues are involved in substrate binding. Reduction of nitrotyrosine residues to aminotyrosine residues failed to restore the enzymatic activity. So, the loss of activity on modification of tyrosine residues was ascribed to conformational perturbances and not simply to the changes in the ionic character of tyrosine residues.