Inhibition of Streptomyces griseus aminopeptidase and effects of calcium ions on catalysis and binding - Comparisons with the homologous enzyme Aeromonas proteolytica aminopeptidase

Streptomyces griseus aminopeptidase is a zinc metalloenzyme containing 2 mo l zinc/mol protein, similar to the homologous enzyme Aeromonas proteolytica aminopeptidase. In addition, a unique Ca2+-binding site has been identifie d in the Streptomyces enzyme, which is absent in the Aeromonas enzyme. Bind ing of Ca2+ enhances stability of the Streptomyces enzyme and modulates its activity and affinity towards substrates and inhibitors in a structure-dep endent manner. Among the three hydrophobic 4-nitroanilides of alanine, vali ne and leucine, the latter displays the largest overall activation (increas e in k(cat)/ K-m). Large enhancements in affinity (1/K-i) upon Ca2+ binding have been observed for inhibitors with flexible (leucine-like) residues at their N-termini and smaller enhancements for inhibitors with rigid (phenyl alanine-like) residues.