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 mol zinc/mol protein, similar to the homologous enzyme Aeromonas proteolytica aminopeptidase. In addition, a unique Ca²⁺-binding site has been identified in the Streptomyces enzyme, which is absent in the Aeromonas enzyme. Binding of Ca²⁺ enhances stability of the Streptomyces enzyme and modulates its activity and affinity towards substrates and inhibitors in a structure- dependent manner. Among the three hydrophobic 4-nitroanilides of alanine, valine and leucine, the latter displays the largest overall activation (increase 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 (phenylalanine-like) residues.