Modulation of Streptomyces leucine aminopeptidase by calcium: identification and functional analysis of key residues in activation and stabilization by calcium.

Streptomyces griseus leucine aminopeptidase (SGAP), which has two zinc atoms in its active site, is clinically important as a model for understanding the structure and mechanism of action of other metallopeptidases. SGAP is a calcium-activated and calcium-stabilized enzyme, and its activation by calcium correlates with substrate specificity. In our previous study, we found a non-calcium-modulated leucine aminopeptidase secreted by Streptomyces septatus, the primary structure of which showed 71% identity with SGAP. In this study, we constructed chimeras of SGAP and S. septatus aminopeptidase by using an in vivo DNA shuffling system and several mutant enzymes by site-directed mutagenesis to identify the key residues in this modulation by calcium. We identified the key residues Asp-173 and Asp-174 of SGAP associated with both SGAP activation and stabilization by calcium. We also showed that the known calcium-binding site, which is composed of Asp-3, Ile-4, Asp-262, and Asp-266 of SGAP, only contributes to SGAP stabilization by calcium. Furthermore, we identified an important residue, Glu-196, that functions in cooperation with Asp-173, Asp-174, and calcium to increase the catalytic activity of SGAP.