Different phosphate binding modes of Streptomyces griseus aminopeptidase between crystal and solution states and the status of zinc-bound water.

Phosphate shows a non-competitive inhibition toward a Streptomyces aminopeptidase (sAP) between pH 5.85 (Ki = 0.48 mM) and 9.0 (110 mM), with a pKa of 7.1 likely due to ionization of H2PO4-. This non-competitive inhibition pattern indicates that phosphate binding to sAP in solution is different from that in the crystal structure, where phosphate is bound to the active site Zn(II) ions. Fluoride uncompetitively inhibits sAP from pH 5.5 (Ki = 3.72 mM) to 9.0 (43.6 mM), with a pKa of approximately 6.2 likely due to a coordinated water. The different inhibition natures and pKa values indicate that the two inhibitors bind at different locations.