The catalytic mechanism underlying the aminopeptidase fromStreptomyces griseus (SGAP) was investigated. pH-dependent activity profiles revealed the enthalpy of ionization for the hydrolysis of leucine-para-nitroanilide by SGAP. The value obtained (30 ± 5 kJÆmol )1 ) is typical of a zinc-bound water molecule, suggesting that the zinc-bound water⁄hydroxide molecule acts as the reaction nucleophile.