Enzyme catalysis: the case of the prostate-specific antigen

Proteases are a class of enzymes that lower the activation energy for the cleavage of the peptide bonds by polarizing the carbonyl group. The catalytic mechanism of proteases is characterized by the formation and the dissociation of a tetrahedral acyl-intermediate. The rate-limiting step in catalysis is either the acylation process (leading to the release of the newly formed -NH3+ terminal) or the subsequent deacylation step (leading to the release of the newly formed –COO− terminal). As a case, the detailed kinetic analysis for the hydrolysis of the chromogenic substrate Mu-His-Ser-Ser-Lys-Leu-Gln-AMC (wherein Mu is the morpholinocarbonyl protecting group and AMC is the 7-amino-4-methylcoumarin chromophoric group) by the prostate-specific antigen (PSA) is reported here. The pH dependence of the catalytic parameters clearly indicates the existence of protonation/deprotonation processes involving (at least) two ionizing groups in the proximity of the active site. In view of the physio-pathological relevance of PSA in prostate diseases (including cancer), the detailed analysis of the catalytic parameters opens new scenarios for the design of selective inhibitors, which might influence the “in vivo” activity of this protease.