Kinetic and Molecular -Modelling study of the interaction between Staphylococcus aureus PC1 enzyme and Imipenem

[eng] The interactions between imipenem (3), a clinically significant carbapenem antibiotic, and Staphylococcus aureus PC1 enzyme, were studied in detail. Imipenem behaves as a slow substrate that reacts by a branched pathway, which suggests the formation of a second acyl-enzyme intermediate. The individual microscopic rate constants for the process were determined. The results were analysed in the light of molecular-modelling considerations. Based on the analysis, the Ser-70(O ) group in the Michaelis-Menten complex formed between 3 and PC1 is very distant from the carbonyl group of the beta-lactam ring of 3, which is consistent with the decreased value of k2 (Model 2, see Scheme 2) for imipenem relative to an appropriate substrate such as benzylpenicillin (2). The deacylation is the rate-determining step of the turnover process. This can be ascribed to the fact that in the deacylation of the second acyl-enzyme, the H2O molecule lying closest to the ester group, Wat81, is in an unfavorable orientation to hydrolyse the intermediate