A QM/MM study on the enzymatic inactivation of cefotaxime

The reaction between the antibiotic cefotaxime and the CTX-M-14 class A serine hydrolase is addressed from a theoretical point of view, by means of hybrid quantum mechanics/molecular mechanical (QM/MM) calculations, adopting a new approach that postulates that the residue Ser70 itself should play the role of the acid-base species required for the cefotaxime acylation. The proposed mechanism differs from earlier proposals existing in literature for other class A beta-lactamases. The results confirm the hypothesis, and show that the reaction should occur via a concerted mechanism in which the acylation of the lactam carbonyl carbon, protonation of the N-7 lactam atom, and opening of the beta-lactam ring occurs simultaneously. Exploration of the potential energy surface shows three critical points, associated with reactants, transition state and product. The transition state is characterized by frequency, intrinsic reaction coordinate, atomic charge, and bond orders calculations. The calculated activation barrier is 20 kcal mol(-1), and the reaction appears to be slightly endothermic by about 12 kcal mol(-1). We conclude that this approach is feasible, and should be considered as an alternative mechanism or may exist in competition with others already published in the literature. This information should be useful for the design of novel antibiotics and beta-lactamase inhibitors.