The breakdown of beta-lactam antibiotics by beta-lactamases is the most important resistance mechanism of Gram negative bacteria against these drugs. The reaction mechanism of class A beta-lactamase, the most widespread family of these enzymes, consists of two main steps: acylation of an active site serine by the antibiotic, followed by deacylation and release of the cleaved compound. We have investigated the first step in acylation (the formation of the tetrahedral intermediate) for the reaction of benzylpenicillin in the TEM-1 enzyme using high level combined quantum mechanics/molecular mechanics (QM/MM) methods. Structures were optimized at the B3LYP/6-31+G(d)/CHARMM27 level, with energies for key points calculated up to the ab initio SCS-MP2/aug-cc-pVTZ/CHARMM27 level. The results support a mechanism in which Glu 166 removes a proton (via an intervening water molecule) from Ser70, which in turn attacks the beta-lactam of the antibiotic. Depending on the method used, the calculated barriers range from 3 to 12 kcal mol(-1) for this step, consistent with experimental data. We have also modeled this reaction step in a model of the K73A mutant enzyme. The barrier to reaction in this mutant model is found to be slightly higher: the results indicate that Lys73 stabilizes the transition state, in particular deprotonated Ser70, lowering the barrier by about 1.7 kcal mol(-1). This finding may help to explain the conservation of Lys73, in addition to the role we have previously found for it in the later stages of the reaction (Hermann et al. Org. Biomol. Chem. 2006, 4, 206-210).