Figure 2.

Comparison of mechanisms for hydrolysis of β-lactam antibiotics catalyzed by TEM-1 β-lactamase to illustrate as the initiating event (i) general-base catalyzed nucleophilic attack by S70 and (ii) general-acid catalyzed protonation of the β-lactam N(1). The two schemes differ mechanistically up to formation of the first tetrahedral adduct of the reaction T1. Structural formulae are, therefore, illustrated only for the acylation portion of each reaction cycle. Formation of the T2 tetrahedral adduct (not shown) occurs through nucleophilic attack on the penicilloyl moiety in EY by a water molecule sequestered in the active site. The structures of the tetrahedral adducts T1 and T2 were deduced by close examination of sterically allowed interactions that an incoming nucleophilic S70 side chain, correspondingly, water molecule, may experience in the ES and EY intermediates, respectively. We discuss results for only sequential reaction mechanisms. The corresponding concerted reaction mechanisms were found to be less favorable energetically. T1a and T1b represent transient species in formation and breakdown of T1. T1a was modeled by applying the electronic configurations of active-site atoms in T1 to nuclear positions in ES. T1b was modeled by applying the electronic configurations of EY to the nuclear coordinates of T1. This sequence of changes in electronic structure and nuclear coordinates follows from first-principle considerations of the Born-Oppenheimer approximation and the Franck-Condon rule. A similar procedure was applied to evaluate electrostatic interactions governing deacylation.