LAUSR

KPC-2 (Klebsiella pneumoniae carbapenemase-2) is a globally disseminated serine-β-lactamase (SBL) responsible for extensive β-lactam antibiotic resistance in Gram-negative pathogens. SBLs inactivate β-lactams via a mechanism involving a hydrolytically labile covalent acyl-enzyme intermediate. Carbapenems, the most potent β-lactams, evade activity of many SBLs by forming long-lived inhibitory acyl-enzymes; however, carbapenemases such as KPC-2 efficiently catalyze deacylation of carbapenem-derived acyl-enzymes. We present high-resolution (1.25-1.4 Å) crystal structures of KPC-2 acyl-enzymes with representative penicillins (ampicillin), cephalosporins (cefalothin) and carbapenems (imipenem, meropenem and ertapenem), obtained utilizing an isosteric deacylation-deficient mutant (E166Q). Mobility of the Ω-loop (residues 165–170) negatively correlates with antibiotic turnover rates (kcat), highlighting the role of this region in positioning catalytic residues for efficient hydrolysis of different β-lactams. Carbapenem-derived acyl-enzyme structures reveal predominance of the Δ1-(2R) imine tautomer, except for the imipenem acyl-enzyme, which is present in dual occupancy in both Δ1-(2R) and (2S) configurations. Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations of deacylation of the KPC-2:meropenem acyl-enzyme, using an adaptive string method (ASM), show that the Δ1-(2R) isomer has a 7 kcal/mol higher barrier for the (rate-determining) formation of the tetrahedral deacylation intermediate than the Δ2 tautomer. The simulations identify tautomer-specific differences in hydrogen bonding networks involving the carbapenem C-3 carboxylate and the deacylating water, that, together with stabilization by protonated N-4 of accumulating negative charge during oxyanion formation, accelerate deacylation of the Δ2-enamine acyl-enzyme compared to the Δ1-imine. Taken together, our data show how the flexible Ω-loop helps confer broad spectrum activity upon KPC-2, while carbapenemase activity stems from efficient deacylation of the Δ2-enamine acyl-enzyme tautomer. Differentiation of the barriers associated with deacylation of these subtly different β-lactam isomers further identifies ASM as a sensitive method for calculation of reaction energetics that can accurately model turnover and, potentially, predict the impact of substrate modifications or point mutations upon activity.