Computer modeling on the tautomerization of sulbactam intermediate in SHV-1 β-lactamases: E166A mutant vs. wild type

We present a theoretical study for the tautomerization of sulbactam intermediates in different SHV-1 β-lactamases: E166A and wild-type (WT). Molecular dynamics (MD) simulations were employed and hydrogen bonds network around active site was found different between the WT and E166A acyl-enzymes. In E166A, Asn170 restricts the C5--C6 bond rotation, thus stabilizes the dihedral angle N4--C5--C6--C7 of imine to a trans conformation. The DFT calculations (B3LYP/6-31+G* and B3LYP/6-31++G**) were performed on tautomerization reactions. Two mechanisms including direct and stepwise proton transfer reactions were proposed based on the MD results. In E166A, the substrate carboxyl group acts as a relay station which assists the proton transfer with a very low energy barrier. However, in WT, such stepwise mechanism is difficult to proceed because of the large separation between C6 and substrate carboxyl group. Our results explain why E166A SHV-1 β-lactamases forms greater population of trans-enamine than WT.