Asp120Asn mutation impairs the catalytic activity of NDM-1 metallo-β-lactamase: experimental and computational study.

New Delhi metallo-β-lactamase-1 (NDM-1) has attracted extensive attention in recent years for its high activity for hydrolyzing almost all β-lactam antibiotics. Like other metallo-β-lactamases (MβLs), NDM-1 features an invariant Asp120 that ligates the zinc ion (ZN2) in the active site. Previous studies showed that substitutions of Asp120 with residues such as Ala, Ser, Asn and Glu dramatically impaired the MβL (BcII, IMP-1, L1) activity, but no consensus about the exact role of Asp120 has reached. Here we constructed D120N mutant of NDM-1 by site-directed mutagenesis. The replacement of Asp120 with Asn, which has much weaker metal ligating capabilities than Asp, severely impaired the lactamase activity without abolishing the ZN2 site. Molecular dynamics simulations suggested that the ZN1-ZN2 distance increased because of mutation, leading to a rearrangement of the active site, including the bridging OH(-). Thereby, the Mulliken charges of ZN1 and ZN2 redistributed, especially for ZN2, which might be the major cause of the impaired activity. Reducing the point charges of Asp120 carboxyl oxygens weakened the ionic interactions between Asp120 and ZN2, and the positions of the zinc ions were also changed as a result. It is proposed that Asp120 acts as a strong ZN2 ligand, positioning ZN2 for catalytically important interactions with the substrate, stabilizing the negatively charged amide nitrogen of the hydrolyzed intermediate, and more importantly, orienting the ZN-bound OH(-) for nucleophilic attacks and protonation. These functions are of general importance for catalyzing β-lactam antibiotics by NDM-1 as well as other MβLs.