Structural and biochemical characterization of VIM-26 shows that Leu224 has implications for the substrate specificity of VIM metallo-ß-lactamases.

During the last decades antimicrobial resistance has become a global health problem. Metallo-ß-lactamases (MBLs) which are broad-spectrum ß-lactamases that inactivate virtually all ß-lactams including carbapenems, are contributing to this health problem. In this study a novel MBL variant, termed VIM-26, identified in a Klebsiella pneumoniae isolate was studied. VIM-26 belongs to the Verona integron-encoded metallo-ß-lactamase (VIM) family of MBLs and is a His224Leu variant of the well-characterized VIM-1 variant. In this study, we report the kinetic parameters, minimum inhibitory concentrations and crystal structures of a recombinant VIM-26 protein, and compare them to previously published data on VIM-1, VIM-2 and VIM-7. The kinetic parameters and minimum inhibitory concentration determinations show that VIM-26, like VIM-7, has higher penicillinase activity but lower cephalosporinase activity than VIM-1 and VIM-2. The four determined VIM-26 crystal structures revealed mono- and di-zinc forms, where the Zn1 ion has distorted tetrahedral coordination geometry with an additional water molecule (W2) at a distance of 2.6-3.7 Å, which could be important during catalysis. The R2 drug binding site in VIM-26 is more open compared to VIM-2 and VIM-7 and neutrally charged due to Leu224 and Ser228. Thus, the VIM-26 drug binding properties are different from the VIM-2 (Tyr224/Arg228) and VIM-7 (His224/Arg228) structures, indicating a role of these residues in the substrate specificity.

His224 alters the R2 drug binding site and Phe218 influences the catalytic efficiency of the metallo-ß-lactamase VIM-7.

Metallo-ß-lactamases (MBLs) are the causative mechanism for resistance to ß-lactams, including carbapenems, in many Gram-negative pathogenic bacteria. One important family of MBLs is the Verona integron-encoded MBLs (VIM). In this study, the importance of residues Asp120, Phe218, and His224 in the most divergent VIM variant, VIM-7, was investigated to better understand the roles of these residues in VIM enzymes through mutations, enzyme kinetics, crystal structures, thermostability, and docking experiments. The tVIM-7-D120A mutant with a tobacco etch virus (TEV) cleavage site was enzymatically inactive, and its structure showed the presence of only the Zn1 ion. The mutant was less thermostable, with a melting temperature (T(m)) of 48.5°C, compared to 55.3 °C for the wild-type tVIM-7. In the F218Y mutant, a hydrogen bonding cluster was established involving residues Asn70, Asp84, and Arg121. The tVIM-7-F218Y mutant had enhanced activity compared to wild-type tVIM-7, and a slightly higher Tm (57.1 °C) was observed, most likely due to the hydrogen bonding cluster. Furthermore, the introduction of two additional hydrogen bonds adjacent to the active site in the tVIM-7-H224Y mutant gave a higher thermostability (T(m), 62.9 °C) and increased enzymatic activity compared to those of the wild-type tVIM-7. Docking of ceftazidime in to the active site of tVIM-7, tVIM-7-H224Y, and VIM-7-F218Y revealed that the side-chain conformations of residue 224 and Arg228 in the L3 loop and Tyr67 in the L1 loop all influence possible substrate binding conformations. In conclusion, the residue composition of the L3 loop, as shown with the single H224Y mutation, is important for activity particularly toward the positively charged cephalosporins like cefepime and ceftazidime.

Crystal structure of the mobile metallo-ß-lactamase AIM-1 from Pseudomonas aeruginosa: insights into antibiotic binding and the role of Gln157.

Metallo-ß-lactamase (MBL) genes confer resistance to virtually all ß-lactam antibiotics and are rapidly disseminated by mobile genetic elements in Gram-negative bacteria. MBLs belong to three different subgroups, B1, B2, and B3, with the mobile MBLs largely confined to subgroup B1. The B3 MBLs are a divergent subgroup of predominantly chromosomally encoded enzymes. AIM-1 (Adelaide IMipenmase 1) from Pseudomonas aeruginosa was the first B3 MBL to be identified on a readily mobile genetic element. Here we present the crystal structure of AIM-1 and use in silico docking and quantum mechanics and molecular mechanics (QM/MM) calculations, together with site-directed mutagenesis, to investigate its interaction with ß-lactams. AIM-1 adopts the characteristic αß/ßα sandwich fold of MBLs but differs from other B3 enzymes in the conformation of an active site loop (residues 156 to 162) which is involved both in disulfide bond formation and, we suggest, interaction with substrates. The structure, together with docking and QM/MM calculations, indicates that the AIM-1 substrate binding site is narrower and more restricted than those of other B3 MBLs, possibly explaining its higher catalytic efficiency. The location of Gln157 adjacent to the AIM-1 zinc center suggests a role in drug binding that is supported by our in silico studies. However, replacement of this residue by either Asn or Ala resulted in only modest reductions in AIM-1 activity against the majority of ß-lactam substrates, indicating that this function is nonessential. Our study reveals AIM-1 to be a subclass B3 MBL with novel structural and mechanistic features.