The efficacy of β-lactam antibiotics is threatened by the emergence and global spread of metallo-β-lactamase-(MBL) mediated resistance, specifically New Delhi-Metallo-β-lactamase-1 (NDM-1). Utilizing fragment-based drug discovery (FBDD), a new class of inhibitors for NDM-1 and two related β-lactamases, IMP-1 and VIM-2, was identified. Based on 2,6-dipicolinic acid (DPA), several libraries were synthesized for structure-activity relationship (SAR) analysis. Inhibitor 36 (IC50 = 80 nM) was identified to be highly selective for MBLs when compared to other Zn(II) metalloenzymes. While DPA displayed a propensity to chelate metal ions from NDM-1, 36 formed a stable NDM-1:Zn(II):inhibitor ternary complex, as demonstrated by 1H NMR, electron paramagnetic resonance (EPR) spectroscopy, equilibrium dialysis, intrinsic tryptophan fluorescence emission, and UV-Vis spectroscopy. When co-administered with 36 (at concentrations non-toxic to mammalian cells), the minimum inhibitory concentration (MIC) of imipenem against clinical isolates of Eschericia coli and Klebsiella pneumoniae harboring NDM-1 were reduced to susceptible levels.
Infections by carbapenem-resistant Enterobacteriaceae (CRE) are difficult to manage owing to broad antibiotic resistance profiles and because of the inability of clinically-used β-lactamase inhibitors to counter the activity of metallo-β-lactamases often harbored by these pathogens. Of particular importance is New Delhi metallo-β-lactamase (NDM), which requires a dinuclear zinc ion cluster for catalytic activity. Here, we compare the structures and functions of clinical NDM variants 1-17. The impact of NDM variants on structure is probed by comparing comparing melting temperature and refolding efficiency and also by spectroscopy (UV-Vis, 1 H-NMR, and EPR) of di-cobalt metalloforms. The impact of NDM variants on function is probed by determining of minimum inhibitory concentrations of various antibiotics, pre-steady state and steadystate kinetics, inhibitor binding, and zincdependence of resistance and activity. We observed only minor differences among the fullloaded dizinc enzymes, but most NDM variants had more distinguishable selective advantages in experiments that mimicked zinc scarcity imposed by typical host defenses. Most NDM variants http://www.jbc.org/ Downloaded from 2 exhibited improved thermostability (up to ~10 °C increased Tm) and improved zinc affinity (up to ~10-fold decreased Kd, Zn2). We also provide first evidence that some NDM variants have evolved the ability to function as monozinc enzymes with high catalytic efficiency (NDM-15, ampicillin: kcat/KM = 5 × 10 6 M -1 s -1 ). These findings reveal the molecular mechanisms that NDM variants have evolved to overcome the combined selective pressures of β-lactam antibiotics and zinc deprivation.Carbapenem-resistant Enterobacteriaceae (CRE) continue to be classified as an "urgent threat," the highest hazard level assigned by the Centers for Disease Control and Prevention(1). The five carbapenemases currently of primary public concern include Klebsiella pneumonia carbapanemase (KPC), New Delhi metallo-β-lactamase (NDM), Verona integrin encoded metallo-β-lactamase (VIM), imipenemase (IMP), and oxacillinase-48-like carbapenemase (OXA-48)(2). Three of these carbapenemases (NDM, VIM, and IMP) are metal-dependent β-lactamases that are not susceptible to any of the β-lactamase inhibitors incorporated into combination drugs used in the clinic. Of these three β-lactamases, NDM is the most widespread in U.S. patients, with infections bearing a blaNDM gene reported in 34 / 50 states (as of December 2017)(3).The genes encoding NDM continue to evolve, with discovery of more than 20 variants (NDM-1 through NDM-21 at the time of writing, 16 at the start of this project). Most of these mutations occur at sites distant from the active site, and the functions they confer are not immediately obvious. A comparison of NDM-1 through NDM-8 showed only minor differences in kcat/KM values (≤ 5-fold) for a panel of diverse β-lactam drugs(4). However, a considerable increase in thermostability was noted for many of the variants, suggesting the functional impact of NDM...
To understand the evolution of Verona integron-encoded metallo-β-lactamase (VIM) genes (blaVIM) and their clinical impact, microbiological, biochemical, and structural studies were conducted. Forty-five clinically derived VIM variants engineered in a uniform background and expressed in Escherichia coli afforded increased resistance toward all tested antibiotics; the variants belonging to the VIM-1-like and VIM-4-like families exhibited higher MICs toward five out of six antibiotics than did variants belonging to the widely distributed and clinically important VIM-2-like family. Generally, maximal MIC increases were observed when cephalothin and imipenem were tested. Additionally, MIC determinations under conditions with low zinc availability suggested that some VIM variants are also evolving to overcome zinc deprivation. The most profound increase in resistance was observed in VIM-2-like variants (e.g., VIM-20 H229R) at low zinc availability. Biochemical analyses reveal that VIM-2 and VIM-20 exhibited similar metal binding properties and steady-state kinetic parameters under the conditions tested. Crystal structures of VIM-20 in the reduced and oxidized forms at 1.25 Å and 1.37 Å resolution, respectively, show that Arg229 forms an additional salt bridge with Glu171. Differential scanning fluorimetry of purified proteins and immunoblots of periplasmic extracts revealed that this difference increases thermostability and resistance to proteolytic degradation when zinc availability is low. Therefore, zinc scarcity appears to be a selective pressure driving the evolution of multiple metallo-β-lactamase families, although compensating mutations use different mechanisms to enhance resistance. IMPORTANCE Antibiotic resistance is a growing clinical threat. One of the most serious areas of concern is the ability of some bacteria to degrade carbapenems, drugs that are often reserved as last-resort antibiotics. Resistance to carbapenems can be conferred by a large group of related enzymes called metallo-β-lactamases that rely on zinc ions for function and for overall stability. Here, we studied an extensive panel of 45 different metallo-β-lactamases from a subfamily called VIM to discover what changes are emerging as resistance evolves in clinical settings. Enhanced resistance to some antibiotics was observed. We also found that at least one VIM variant developed a new ability to remain more stable under conditions where zinc availability is limited, and we determined the origin of this stability in atomic detail. These results suggest that zinc scarcity helps drive the evolution of this resistance determinant.
In an effort to evaluate whether a recently reported putative metallo-β-lactamase (MβL) contains a novel MβL active site, SPS-1 from Sediminispirochaeta smaragdinae was overexpressed, purified, and characterized using spectroscopic and crystallographic studies. Metal analyses demonstrate that recombinant SPS-1 binds nearly 2 equiv of Zn(II), and steady-state kinetic studies show that the enzyme hydrolyzes carbapenems and certain cephalosporins but not β-lactam substrates with bulky substituents at the 6/7 position. Spectroscopic studies of Co(II)-substituted SPS-1 suggest a novel metal center in SPS-1, with a reduced level of spin coupling between the metal ions and a novel Zn metal binding site. This site was confirmed with a crystal structure of the enzyme. The structure shows a Zn site that is similar to that in NDM-1 and other subclass B1 MβLs; however, the Zn metal ion is coordinated by two histidine residues and a water molecule, which is held in position by a hydrogen bond network. The Zn metal is displaced nearly 1 Å from the position reported in other MβLs. The structure also shows extended helices above the active site, which create a binding pocket that precludes the binding of substrates with large, bulky substituents at the 6/7 position of β-lactam antibiotics. This study reveals a novel metal binding site in MβLs and suggests that the targeting of metal binding sites in MβLs with inhibitors is now more challenging with the identification of this new MβL.
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