Studies on enmetazobactam clarify mechanisms of widely used ß-lactamase inhibitors.

ß-Lactams are the most important class of antibacterials, but their use is increasingly compromised by resistance, most importantly via serine ß-lactamase (SBL)-catalyzed hydrolysis. The scope of ß-lactam antibacterial activity can be substantially extended by coadministration with a penicillin-derived SBL inhibitor (SBLi), i.e., the penam sulfones tazobactam and sulbactam, which are mechanism-based inhibitors working by acylation of the nucleophilic serine. The new SBLi enmetazobactam, an N-methylated tazobactam derivative, has recently completed clinical trials. Biophysical studies on the mechanism of SBL inhibition by enmetazobactam reveal that it inhibits representatives of all SBL classes without undergoing substantial scaffold fragmentation, a finding that contrasts with previous reports on SBL inhibition by tazobactam and sulbactam. We therefore reinvestigated the mechanisms of tazobactam and sulbactam using mass spectrometry under denaturing and nondenaturing conditions, X-ray crystallography, and NMR spectroscopy. The results imply that the reported extensive fragmentation of penam sulfone­derived acyl­enzyme complexes does not substantially contribute to SBL inhibition. In addition to observation of previously identified inhibitor-induced SBL modifications, the results reveal that prolonged reaction of penam sulfones with SBLs can induce dehydration of the nucleophilic serine to give a dehydroalanine residue that undergoes reaction to give a previously unobserved lysinoalanine cross-link. The results clarify the mechanisms of action of widely clinically used SBLi, reveal limitations on the interpretation of mass spectrometry studies concerning mechanisms of SBLi, and will inform the development of new SBLi working by reaction to form hydrolytically stable acyl­enzyme complexes.

How Clavulanic Acid Inhibits Serine ß-Lactamases.

Clavulanic acid is a medicinally important inhibitor of serine ß-lactamases (SBLs). We report studies on the mechanisms by which clavulanic acid inhibits representative Ambler class A (TEM-116), C (Escherichia coli AmpC), and D (OXA-10) SBLs using denaturing and non-denaturing mass spectrometry (MS). Similarly to observations with penam sulfones, most of the results support a mechanism involving acyl enzyme complex formation, followed by oxazolidine ring opening without efficient subsequent scaffold fragmentation (at pH 7.5). This observation contrasts with previous MS studies, which identified clavulanic acid scaffold fragmented species as the predominant SBL bound products. In all the SBLs studied here, fragmentation was promoted by acidic conditions, which are commonly used in LC­MS analyses. Slow fragmentation was, however, observed under neutral conditions with TEM-116 on prolonged reaction with clavulanic acid. Although our results imply clavulanic acid scaffold fragmentation is likely not crucial for SBL inhibition in vivo, development of inhibitors that fragment to give stable covalent complexes is of interest.

Structural Investigations of the Inhibition of Escherichia coli AmpC ß-Lactamase by Diazabicyclooctanes.

ß-Lactam antibiotics are presently the most important treatments for infections by pathogenic Escherichia coli, but their use is increasingly compromised by ß-lactamases, including the chromosomally encoded class C AmpC serine-ß-lactamases (SBLs). The diazabicyclooctane (DBO) avibactam is a potent AmpC inhibitor; the clinical success of avibactam combined with ceftazidime has stimulated efforts to optimize the DBO core. We report kinetic and structural studies, including four high-resolution crystal structures, concerning inhibition of the AmpC serine-ß-lactamase from E. coli (AmpC EC ) by clinically relevant DBO-based inhibitors: avibactam, relebactam, nacubactam, and zidebactam. Kinetic analyses and mass spectrometry-based assays were used to study their mechanisms of AmpC EC inhibition. The results reveal that, under our assay conditions, zidebactam manifests increased potency (apparent inhibition constant [Kiapp], 0.69 µM) against AmpC EC compared to that of the other DBOs (Kiapp = 5.0 to 7.4 µM) due to an ∼10-fold accelerated carbamoylation rate. However, zidebactam also has an accelerated off-rate, and with sufficient preincubation time, all the DBOs manifest similar potencies. Crystallographic analyses indicate a greater conformational freedom of the AmpC EC -zidebactam carbamoyl complex compared to those for the other DBOs. The results suggest the carbamoyl complex lifetime should be a consideration in development of DBO-based SBL inhibitors for the clinically important class C SBLs.

The Chemical Biology of Human Metallo-ß-Lactamase Fold Proteins.

The αßßα metallo ß-lactamase (MBL) fold (MBLf) was first observed in bacterial enzymes that catalyze the hydrolysis of almost all ß-lactam antibiotics, but is now known to be widely distributed. The MBL core protein fold is present in human enzymes with diverse biological roles, including cell detoxification pathways and enabling resistance to clinically important anticancer medicines. Human (h)MBLf enzymes can bind metals, including zinc and iron ions, and catalyze a range of chemically interesting reactions, including both redox (e.g., ETHE1) and hydrolytic processes (e.g., Glyoxalase II, SNM1 nucleases, and CPSF73). With a view to promoting basic research on MBLf enzymes and their medicinal targeting, here we summarize current knowledge of the mechanisms and roles of these important molecules.

A Fluorescence-Based Assay for Screening ß-Lactams Targeting the Mycobacterium tuberculosis Transpeptidase LdtMt2.

Mycobacterium tuberculosis l,d-transpeptidases (Ldts), which are involved in cell-wall biosynthesis, have emerged as promising targets for the treatment of tuberculosis. However, an efficient method for testing inhibition of these enzymes is not currently available. We present a fluorescence-based assay for LdtMt2 , which is suitable for high-throughput screening. Two fluorogenic probes were identified that release a fluorophore upon reaction with LdtMt2 , thus making it possible to assess the availability of the catalytic site in the presence of inhibitors. The assay was applied to a panel of ß-lactam antibiotics and related inhibitors; the results validate observations that the (carba)penem subclass of ß-lactams are more potent Ldt inhibitors than other ß-lactam classes, though unexpected variations in potency were observed. The method will enable systematic structure-activity relationship studies on Ldts, thereby facilitating the identification of new antibiotics active against M. tuberculosis.

Molecular Basis of Class A ß-Lactamase Inhibition by Relebactam.

ß-Lactamase production is the major ß-lactam resistance mechanism in Gram-negative bacteria. ß-Lactamase inhibitors (BLIs) efficacious against serine ß-lactamase (SBL) producers, especially strains carrying the widely disseminated class A enzymes, are required. Relebactam, a diazabicyclooctane (DBO) BLI, is in phase 3 clinical trials in combination with imipenem for the treatment of infections by multidrug-resistant Enterobacteriaceae We show that relebactam inhibits five clinically important class A SBLs (despite their differing spectra of activity), representing both chromosomal and plasmid-borne enzymes, i.e., the extended-spectrum ß-lactamases L2 (inhibition constant 3 µM) and CTX-M-15 (21 µM) and the carbapenemases KPC-2, -3, and -4 (1 to 5 µM). Against purified class A SBLs, relebactam is an inferior inhibitor compared with the clinically approved DBO avibactam (9- to 120-fold differences in half maximal inhibitory concentration [IC50]). MIC assays indicate relebactam potentiates ß-lactam (imipenem) activity against KPC-producing Klebsiella pneumoniae, with similar potency to avibactam (with ceftazidime). Relebactam is less effective than avibactam in combination with aztreonam against Stenotrophomonas maltophilia K279a. X-ray crystal structures of relebactam bound to CTX-M-15, L2, KPC-2, KPC-3, and KPC-4 reveal its C2-linked piperidine ring can sterically clash with Asn104 (CTX-M-15) or His/Trp105 (L2 and KPCs), rationalizing its poorer inhibition activity than that of avibactam, which has a smaller C2 carboxyamide group. Mass spectrometry and crystallographic data show slow, pH-dependent relebactam desulfation by KPC-2, -3, and -4. This comprehensive comparison of relebactam binding across five clinically important class A SBLs will inform the design of future DBOs, with the aim of improving clinical efficacy of BLI-ß-lactam combinations.

19 F†NMR Monitoring of Reversible Protein Post-Translational Modifications: Class†D ß-Lactamase Carbamylation and Inhibition.

Bacterial production of ß-lactamases with carbapenemase activity is a global health threat. The active sites of class D carbapenemases such as OXA-48, which is of major clinical importance, uniquely contain a carbamylated lysine residue which is essential for catalysis. Although there is significant interest in characterizing this post-translational modification, and it is a promising inhibition target, protein carbamylation is challenging to monitor in solution. We report the use of 19 F NMR spectroscopy to monitor the carbamylation state of 19 F-labelled OXA-48. This method was used to investigate the interactions of OXA-48 with clinically used serine ß-lactamase inhibitors, including avibactam and vaborbactam. Crystallographic studies on 19 F-labelled OXA-48 provide a structural rationale for the sensitivity of the 19 F label to active site interactions. The overall results demonstrate the use of 19 F NMR to monitor reversible covalent post-translational modifications.

Profiling interactions of vaborbactam with metallo-ß-lactamases.

ß-Lactams are the most successful antibacterials, yet their use is threatened by resistance, importantly as caused by ß-lactamases. ß-Lactamases fall into two mechanistic groups: the serine ß-lactamases that utilise a covalent acyl-enzyme mechanism and the metallo ß-lactamases that utilise a zinc-bound water nucleophile. Achieving simultaneous inhibition of both ß-lactamase classes remains a challenge in the field. Vaborbactam is a boronate-based inhibitor that reacts with serine-ß-lactamases to form covalent complexes that mimic tetrahedral intermediates in catalysis. Vaborbactam has recently been approved for clinical use in combination with the carbapenem meropenem. Here we show that vaborbactam moderately inhibits metallo-ß-lactamases from all 3 subclasses (B1, B2 and B3), with a potency of around 20-100 fold below that by which it inhibits its current clinical targets, the Class A serine ß-lactamases. This result contrasts with recent investigations of bicyclic boronate inhibitors, which potently inhibit subclass B1 MBLs but which presently lack activity against B2 and B3 enzymes. These findings indicate that cyclic boronate scaffolds have the potential to inhibit the full range of ß-lactamases and justify further work on the development of boronates as broad-spectrum ß-lactamase inhibitors.

Non-Hydrolytic ß-Lactam Antibiotic Fragmentation by l,d-Transpeptidases and Serine ß-Lactamase Cysteine Variants.

Enzymes often use nucleophilic serine, threonine, and cysteine residues to achieve the same type of reaction; the underlying reasons for this are not understood. While bacterial d,d-transpeptidases (penicillin-binding proteins) employ a nucleophilic serine, l,d-transpeptidases use a nucleophilic cysteine. The covalent complexes formed by l,d-transpeptidases with some ß-lactam antibiotics undergo non-hydrolytic fragmentation. This is not usually observed for penicillin-binding proteins, or for the related serine ß-lactamases. Replacement of the nucleophilic serine of serine ß-lactamases with cysteine yields enzymes which fragment ß-lactams via a similar mechanism as the l,d-transpeptidases, implying the different reaction outcomes are principally due to the formation of thioester versus ester intermediates. The results highlight fundamental differences in the reactivity of nucleophilic serine and cysteine enzymes, and imply new possibilities for the inhibition of nucleophilic enzymes.

Structural/mechanistic insights into the efficacy of nonclassical ß-lactamase inhibitors against extensively drug resistant Stenotrophomonas maltophilia clinical isolates.

Clavulanic acid and avibactam are clinically deployed serine ß-lactamase inhibitors, important as a defence against antibacterial resistance. Bicyclic boronates are recently discovered inhibitors of serine and some metallo ß-lactamases. Here, we show that avibactam and a bicyclic boronate inhibit L2 (serine ß-lactamase) but not L1 (metallo ß-lactamase) from the extensively drug resistant human pathogen Stenotrophomonas maltophilia. X-ray crystallography revealed that both inhibitors bind L2 by covalent attachment to the nucleophilic serine. Both inhibitors reverse ceftazidime resistance in S. maltophilia because, unlike clavulanic acid, they do not induce L1 production. Ceftazidime/inhibitor resistant mutants hyperproduce L1, but retain aztreonam/inhibitor susceptibility because aztreonam is not an L1 substrate. Importantly, avibactam, but not the bicyclic boronate is deactivated by L1 at a low rate; the utility of avibactam might be compromised by mutations that increase this deactivation rate. These data rationalize the observed clinical efficacy of ceftazidime/avibactam plus aztreonam as combination therapy for S. maltophilia infections and confirm that aztreonam-like ß-lactams plus nonclassical ß-lactamase inhibitors, particularly avibactam-like and bicyclic boronate compounds, have potential for treating infections caused by this most intractable of drug resistant pathogens.

Cyclobutanone Mimics of Intermediates in Metallo-ß-Lactamase Catalysis.

The most important resistance mechanism to ß-lactam antibiotics involves hydrolysis by two ß-lactamase categories: the nucleophilic serine and the metallo-ß-lactamases (SBLs and MBLs, respectively). Cyclobutanones are hydrolytically stable ß-lactam analogues with potential to inhibit both SBLs and MBLs. We describe solution and crystallographic studies on the interaction of a cyclobutanone penem analogue with the clinically important MBL SPM-1. NMR experiments using 19 F-labeled SPM-1 imply the cyclobutanone binds to SPM-1 with micromolar affinity. A crystal structure of the SPM-1:cyclobutanone complex reveals binding of the hydrated cyclobutanone through interactions with one of the zinc ions, stabilisation of the hydrate by hydrogen bonding to zinc-bound water, and hydrophobic contacts with aromatic residues. NMR analyses using a 13 C-labeled cyclobutanone support assignment of the bound species as the hydrated ketone. The results inform on how MBLs bind substrates and stabilize tetrahedral intermediates. They support further investigations on the use of transition-state and/or intermediate analogues as inhibitors of all ß-lactamase classes.

Structure activity relationship studies on rhodanines and derived enethiol inhibitors of metallo-ß-lactamases.

Metallo-ß-lactamases (MBLs) enable bacterial resistance to almost all classes of ß-lactam antibiotics. We report studies on enethiol containing MBL inhibitors, which were prepared by rhodanine hydrolysis. The enethiols inhibit MBLs from different subclasses. Crystallographic analyses reveal that the enethiol sulphur displaces the di-Zn(II) ion bridging 'hydrolytic' water. In some, but not all, cases biophysical analyses provide evidence that rhodanine/enethiol inhibition involves formation of a ternary MBL enethiol rhodanine complex. The results demonstrate how low molecular weight active site Zn(II) chelating compounds can inhibit a range of clinically relevant MBLs and provide additional evidence for the potential of rhodanines to be hydrolysed to potent inhibitors of MBL protein fold and, maybe, other metallo-enzymes, perhaps contributing to the complex biological effects of rhodanines. The results imply that any medicinal chemistry studies employing rhodanines (and related scaffolds) as inhibitors should as a matter of course include testing of their hydrolysis products.

A New Mechanism for ß-Lactamases: Class D Enzymes Degrade 1ß-Methyl Carbapenems through Lactone Formation.

ß-Lactamases threaten the clinical use of carbapenems, which are considered antibiotics of last resort. The classical mechanism of serine carbapenemase catalysis proceeds through hydrolysis of an acyl-enzyme intermediate. We show that class D ß-lactamases also degrade clinically used 1ß-methyl-substituted carbapenems through the unprecedented formation of a carbapenem-derived ß-lactone. ß-Lactone formation results from nucleophilic attack of the carbapenem hydroxyethyl side chain on the ester carbonyl of the acyl-enzyme intermediate. The carbapenem-derived lactone products inhibit both serine ß-lactamases (particularly class D) and metallo-ß-lactamases. These results define a new mechanism for the class D carbapenemases, in which a hydrolytic water molecule is not required.

Crystal structure of human persulfide dioxygenase: structural basis of ethylmalonic encephalopathy.

The ethylmalonic encephalopathy protein 1 (ETHE1) catalyses the oxygen-dependent oxidation of glutathione persulfide (GSSH) to give persulfite and glutathione. Mutations to the hETHE1 gene compromise sulfide metabolism leading to the genetic disease ethylmalonic encephalopathy. hETHE1 is a mono-iron binding member of the metallo-ß-lactamase (MBL) fold superfamily. We report crystallographic analysis of hETHE1 in complex with iron to 2.6 Å resolution. hETHE1 contains an αßßα MBL-fold, which supports metal-binding by the side chains of an aspartate and two histidine residues; three water molecules complete octahedral coordination of the iron. The iron binding hETHE1 enzyme is related to the 'classical' di-zinc binding MBL hydrolases involved in antibiotic resistance, but has distinctive features. The histidine and aspartate residues involved in iron-binding in ETHE1, occupy similar positions to those observed across both the zinc 1 and zinc 2 binding sites in classical MBLs. The active site of hETHE1 is very similar to an ETHE1-like enzyme from Arabidopsis thaliana (60% sequence identity). A channel leading to the active site is sufficiently large to accommodate a GSSH substrate. Some of the observed hETHE1 clinical mutations cluster in the active site region. The structure will serve as a basis for detailed functional and mechanistic studies on ETHE1 and will be useful in the development of selective MBL inhibitors.

Cyclic Boronates Inhibit All Classes of ß-Lactamases.

ß-Lactamase-mediated resistance is a growing threat to the continued use of ß-lactam antibiotics. The use of the ß-lactam-based serine-ß-lactamase (SBL) inhibitors clavulanic acid, sulbactam, and tazobactam and, more recently, the non-ß-lactam inhibitor avibactam has extended the utility of ß-lactams against bacterial infections demonstrating resistance via these enzymes. These molecules are, however, ineffective against the metallo-ß-lactamases (MBLs), which catalyze their hydrolysis. To date, there are no clinically available metallo-ß-lactamase inhibitors. Coproduction of MBLs and SBLs in resistant infections is thus of major clinical concern. The development of "dual-action" inhibitors, targeting both SBLs and MBLs, is of interest, but this is considered difficult to achieve due to the structural and mechanistic differences between the two enzyme classes. We recently reported evidence that cyclic boronates can inhibit both serine- and metallo-ß-lactamases. Here we report that cyclic boronates are able to inhibit all four classes of ß-lactamase, including the class A extended spectrum ß-lactamase CTX-M-15, the class C enzyme AmpC from Pseudomonas aeruginosa, and class D OXA enzymes with carbapenem-hydrolyzing capabilities. We demonstrate that cyclic boronates can potentiate the use of ß-lactams against Gram-negative clinical isolates expressing a variety of ß-lactamases. Comparison of a crystal structure of a CTX-M-15:cyclic boronate complex with structures of cyclic boronates complexed with other ß-lactamases reveals remarkable conservation of the small-molecule binding mode, supporting our proposal that these molecules work by mimicking the common tetrahedral anionic intermediate present in both serine- and metallo-ß-lactamase catalysis.

13C-Carbamylation as a mechanistic probe for the inhibition of class D ß-lactamases by avibactam and halide ions.

The class D (OXA) serine ß-lactamases are a major cause of resistance to ß-lactam antibiotics. The class D enzymes are unique amongst ß-lactamases because they have a carbamylated lysine that acts as a general acid/base in catalysis. Previous crystallographic studies led to the proposal that ß-lactamase inhibitor avibactam targets OXA enzymes in part by promoting decarbamylation. Similarly, halide ions are proposed to inhibit OXA enzymes via decarbamylation. NMR analyses, in which the carbamylated lysines of OXA-10, -23 and -48 were 13C-labelled, indicate that reaction with avibactam does not ablate lysine carbamylation in solution. While halide ions did not decarbamylate the 13C-labelled OXA enzymes in the absence of substrate or inhibitor, avibactam-treated OXA enzymes were susceptible to decarbamylation mediated by halide ions, suggesting halide ions may inhibit OXA enzymes by promoting decarbamylation of acyl-enzyme complex. Crystal structures of the OXA-10 avibactam complex were obtained with bromide, iodide, and sodium ions bound between Trp-154 and Lys-70. Structures were also obtained wherein bromide and iodide ions occupy the position expected for the 'hydrolytic water' molecule. In contrast with some solution studies, Lys-70 was decarbamylated in these structures. These results reveal clear differences between crystallographic and solution studies on the interaction of class D ß-lactamases with avibactam and halides, and demonstrate the utility of 13C-NMR for studying lysine carbamylation in solution.

19 F-NMR Reveals the Role of Mobile Loops in Product and Inhibitor Binding by the São Paulo Metallo-ß-Lactamase.

Resistance to ß-lactam antibiotics mediated by metallo-ß-lactamases (MBLs) is a growing problem. We describe the use of protein-observe 19 F-NMR (PrOF NMR) to study the dynamics of the São Paulo MBL (SPM-1) from ß-lactam-resistant Pseudomonas aeruginosa. Cysteinyl variants on the α3 and L3 regions, which flank the di-ZnII active site, were selectively 19 F-labeled using 3-bromo-1,1,1-trifluoroacetone. The PrOF NMR results reveal roles for the mobile α3 and L3 regions in the binding of both inhibitors and hydrolyzed ß-lactam products to SPM-1. These results have implications for the mechanisms and inhibition of MBLs by ß-lactams and non-ß-lactams and illustrate the utility of PrOF NMR for efficiently analyzing metal chelation, identifying new binding modes, and studying protein binding from a mixture of equilibrating isomers.

Interaction of Avibactam with Class B Metallo-ß-Lactamases.

ß-Lactamases are the most important mechanisms of resistance to the ß-lactam antibacterials. There are two mechanistic classes of ß-lactamases: the serine ß-lactamases (SBLs) and the zinc-dependent metallo-ß-lactamases (MBLs). Avibactam, the first clinically useful non-ß-lactam ß-lactamase inhibitor, is a broad-spectrum SBL inhibitor, which is used in combination with a cephalosporin antibiotic (ceftazidime). There are multiple reports on the interaction of avibactam with SBLs but few such studies with MBLs. We report biochemical and biophysical studies on the binding and reactivity of avibactam with representatives from all 3 MBL subfamilies (B1, B2, and B3). Avibactam has only limited or no activity versus MBL-mediated resistance in pathogens. Avibactam does not inhibit MBLs and binds only weakly to most of the MBLs tested; in some cases, avibactam undergoes slow hydrolysis of one of its urea N-CO bonds followed by loss of CO2, in a process different from that observed with the SBLs studied. The results suggest that while the evolution of MBLs that more efficiently catalyze avibactam hydrolysis should be anticipated, pursuing the development of dual-action SBL and MBL inhibitors based on the diazabicyclooctane core of avibactam may be productive.

Structural and Biochemical Characterization of Rm3, a Subclass B3 Metallo-ß-Lactamase Identified from a Functional Metagenomic Study.

ß-Lactamase production increasingly threatens the effectiveness of ß-lactams, which remain a mainstay of antimicrobial chemotherapy. New activities emerge through both mutation of previously known ß-lactamases and mobilization from environmental reservoirs. The spread of metallo-ß-lactamases (MBLs) represents a particular challenge because of their typically broad-spectrum activities encompassing carbapenems, in addition to other ß-lactam classes. Increasingly, genomic and metagenomic studies have revealed the distribution of putative MBLs in the environment, but in most cases their activity against clinically relevant ß-lactams and, hence, the extent to which they can be considered a resistance reservoir remain uncharacterized. Here we characterize the product of one such gene, blaRm3, identified through functional metagenomic sampling of an environment with high levels of biocide exposure. blaRm3 encodes a subclass B3 MBL that, when expressed in a recombinant Escherichia coli strain, is exported to the bacterial periplasm and hydrolyzes clinically used penicillins, cephalosporins, and carbapenems with an efficiency limited by high Km values. An Rm3 crystal structure reveals the MBL superfamily αß/ßα fold, which more closely resembles that in mobilized B3 MBLs (AIM-1 and SMB-1) than other chromosomal enzymes (L1 or FEZ-1). A binuclear zinc site sits in a deep channel that is in part defined by a relatively extended N terminus. Structural comparisons suggest that the steric constraints imposed by the N terminus may limit its affinity for ß-lactams. Sequence comparisons identify Rm3-like MBLs in numerous other environmental samples and species. Our data suggest that Rm3-like enzymes represent a distinct group of B3 MBLs with a wide distribution and can be considered an environmental reservoir of determinants of ß-lactam resistance.

Structural Basis of Metallo-ß-Lactamase Inhibition by Captopril Stereoisomers.

ß-Lactams are the most successful antibacterials, but their effectiveness is threatened by resistance, most importantly by production of serine- and metallo-ß-lactamases (MBLs). MBLs are of increasing concern because they catalyze the hydrolysis of almost all ß-lactam antibiotics, including recent-generation carbapenems. Clinically useful serine-ß-lactamase inhibitors have been developed, but such inhibitors are not available for MBLs. l-Captopril, which is used to treat hypertension via angiotensin-converting enzyme inhibition, has been reported to inhibit MBLs by chelating the active site zinc ions via its thiol(ate). We report systematic studies on B1 MBL inhibition by all four captopril stereoisomers. High-resolution crystal structures of three MBLs (IMP-1, BcII, and VIM-2) in complex with either the l- or d-captopril stereoisomer reveal correlations between the binding mode and inhibition potency. The results will be useful in the design of MBL inhibitors with the breadth of selectivity required for clinical application against carbapenem-resistant Enterobacteriaceae and other organisms causing MBL-mediated resistant infections.