Structures of fungal and plant acetohydroxyacid synthases.

Acetohydroxyacid synthase (AHAS), also known as acetolactate synthase, is a flavin adenine dinucleotide-, thiamine diphosphate- and magnesium-dependent enzyme that catalyses the first step in the biosynthesis of branched-chain amino acids1. It is the target for more than 50 commercial herbicides2. AHAS requires both catalytic and regulatory subunits for maximal activity and functionality. Here we describe structures of the hexadecameric AHAS complexes of Saccharomyces cerevisiae and dodecameric AHAS complexes of Arabidopsis thaliana. We found that the regulatory subunits of these AHAS complexes form a core to which the catalytic subunit dimers are attached, adopting the shape of a Maltese cross. The structures show how the catalytic and regulatory subunits communicate with each other to provide a pathway for activation and for feedback inhibition by branched-chain amino acids. We also show that the AHAS complex of Mycobacterium tuberculosis adopts a similar structure, thus demonstrating that the overall AHAS architecture is conserved across kingdoms.

Metabolic flux enhancement from the translational fusion of terpene synthases is linked to terpene synthase accumulation.

The end-to-end fusion of enzymes that catalyse successive steps in a reaction pathway is a metabolic engineering strategy that has been successfully applied in a variety of pathways and is particularly common in terpene bioproduction. Despite its popularity, limited work has been done to interrogate the mechanism of metabolic enhancement from enzyme fusion. We observed a remarkable >110-fold improvement in nerolidol production upon translational fusion of nerolidol synthase (a sesquiterpene synthase) to farnesyl diphosphate synthase. This delivered a titre increase from 29.6 mg/L up to 4.2 g/L nerolidol in a single engineering step. Whole-cell proteomic analysis revealed that nerolidol synthase levels in the fusion strains were greatly elevated compared to the non-fusion control. Similarly, the fusion of nerolidol synthase to non-catalytic domains also produced comparable increases in titre, which coincided with improved enzyme expression. When farnesyl diphosphate synthase was fused to other terpene synthases, we observed more modest improvements in terpene titre (1.9- and 3.8-fold), corresponding with increases of a similar magnitude in terpene synthase levels. Our data demonstrate that increased in vivo enzyme levels - resulting from improved expression and/or improved protein stability - is a major driver of catalytic enhancement from enzyme fusion.

Structural and Functional Insight into the Mechanism of the Fe-S Cluster-Dependent Dehydratase from Paralcaligenes ureilyticus.

Enzyme-catalyzed reaction cascades play an increasingly important role for the sustainable manufacture of diverse chemicals from renewable feedstocks. For instance, dehydratases from the ilvD/EDD superfamily have been embedded into a cascade to convert glucose via pyruvate to isobutanol, a platform chemical for the production of aviation fuels and other valuable materials. These dehydratases depend on the presence of both a Fe-S cluster and a divalent metal ion for their function. However, they also represent the rate-limiting step in the cascade. Here, catalytic parameters and the crystal structure of the dehydratase from Paralcaligenes ureilyticus (PuDHT, both in presence of Mg2+ and Mn2+ ) were investigated. Rate measurements demonstrate that the presence of stoichiometric concentrations Mn2+ promotes higher activity than Mg2+ , but at high concentrations the former inhibits the activity of PuDHT. Molecular dynamics simulations identify the position of a second binding site for the divalent metal ion. Only binding of Mn2+ (not Mg2+ ) to this site affects the ligand environment of the catalytically essential divalent metal binding site, thus providing insight into an inhibitory mechanism of Mn2+ at higher concentrations. Furthermore, in silico docking identified residues that play a role in determining substrate binding and selectivity. The combined data inform engineering approaches to design an optimal dehydratase for the cascade.

Engineering indel and substitution variants of diverse and ancient enzymes using Graphical Representation of Ancestral Sequence Predictions (GRASP).

Ancestral sequence reconstruction is a technique that is gaining widespread use in molecular evolution studies and protein engineering. Accurate reconstruction requires the ability to handle appropriately large numbers of sequences, as well as insertion and deletion (indel) events, but available approaches exhibit limitations. To address these limitations, we developed Graphical Representation of Ancestral Sequence Predictions (GRASP), which efficiently implements maximum likelihood methods to enable the inference of ancestors of families with more than 10,000 members. GRASP implements partial order graphs (POGs) to represent and infer insertion and deletion events across ancestors, enabling the identification of building blocks for protein engineering. To validate the capacity to engineer novel proteins from realistic data, we predicted ancestor sequences across three distinct enzyme families: glucose-methanol-choline (GMC) oxidoreductases, cytochromes P450, and dihydroxy/sugar acid dehydratases (DHAD). All tested ancestors demonstrated enzymatic activity. Our study demonstrates the ability of GRASP (1) to support large data sets over 10,000 sequences and (2) to employ insertions and deletions to identify building blocks for engineering biologically active ancestors, by exploring variation over evolutionary time.

Inhibitors for metallo-ß-lactamases from the B1 and B3 subgroups provide an avenue to combat a major mechanism of antibiotic resistance.

Metallo-ß-lactamases (MBLs) are a group of Zn(II)-dependent enzymes that pose a major threat to global health. They are linked to an increasing number of multi-drug resistant bacterial pathogens, but no clinically useful inhibitor is yet available. Since ß-lactam antibiotics, which are inactivated by MBLs, constitute ∼65% of all antibiotics used to treat infections, the search for clinically relevant MBL inhibitors is urgent. Here, derivatives of a 2-amino-1-benzyl-4,5-diphenyl-1H-pyrrole-3-carbonitrile (1a) were synthesised and their inhibitory effects assessed against prominent representatives of the MBL family. Several compounds are potent inhibitors of each MBL tested, making them promising candidates for the development of broad-spectrum drug leads. In particular, compound 5f is highly potent across the MBL family, with Ki values in the low µM range. Furthermore, this compound also appears to display synergy in combination with antibiotics such as penicillin G, cefuroxime or meropenem. This molecule thus represents a promising starting point to develop new drugs to inhibit a major mechanism of antibiotic resistance.

Sequence- and structure-guided improvement of the catalytic performance of a GH11 family xylanase from Bacillus subtilis.

Xylanases produce xylooligosaccharides from xylan and have thus attracted increasing attention for their usefulness in industrial applications. Previously, we demonstrated that the GH11 xylanase XynLC9 from Bacillus subtilis formed xylobiose and xylotriose as the major products with negligible production of xylose when digesting corncob-extracted xylan. Here, we aimed to improve the catalytic performance of XynLC9 via protein engineering. Based on the sequence and structural comparisons of XynLC9 with the xylanases Xyn2 from Trichoderma reesei and Xyn11A from Thermobifida fusca, we identified the N-terminal residues 5-YWQN-8 in XynLC9 as engineering hotspots and subjected this sequence to site saturation and iterative mutagenesis. The mutants W6F/Q7H and N8Y possessed a 2.6- and 1.8-fold higher catalytic activity than XynLC9, respectively, and both mutants were also more thermostable. Kinetic measurements suggested that W6F/Q7H and N8Y had lower substrate affinity, but a higher turnover rate (kcat), which resulted in increased catalytic efficiency than WT XynLC9. Furthermore, the W6F/Q7H mutant displayed a 160% increase in the yield of xylooligosaccharides from corncob-extracted xylan. Molecular dynamics simulations revealed that the W6F/Q7H and N8Y mutations led to an enlarged volume and surface area of the active site cleft, which provided more space for substrate entry and product release and thus accelerated the catalytic activity of the enzyme. The molecular evolution approach adopted in this study provides the design of a library of sequences that captures functional diversity in a limited number of protein variants.

Structure-Guided Modulation of the Catalytic Properties of [2Fe-2S]-Dependent Dehydratases.

The FeS cluster-dependent dihydroxyacid dehydratases (DHADs) and sugar acid-specific dehydratases (DHTs) from the ilvD/EDD superfamily are key enzymes in the bioproduction of a wide variety of chemicals. We analyzed [2Fe-2S]-dependent dehydratases in silico and in vitro, deduced functionally relevant sequence, structure, and activity relationships within the ilvD/EDD superfamily, and we propose a new classification based on their evolutionary relationships and substrate profiles. In silico simulations and analyses identified several key positions for specificity, which were experimentally investigated with site-directed and saturation mutagenesis. We thus increased the promiscuity of DHAD from Fontimonas thermophila (FtDHAD), showing >10-fold improved activity toward D-gluconate, and shifted the substrate preference of DHT from Paralcaligenes ureilyticus (PuDHT) toward shorter sugar acids (recording a six-fold improved activity toward the non-natural substrate D-glycerate). The successful elucidation of the role of important active site residues of the ilvD/EDD superfamily will further guide developments of this important biocatalyst for industrial applications.

Dihydroxy-Acid Dehydratases From Pathogenic Bacteria: Emerging Drug Targets to Combat Antibiotic Resistance.

There is an urgent global need for the development of novel therapeutics to combat the rise of various antibiotic-resistant superbugs. Enzymes of the branched-chain amino acid (BCAA) biosynthesis pathway are an attractive target for novel anti-microbial drug development. Dihydroxy-acid dehydratase (DHAD) is the third enzyme in the BCAA biosynthesis pathway. It relies on an Fe-S cluster for catalytic activity and has recently also gained attention as a catalyst in cell-free enzyme cascades. Two types of Fe-S clusters have been identified in DHADs, i.e. [2Fe-2S] and [4Fe-4S], with the latter being more prone to degradation in the presence of oxygen. Here, we characterise two DHADs from bacterial human pathogens, Staphylococcus aureus and Campylobacter jejuni (SaDHAD and CjDHAD). Purified SaDHAD and CjDHAD are virtually inactive, but activity could be reversibly reconstituted in vitro (up to ∼19,000-fold increase with kcat as high as ∼6.7 s-1 ). Inductively-coupled plasma-optical emission spectroscopy (ICP-OES) measurements are consistent with the presence of [4Fe-4S] clusters in both enzymes. N-isopropyloxalyl hydroxamate (IpOHA) and aspterric acid are both potent inhibitors for both SaDHAD (Ki =7.8 and 51.6 µM, respectively) and CjDHAD (Ki =32.9 and 35.1 µM, respectively). These compounds thus present suitable starting points for the development of novel anti-microbial chemotherapeutics.

Biotechnological potential and initial characterization of two novel sesquiterpene synthases from Basidiomycota Coniophora puteana for heterologous production of δ-cadinol.

BACKGROUND: Terpene synthases are versatile catalysts in all domains of life, catalyzing the formation of an enormous variety of different terpenoid secondary metabolites. Due to their diverse bioactive properties, terpenoids are of great interest as innovative ingredients in pharmaceutical and cosmetic applications. Recent advances in genome sequencing have led to the discovery of numerous terpene synthases, in particular in Basidiomycota like the wood rotting fungus Coniophora puteana, which further enhances the scope for the manufacture of terpenes for industrial purposes. RESULTS: In this study we describe the identification of two novel (+)-δ-cadinol synthases from C. puteana, Copu5 and Copu9. The sesquiterpene (+)-δ-cadinol was previously shown to exhibit cytotoxic activity therefore having an application as possible, new, and sustainably sourced anti-tumor agent. In an Escherichia coli strain, optimized for sesquiterpene production, titers of 225 mg l-1 and 395 mg l-1, respectively, could be achieved. Remarkably, both enzymes share the same product profile thereby representing the first two terpene synthases from Basidiomycota with identical product profiles. We solved the crystal structure of Copu9 in its closed conformation, for the first time providing molecular details of sesquiterpene synthase from Basidiomycota. Based on the Copu9 structure, we conducted structure-based mutagenesis of amino acid residues lining the active site, thereby altering the product profile. Interestingly, the mutagenesis study also revealed that despite the conserved product profiles of Copu5 and Copu9 different conformational changes may accompany the catalytic cycle of the two enzymes. This observation suggests that the involvement of tertiary structure elements in the reaction mechanism(s) employed by terpene synthases may be more complex than commonly expected. CONCLUSION: The presented product selectivity and titers of Copu5 and Copu9 may pave the way towards a sustainable, biotechnological production of the potentially new bioactive (+)-δ-cadinol. Furthermore, Copu5 and Copu9 may serve as model systems for further mechanistic studies of terpenoid catalysis.

Kinetic and Structural Characterization of the First B3 Metallo-ß-Lactamase with an Active-Site Glutamic Acid.

The structural diversity in metallo-ß-lactamases (MBLs), especially in the vicinity of the active site, has been a major hurdle in the development of clinically effective inhibitors. Representatives from three variants of the B3 MBL subclass, containing either the canonical HHH/DHH active-site motif (present in the majority of MBLs in this subclass) or the QHH/DHH (B3-Q) or HRH/DQK (B3-RQK) variations, were reported previously. Here, we describe the structure and kinetic properties of the first example (SIE-1) of a fourth variant containing the EHH/DHH active-site motif (B3-E). SIE-1 was identified in the hexachlorocyclohexane-degrading bacterium Sphingobium indicum, and kinetic analyses demonstrate that although it is active against a wide range of antibiotics, its efficiency is lower than that of other B3 MBLs but has increased efficiency toward cephalosporins relative to other ß-lactam substrates. The overall fold of SIE-1 is characteristic of the MBLs; the notable variation is observed in the Zn1 site due to the replacement of the canonical His116 by a glutamate. The unusual preference of SIE-1 for cephalosporins and its occurrence in a widespread environmental organism suggest the scope for increased MBL-mediated ß-lactam resistance. Thus, it is relevant to include SIE-1 in MBL inhibitor design studies to widen the therapeutic scope of much needed antiresistance drugs.

Recent Advances in Heterogeneous Catalytic Systems Containing Metal Ions for Phosphate Ester Hydrolysis.

Organophosphates are a class of organic compounds that are important for living organisms, forming the building blocks for DNA, RNA, and some essential cofactors. Furthermore, non-natural organophosphates are widely used in industrial applications, including as pesticides; in laundry detergents; and, unfortunately, as chemical weapons agents. In some cases, the natural degradation of organophosphates can take thousands of years; this longevity creates problems associated with handling and the storage of waste generated by such phosphate esters, in particular. Efforts to develop new catalysts for the cleavage of phosphate esters have progressed in recent decades, mainly in the area of homogeneous catalysis. In contrast, the development of heterogeneous catalysts for the hydrolysis of organophosphates has not been as prominent. Herein, examples of heterogeneous systems are described and the importance of the development of heterogeneous catalysts applicable to organophosphate hydrolysis is highlighted, shedding light on recent advances related to different solid matrices that have been employed.

Discovery of a Pyrimidinedione Derivative with Potent Inhibitory Activity against Mycobacterium tuberculosis Ketol-Acid Reductoisomerase.

New drugs aimed at novel targets are urgently needed to combat the increasing rate of drug-resistant tuberculosis (TB). Herein, the National Cancer Institute Developmental Therapeutic Program (NCI-DTP) chemical library was screened against a promising new target, ketol-acid reductoisomerase (KARI), the second enzyme in the branched-chain amino acid (BCAA) biosynthesis pathway. From this library, 6-hydroxy-2-methylthiazolo[4,5-d]pyrimidine-5,7(4H,6H)-dione (NSC116565) was identified as a potent time-dependent inhibitor of Mycobacterium tuberculosis (Mt) KARI with a Ki of 95.4 nm. Isothermal titration calorimetry studies showed that this inhibitor bound to MtKARI in the presence and absence of the cofactor, nicotinamide adenine dinucleotide phosphate (NADPH), which was confirmed by crystal structures of the compound in complex with closely related Staphylococcus aureus KARI. It is also shown that NSC116565 inhibits the growth of H37Ra and H37Rv strains of Mt with MIC50 values of 2.93 and 6.06 µm, respectively. These results further validate KARI as a TB drug target and show that NSC116565 is a promising lead for anti-TB drug development.

Discovery, Synthesis and Evaluation of a Ketol-Acid Reductoisomerase Inhibitor.

Ketol-acid reductoisomerase (KARI), the second enzyme in the branched-chain amino acid biosynthesis pathway, is a potential drug target for bacterial infections including Mycobacterium tuberculosis. Here, we have screened the Medicines for Malaria Venture Pathogen Box against purified M. tuberculosis (Mt) KARI and identified two compounds that have Ki values below 200 nm. In Mt cell susceptibility assays one of these compounds exhibited an IC50 value of 0.8 µm. Co-crystallization of this compound, 3-((methylsulfonyl)methyl)-2H-benzo[b][1,4]oxazin-2-one (MMV553002), in complex with Staphylococcus aureus KARI, which has 56 % identity with Mt KARI, NADPH and Mg2+ yielded a structure to 1.72 Šresolution. However, only a hydrolyzed product of the inhibitor (i.e. 3-(methylsulfonyl)-2-oxopropanic acid, missing the 2-aminophenol attachment) is observed in the active site. Surprisingly, Mt cell susceptibility assays showed that the 2-aminophenol product is largely responsible for the anti-TB activity of the parent compound. Thus, 3-(methylsulfonyl)-2-oxopropanic acid was identified as a potent KARI inhibitor that could be further explored as a potential biocidal agent and we have shown 2-aminophenol, as an anti-TB drug lead, especially given it has low toxicity against human cells. The study highlights that careful analysis of broad screening assays is required to correctly interpret cell-based activity data.

Inhibition studies of ketol-acid reductoisomerases from pathogenic microorganisms.

Ketol-acid reductoisomerase (KARI), the second enzyme in the branched-chain amino acid (BCAA) biosynthesis pathway, is an emerging target for the discovery of biocides. Here, we demonstrate that cyclopropane-1,1-dicarboxylate (CPD) inhibits KARIs from the pathogens Mycobacterium tuberculosis (Mt) and Campylobacter jejuni (Cj) reversibly with Ki values of 3.03 µM and 0.59 µM, respectively. Another reversible inhibitor of both KARIs, Hoe 704, is more potent than CPD with Ki values of 300 nM and 110 nM for MtKARI and CjKARI, respectively. The most potent inhibitor tested here is N-hydroxy-N-isopropyloxamate (IpOHA). It has a Ki of ~26 nM for MtKARI, but binds rather slowly (kon ~900 M-1s-1). In contrast, IpOHA binds more rapidly (kon ~7000 M-1s-1) to CjKARI and irreversibly.

Adaptation of a continuous, calorimetric kinetic assay to study the agmatinase-catalyzed hydrolytic reaction.

Ureohydrolases are members of the metallohydrolase family of enzymes. Here, a simple continuous assay for agmatinase (AGM) activity was established by following the degradation of agmatine to urea and putrescine using isothermal titration calorimetry (ITC). ITC is particularly useful for kinetic assays when substrates of interest do not possess suitable chromophores that facilitate the continuous spectrophotometric detection of substrate depletion and/or product formation. In order to assess the accuracy of the ITC-based assay, catalytic parameters were also determined using a discontinuous, colorimetric assay. Both methods resulted in comparable kinetic parameters. From the colorimetric assay the kcat and KM values are 131 s-1 and 0.25 mM, respectively, and from the ITC assay the corresponding parameters are 30 s-1 and 0.45 mM, respectively. The continuous ITC-based assay will facilitate functional studies for an enzyme that is an emerging target for the development of addiction treatments.

The Binding Mode of an ADP Analogue to a Metallohydrolase Mimics the Likely Transition State.

Purple acid phosphatases (PAPs) are members of the large family of metallohydrolases, a group of enzymes that perform a wide range of biological functions, while employing a highly conserved catalytic mechanism. PAPs are found in plants, animals and fungi; in humans they play an important role in bone turnover and are thus of interest for developing treatments for osteoporosis. The majority of metallohydrolases use a metal-bound hydroxide to initiate catalysis, which leads to the formation of a proposed five-coordinate oxyphosphorane species in the transition state. In this work, we crystallized PAP from red kidney beans (rkbPAP) in the presence of both adenosine and vanadate. The in crystallo-formed vanadate analogue of ADP provides detailed insight into the binding mode of a PAP substrate, captured in a structure that mimics the putative fivecoordinate transition state. Our observations not only provide unprecedented insight into the mechanism of metallohydrolases, but might also guide the structure-based design of inhibitors for application in the treatment of several human illnesses.

Guanidine- and purine-functionalized ligands of FeIIIZnII complexes: effects on the hydrolysis of DNA.

In this paper, the catalytic effects of aminoguanidine and aminopurine groups in the second sphere of a FeIIIZnII complex that mimics the active site of the metallohydrolase purple acid phosphatase (PAP) are investigated, with a particular view on DNA as substrate. The ligand 3-(((3-((bis(2-(pyridin-2-yl)ethyl)amino)methyl)-2-hydroxy-5-methylbenzyl)(pyridin-2-ylmethyl)amino)meth-yl)-2 hydroxy-5-methylbenzaldehyde-(H2L1bpea) was synthesized and its complex [(OH)FeIII(µ-OH)ZnII(H2O)(L1bpea)](ClO4) was used as a base for comparison with similar complexes previously published in the literature. Subsequent modifications were conducted in the aldehyde group, where aminoguanidine (amig) and aminopurine (apur) were used as side chain derivatives. The complexes [(OH)FeIII(µ-OH)ZnII(H2O)(L1bpea)](ClO4) (1), [(OH)FeIII(µ-OH)ZnII(H2O)(L1bpea-amig)](ClO4) (2) and [(OH)FeIII(µ-OH)ZnII(H2O)(L1bpea-apur)](ClO4) (3) were characterized by spectroscopic methods (infrared, UV-Vis) and ESI-MS spectrometry. Density functional theory (DFT) was also used to better understand the structure of the complexes. The hydrolytic activity of complexes 1, 2 and 3 was analyzed using both the model substrate 2,4-BDNPP (bis-(2,4-dinitrophenyl)phosphate) and DNA. Complexes 2 and 3, containing the derivatized ligands, have a significantly higher association constant (Kassoc≅ 1/KM) for the activated substrate 2,4-BDNPP compared to complex 1. The catalytic efficiency (kcat/KM) is also higher due to hydrogen bonds and/or π-stacking interactions between the substrate and the aminoguanidine or aminopurine groups present in 2 and 3, respectively. In the DNA cleavage assays, all complexes were able to cleave DNA, with 1 and 2 having higher catalytic activity than 3. In addition, when compared to previously analyzed complexes, complex 2 is one of the most active, having a kcat of 0.21 h-1.

Investigating coordination flexibility of glycerophosphodiesterase (GpdQ) through interactions with mono-, di-, and triphosphoester (NPP, BNPP, GPE, and paraoxon) substrates.

In this study, interactions of the catalytically active binuclear form of glycerophosphodiesterase (GpdQ) with four chemically diverse substrates, i.e. NPP (a phosphomonoester), BNPP and GPE (both phosphodiesters), and paraoxon (a phosphotriester) have been investigated using all-atom molecular dynamics (MD) simulations. The roles of metal ions and key amino acid residues, coordination flexibility, and dynamic transformations in all enzyme-substrate complexes have been elucidated. The roles of important first and second coordination shell residues in substrate binding and coordination flexibility of the enzyme suggested by simulations are supported by experimental data. The chemical nature of the substrate is found to influence the mode of binding, electrostatic surface potential, metal-metal distance, and reorganization of the active site. The experimentally proposed association between the substrate binding and coordination flexibility is analyzed using principal component analysis (PCA), movements of loops, and root-mean-square-fluctuations (RMSF) as parameters. The PCA of these substrates provides different energy basins, i.e. one, three, two and five for NPP, BNPP, GPE, and paraoxon, respectively. Additionally, the area of an irregular hexagon (268.3, 288.9, 350.8, and 362.5 Å2) formed by the residues on these loops illustrates their distinct motions. The substrate binding free energies of NPP, BNPP, and GPE are quite close (22.4-24.3 kcal mol-1), but paraoxon interacts with the smallest binding free energy (14.1 kcal mol-1). The metal binding energies in the presence of these substrates are substantially different, i.e. the lowest for NPP and the highest for paraoxon. These results thus provide deeper insight into the chemical promiscuity and coordination flexibility of this important enzyme.

Copper Ions and Coordination Complexes as Novel Carbapenem Adjuvants.

Carbapenem-resistant Enterobacteriaceae are urgent threats to global human health. These organisms produce ß-lactamases with carbapenemase activity, such as the metallo-ß-lactamase NDM-1, which is notable due to its association with mobile genetic elements and the lack of a clinically useful inhibitor. Here we examined the ability of copper to inhibit the activity of NDM-1 and explored the potential of a copper coordination complex as a mechanism to efficiently deliver copper as an adjuvant in clinical therapeutics. An NDM-positive Escherichia coli isolate, MS6192, was cultured from the urine of a patient with a urinary tract infection. MS6192 was resistant to antibiotics from multiple classes, including diverse ß-lactams (penicillins, cephalosporins, and carbapenems), aminoglycosides, and fluoroquinolones. In the presence of copper (range, 0 to 2 mM), however, the susceptibility of MS6192 to the carbapenems ertapenem and meropenem increased markedly. In standard checkerboard assays, copper decreased the MICs of ertapenem and meropenem against MS6192 in a dose-dependent manner, suggesting a synergistic mode of action. To examine the inhibitory effect of copper in the absence of other ß-lactamases, the blaNDM-1 gene from MS6192 was cloned and expressed in a recombinant E. coli K-12 strain. Analysis of cell extracts prepared from this strain revealed that copper directly inhibited NDM-1 activity, which was confirmed using purified recombinant NDM-1. Finally, delivery of copper at a low concentration of 10 µM by using the FDA-approved coordination complex copper-pyrithione sensitized MS6192 to ertapenem and meropenem in a synergistic manner. Overall, this work demonstrates the potential use of copper coordination complexes as novel carbapenemase adjuvants.

Second-Sphere Effects in Dinuclear FeIIIZnII Hydrolase Biomimetics: Tuning Binding and Reactivity Properties.

Herein, we report the synthesis and characterization of two dinuclear FeIIIZnII complexes [FeIIIZnIILP1] (1) and [FeIIIZnIILP2] (2), in which LP1 and LP2 are conjugated systems containing one and two pyrene groups, respectively, connected via the diamine -HN(CH2)4NH- spacer to the well-known N5O2-donor H2L ligand (H2L = 2-bis{[(2-pyridylmethyl)aminomethyl]-6-[(2-hydroxybenzyl)(2-pyridylmethyl)]aminomethyl}-4-methylphenol). The complex [FeIIIZnIIL1] (3), in which H2L was modified to H2L1, with a carbonyl group attached to the terminal phenol group, was included in this study for comparison purposes.1 Both complexes 1 and 2 were satisfactorily characterized in the solid state and in solution. Extended X-ray absorption fine structure data for 1 and 3 in an acetonitrile solution show that the multiply bridged structure seen in the solid state of 3 is retained in solution. Potentiometric and UV-vis titration of 1 and 2 show that electrostatic interaction between the protonated amino groups and coordinated water molecules significantly decreases the pKa of the iron(III)-bound water compared to those of 3. On the other hand, catalytic activity studies using 1 and 2 in the hydrolysis of the activated substrate bis(2,4-dinitrophenyl)phosphate (BDNPP) resulted in a significant increase in the association of the substrate (Kass ≅ 1/KM) compared to that of 3 because of electrostatic and hydrophobic interactions between BDNPP and the side-chain diaminopyrene of the ligands H2LP1 and H2LP2. In addition, the introduction of the pyrene motifs in 1 and 2 enhanced their activity toward DNA and as effective antitumor drugs, although the biochemical mechanism of the latter effect is currently under investigation. These complexes represent interesting examples of how to promote an increase in the activity of traditional artificial metal nucleases by introducing second-coordination-sphere effects.