Mechanism of inhibition of the class C beta-lactamase of Enterobacter cloacae P99 by cyclic acyl phosph(on)ates: rescue by return

Divergent synthesis of benzoxaphospholenes and phosphacoumarins via the reaction of 2-alkenylphenols with phosphorus(III/V) chlorides

The divergent synthesis of benzo[e]-1,2-oxaphosphinines or benzo[d]-1,2-oxaphospholenes along with spirocyclic quasiphosphonium compounds based on 2-alkenylphenols and phosphorus(III/V) chlorides is presented. The reaction is condition-dependent and determined by the biphility of the phosphorus(III) derivative and the dual reactivity of 2-alkenylphenol. The procedures are applicable for obtaining benzo[e]-1,2-oxaphosphinines substituted at position 4 and disubstituted at positions 4 and 5 as well as 3,3-disubstituted benzo[d]-1,2-oxaphospholenes with good to high yields.

Transition-Metal-Free Electrochemical Selenylative Cyclization of Alkynyl Phosphonates

Unprecedented regioselective electrochemical tandem selenation/cyclization of alkynyl phosphonates with diselenide is described here. These obtained selenoether products can be chemo-selectively converted into halogen-functionalized cyclic enol phosphonates under our electrochemical conditions. These protocols provide straightforward access to valuable cyclic enol phosphonate or phosphaisocoumarins under the electrochemical and transition-metal-free conditions. The robustness of these transformations was illustrated by their compatibility with various complex natural products and bioactive molecules. The selenoether and halogen functional groups allow the further diversification of the phosphorus heterocycles thus obtained.

Uranyl phosphonates: crystalline materials and nanosheets for temperature sensing

Ultrathin nanosheets of luminescent metal-organic frameworks or coordination polymers have been widely used for sensing ions, solvents and biomolecules but, as far as we are aware, not yet used for temperature sensing. Herein we report two luminescent uranyl phosphonates based on 2-(phosphonomethyl)benzoic acid (2-pmbH₃), namely (UO₂)(2-pmbH₂)₂ (1) and (H₃O)[(UO₂)₂(2-pmb)(2-pmbH)] (2). The former has a supramolecular layer structure, composed of chains of corner-sharing {UO₆} octahedra and {PO₃C} tetrahedra which are connected by hydrogen bonds between phosphonate and carboxylic groups. Compound 2 possesses a unique 2D anionic framework structure, where the inorganic uranyl phosphonate chains made up of {UO₇} and {PO₃C} polyhedra are cross-linked by 2-pmb^3- ligands. The carboxylic groups of 2-pmbH^2- ligands are pendant on the two sides of the layers and form hydrogen bonds between the layers. Both compounds can be exfoliated in acetone via a top-down freeze-thaw method, resulting in nanosheets of two-layer thickness. Interestingly, the photoluminescence (PL) of 1 and 2 is highly temperature sensitive. Variable temperature PL studies revealed that compounds 1 and 2 can be used as thermometers in the temperature ranges 120-300 K and 100-280 K, respectively. By doping the nanosheets into polymer matrix, 1-ns@PMMA and 2-ns@PMMA were prepared. The PL intensity of 1-ns@PMMA is insensitive to temperature, unlike that of the bulk sample. While 2-ns@PMMA exhibits similar temperature-dependent luminescence behaviour to its bulk counterpart, thereby enabling its potential application as a thermometer in the temperature range 100-280 K.

Microwave-assisted periselective annulation of triarylphosphenes with aldehydes and ketones

The reaction of diazo(aryl)methyl(diaryl)phosphine oxides with aldehydes and ketones generates benzo-δ-phosphinolactones in low to good yields with 1,1-diarylalk-1-enes as byproducts under microwave irradiation. Diazo(aryl)methyl(diaryl)phosphine oxides first undergo a Wolff rearrangement to form diaryl(aryl)phosphenes, which further react with aldehydes and ketones to afford benzo-δ-phosphinolactones and β-phosphinolactones. The latter are unstable under heating and fragment into the corresponding 1,1-diarylalk-1-enes and arylphosphine dioxides under reaction conditions. The arylphosphine dioxides become arylphosphonic acids during workup. The periselectivity in the annulation shows that the reaction of diaryl(aryl)phosphenes with most aldehydes and ketones favors phosphene phenyl participation in (4 + 2) annulation over (2 + 2) annulation.

Transition metal-free access to 3,4-dihydro-1,2-oxaphosphinine-2-oxides from phosphonochloridates and chalcones through tandem Michael addition and nucleophilic substitution

3,4-Dihydro-1,2-oxaphosphinine 2-oxides are synthesized through tandem intermolecular nucleophilic addition and cyclization of phosphonochloridates and enones in the presence of base.

Structural insight into potent broad-spectrum inhibition with reversible recyclization mechanism: avibactam in complex with CTX-M-15 and Pseudomonas aeruginosa AmpC β-lactamases

Although β-lactams have been the most effective class of antibacterial agents used in clinical practice for the past half century, their effectiveness on Gram-negative bacteria has been eroded due to the emergence and spread of β-lactamase enzymes that are not affected by currently marketed β-lactam/β-lactamase inhibitor combinations. Avibactam is a novel, covalent, non-β-lactam β-lactamase inhibitor presently in clinical development in combination with either ceftaroline or ceftazidime. In vitro studies show that avibactam may restore the broad-spectrum activity of cephalosporins against class A, class C, and some class D β-lactamases. Here we describe the structures of two clinically important β-lactamase enzymes bound to avibactam, the class A CTX-M-15 extended-spectrum β-lactamase and the class C Pseudomonas aeruginosa AmpC β-lactamase, which together provide insight into the binding modes for the respective enzyme classes. The structures reveal similar binding modes in both enzymes and thus provide a rationale for the broad-spectrum inhibitory activity of avibactam. Identification of the key residues surrounding the binding pocket allows for a better understanding of the potency of this scaffold. Finally, avibactam has recently been shown to be a reversible inhibitor, and the structures provide insights into the mechanism of avibactam recyclization. Analysis of the ultra-high-resolution CTX-M-15 structure suggests how the deacylation mechanism favors recyclization over hydrolysis.

Current challenges in antimicrobial chemotherapy: focus on ß-lactamase inhibition

The use of the three classical beta-lactamase inhibitors (clavulanic acid, tazobactam and sulbactam) in combination with beta-lactam antibacterials is currently the most successful strategy to combat beta-lactamase-mediated resistance. However, these inhibitors are efficient in inactivating only class A beta-lactamases and the efficiency of the inhibitor/antibacterial combination can be compromised by several mechanisms, such as the production of naturally resistant class B or class D enzymes, the hyperproduction of AmpC or even the production of evolved inhibitor-resistant class A enzymes. Thus, there is an urgent need for the development of novel inhibitors. For serine active enzymes (classes A, C and D), derivatives of the beta-lactam ring such as 6-beta-halogenopenicillanates, beta-lactam sulfones, penems and oxapenems, monobactams or trinems seem to be potential starting points to design efficient molecules (such as AM-112 and LK-157). Moreover, a promising non-beta-lactam molecule, NXL-104, is now under clinical development. In contrast, an ideal inhibitor of metallo-beta-lactamases (class B) remains to be found, despite the huge number of potential molecules already described (biphenyl tetrazoles, cysteinyl peptides, mercaptocarboxylates, succinic acid derivatives, etc.). The search for such an inhibitor is complicated by the absence of a covalent intermediate in their catalytic mechanisms and the fact that beta-lactam derivatives often behave as substrates rather than as inhibitors. Currently, the most promising broad-spectrum inhibitors of class B enzymes are molecules presenting chelating groups (thiols, carboxylates, etc.) combined with an aromatic group. This review describes all the types of molecules already tested as potential beta-lactamase inhibitors and thus constitutes an update of the current status in beta-lactamase inhibitor discovery.

Helical peptides derived from lactoferrin bind hepatitis C virus envelope protein E2

Hepatitis C virus is a major cause of chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma infecting more than 170 million people. Hepatitis C virus envelope 2 glycoprotein (E2) binds several cell-surface molecules that act as receptor candidates mediating hepatitis C virus entry into hepatocytes. Peptides derived from human lactoferrin have been shown to bind hepatitis C virus-E2 protein thereby preventing hepatitis C virus entry in cultured hepatocytes. In this study, starting from a 33-residue human lactoferrin-derived peptide, a number of biotin-linked alpha-peptides were synthesized and investigated for their E2 protein binding activity. E2 protein from hepatitis C virus genotype 1b was expressed in 293 human embryonic kidney cells and purified using affinity chromatography. A biotin-streptavidin based binding assay was developed to determine the binding affinity of the synthetic peptides for E2 protein. Two of the peptides bound E2 specifically with submicromolar to low micromolar affinity [equilibrium dissociation constant (K(d)) of 0.569 and 28.8 microM]. Further, these two peptides had the highest helical content in solution as observed by circular dichroism spectroscopy, suggesting that binding affinity increases with increase in helicity. These results have provided new lead peptides for future investigations of hepatitis C virus entry inhibitors that may provide an interesting approach to prevent hepatitis C virus infectivity.

Inhibition of class D beta-lactamases by acyl phosphates and phosphonates

The susceptibility of typical class D beta-lactamases to inhibition by acyl phosph(on)ates has been determined. To a large degree, these class D enzymes behaved very similarly to the class A TEM beta-lactamase towards these reagents. Dibenzoyl phosphate stood out in both cases as a lead compound towards a new class of effective inhibitors.

Modeling kinase-substrate specificity: implication of the distance between substrate nucleophilic oxygen and attacked phosphorus of ATP analog on binding affinity

Molecular dynamics simulations were performed on modeled kinase-substrate complexes in an attempt to establish a relationship between structural features and binding ability of the complexes. We found that the monitored distance between substrate nucleophilic oxygen (OG) and attacked phosphorus (PG) of ATP analog correlated closely with the binding affinity. With reference to 3.3 A, the van der Waals sum of oxygen and phosphorus, the calculated distances of good substrates were close to it whereas those of poor substrates were far apart from it. Therefore, it is reasonable to consider the OG-PG distance as a potential criterion to prefigure the kinase-substrate binding specificity and the simple computational techniques may work as an easy approach to distinguish good substrates from weak or poor substrates.

Novel imidazole substituted 6-methylidene-penems as broad-spectrum β-lactamase inhibitors

Beta-lactamases are serine and metallo-dependent enzymes produced by the bacteria in defense against beta-lactam antibiotics. Production of class-A, class-B, and class-C enzymes by the bacteria make the use of beta-lactam antibiotics ineffective in certain cases. To overcome resistance to beta-lactam antibiotics, several beta-lactamase inhibitors such as clavulanic acid, sulbactam, and tazobactam are widely used in the clinic in combination with beta-lactam antibiotics. However, single point mutations within these enzymes have allowed bacteria to overcome the inhibitory effect of the commercially approved beta-lactamase inhibitors. Although the commercially available beta-lactamase inhibitor/beta-lactam antibiotic combinations are effective against class-A producing bacteria and many extended spectrum beta-lactamase (ESBL's) producing bacteria they are less effective against class-C enzymes expressing bacteria. To circumvent this problem, based on modeling studies several novel imidazole substituted 6-methylidene-penem derivatives were synthesized and tested against various beta-lactamase producing isolates. The present paper deals with the synthesis and structure-activity relationships (SAR) of these compounds.

Toward better antibiotics: crystallographic studies of a novel class of DD-peptidase/beta-lactamase inhibitors

Beta-lactam antibiotics are vital weapons in the treatment of bacterial infections, but their future is under increasing threat from beta-lactamases. These bacterial enzymes hydrolyze and inactivate beta-lactam antibiotics, rendering the host cell resistant to the bactericidal effects of the drugs. Nevertheless, the bacterial D-alanyl-D-alanine transpeptidases (DD-peptidases), the killing targets of beta-lactams, remain attractive targets for antibiotic compounds. Cyclic acyl phosph(on)ates have been developed and investigated as potential inhibitors of both transpeptidases and beta-lactamases. The X-ray crystal structures of the complexes of the Streptomyces strain R61 DD-peptidase inhibited by a bicyclic [1-hydroxy-4,5-benzo-2,6-dioxaphosphorinanone(3)-1-oxide] and a monocyclic [1-hydroxy-4-phenyl-2,6-dioxaphosphorinanone(3)-1-oxide] acyl phosphate were determined to investigate the mode of action of these novel inhibitors. The structures show, first, that these inhibitors form covalent bonds with the active site serine residue of the enzyme and that the refractory complexes thus formed are phosphoryl-enzyme species rather than acyl enzymes. The complexes are long-lived largely because, after ring opening, the ligands adopt conformations that cannot directly recyclize, the latter a phenomenon previously observed with cyclic acyl phosph(on)ates. While the two inhibitors bind in nearly identical conformations, the phosphoryl-enzyme complex formed from the monocyclic compound is significantly less mobile than that formed from the bicyclic compound. Despite this difference, the complex with the bicyclic compound breaks down to regenerate free enzyme somewhat more slowly than that of the monocyclic. This may be because of steric problems associated with the reorientation of the larger bicyclic ligand required for reactivation. The structures are strikingly different in the orientation of the phosphoryl moiety from those generated using more specific phosph(on)ates. Models of the noncovalent complexes of the monocyclic compound with the R61 DD-peptidase and a structurally very similar class C beta-lactamase suggest reasons why the former enzyme is phosphorylated by this compound, while the latter is acylated. Finally, this paper provides information that will help in the design of additional DD-peptidase inhibitors with the potential to serve as leads in the development of novel antibiotics.

A detailed kinetic study of Mox-1, a plasmid-encoded class C beta-lactamase

Surveys of beta-lactamases in different parts of the world show an important increase in class C beta-lactamases, thus the study of these enzymes is becoming an important issue. We created an overproduction system for Mox-1, a plasmid class C beta-lactamase, by cloning the gene encoding this enzyme, and placing it under the control of a T7 promoter, using vector pET 28a. The enzyme, purified by ion exchange chromatography, was used to obtain the molecular mass (38246), the N-terminal sequence (GEASPVDPLRPVV), and pI (8.9), and to perform a detailed kinetic study. Cephalotin was used as reporter substrate in the case of poor substrates. The kinetic study showed that benzylpenicillin, cephalotin, cefcapene and moxalactam were good substrates for Mox-1 (k(cat)/K(m) values >2.5 x 10(6) M(-1) s(-1)). On the other hand, ceftazidime and cefepime were poor substrates for this enzyme (K(m) values >200 microM). Clavulanic acid had no inhibitory effect on Mox-1 (K(m)=30.2 mM), however aztreonam behaved as an inhibitor of Mox-1 (K(i)=2.85 microM).

Potential transition state analogue inhibitors for the penicillin-binding proteins

Penicillin-binding proteins (PBPs) are ubiquitous bacterial enzymes involved in cell wall biosynthesis. The development of new PBP inhibitors is a potentially viable strategy for developing new antibacterial agents. Several potential transition state analogue inhibitors for the PBPs were synthesized, including peptide chloromethyl ketones, trifluoromethyl ketones, aldehydes, and boronic acids. These agents were characterized chemically, stereochemically, and as inhibitors of a set of low molecular mass PBPs: Escherichia coli (EC) PBP 5, Neisseria gonorrhoeae (NG) PBP 3, and NG PBP 4. A peptide boronic acid was the most effective PBP inhibitor in the series, with a preference observed for a d-boroAla-based over an l-boroAla-based inhibitor, as expected given that physiological PBP substrates are based on d-Ala at the cleavage site. The lowest K(I) of 370 nM was obtained for NG PBP 3 inhibition by Boc-l-Lys(Cbz)-d-boroAla (10b). Competitive inhibition was observed for this enzyme-inhibitor pair, as expected for an active site-directed inhibitor. For the three PBPs included in this study, an inverse correlation was observed between the values for log K(I) with 10b and the values for log(k(cat)/K(m)) for activity against the analogous substrate, and K(m)/K(I) ratios were 90, 1900, and 9600 for NG PBP 4, EC PBP 5, and NG PBP 3, respectively. These results demonstrate that peptide boronic acids can be effective transition state analogue inhibitors for the PBPs and provide a basis for the use of these agents as probes of PBP structure, function, and mechanism, as well as a possible basis for the development of new PBP-targeted antibacterial agents.