j Class C ␤-lactamases poorly hydrolyze cephamycins (e.g., cefoxitin, cefotetan, and moxalactam). In the past 2 decades, a new family of plasmid-based AmpC ␤-lactamases conferring resistance to cefoxitin, the FOX family, has grown to include nine unique members descended from the Aeromonas caviae chromosomal AmpC. To understand the basis for the unique cephamycinase activity in the FOX family, we determined the first X-ray crystal structures of FOX-4, apo enzyme and the acyl-enzyme with its namesake compound, cefoxitin, using the Y150F deacylation-deficient variant. Notably, recombinant expression of Nterminally tagged FOX-4 also yielded an inactive adenylylated enzyme form not previously observed in ␤-lactamases. The posttranslational modification (PTM), which occurs on the active site Ser64, would not seem to provide a selective advantage, yet might present an opportunity for the design of novel antibacterial drugs. Substantial ligand-induced changes in the enzyme are seen in the acyl-enzyme complex, particularly the R2 loop and helix H10 (P289 to N297), with movement of F293 by 10.3 Å. Taken together, this study provides the first picture of this highly proficient class C cephamycinase, uncovers a novel PTM, and suggests a possible cephamycin resistance mechanism involving repositioning of the substrate due to the presence of S153P, N289P, and N346I substitutions in the ligand binding pocket.

Diphosphomevalonate (Mev⅐pp) is the founding member of a new class of potential antibiotics targeting the Streptococcus pneumoniae mevalonate (Mev) pathway. We have synthesized a series of Mev⅐pp analogues designed to simultaneously block two steps in this pathway, through allosteric inhibition of mevalonate kinase (MK) and, for five of the analogues, by mechanismbased inactivation of diphosphomevalonate decarboxylase (DPM-DC). The analogue series expands the C 3 -methyl group of Mev⅐pp with hydrocarbons of varying size, shape, and chemical and physical properties. Previously, we established the feasibility of a prodrug strategy in which unphosphorylated Mev analogues could be enzymatically converted to the active Mev⅐pp forms by the endogenous MK and phosphomevalonate kinase. We now report the kinetic parameters for the turnover of non-, mono-, and diphosphorylated analogues as substrates and inhibitors of the three mevalonate pathway enzymes. The inhibition of MK by Mev⅐pp analogues revealed that the allosteric site is selective for compact, electron-rich C 3 -subsitutents. The lack of reactivity of analogues with DPM-DC provided evidence, counter to the existing model, for a decarboxylation transition state that is concerted rather than dissociative. The Mev pathway is composed of three structurally and functionally conserved enzymes that catalyze consecutive steps in a metabolic pathway. The current work reveals that these enzymes exhibit significant differences in specificity toward R-group substitution at C 3 and that these patterns are explained well by changes in the volume of the C 3 R-group-binding pockets of the enzymes.Streptococcus pneumoniae, the primary cause of bacterial meningitis and pneumonia, kills over 4000 people daily worldwide and disproportionately affects children and the elderly (1, 2). Antibiotic resistance is a major problem in fighting this organism, with multiple-drug resistance rates as high as 95% in some regions (3). Although vaccines targeting the 7 or 23 most prevalent strains have shown success in reducing disease incidence in developed countries (4), there is a continual need for new antibiotic strategies to combat unvaccinated strains, which are rapidly filling the biological niches left by the vaccine (5).The mevalonate (Mev) 3 pathway is essential for the survival of S. pneumoniae in lung and serum (6). The bacterium uses this pathway to convert Mev to isopentenyl diphosphate, the "building block" of the isoprenoids: a class of 25,000 unique molecules having a wide range of biological functions. The pathway consists of three GHMP family kinases: Mev kinase (MK), phosphomevalonate kinase (PMK) and diphosphomevalonate decarboxylase (DPM-DC) ( Fig. 1) (7). Mutations that knock out genes in this pathway kill the organism in vivo, suggesting that S. pneumoniae cannot obtain the necessary precursors or downstream products from the host (6). In principle, each of the three enzymes is an antibiotic target, because inhibition of any of them prevents the production of isopentenyl d...

Ceftazidime-avibactam is a "second-generation" β-lactam-β-lactamase inhibitor combination that is effective against expressing class A extended-spectrum β-lactamases, class A carbapenemases, and/or class C cephalosporinases. Knowledge of the interactions of avibactam, a diazabicyclooctane with different β-lactamases, is required to anticipate future resistance threats. FOX family β-lactamases possess unique hydrolytic properties with a broadened substrate profile to include cephamycins, partly as a result of an isoleucine at position 346, instead of the conserved asparagine found in most AmpCs. Interestingly, a single amino acid substitution at N346 in the AmpC is implicated in resistance to the aztreonam-avibactam combination. In order to understand how diverse active-site topologies affect avibactam inhibition, we tested a panel of clinical isolates producing using ceftazidime-avibactam, determined the biochemical parameters for inhibition using the FOX-4 variant, and probed the atomic structure of avibactam with FOX-4. Avibactam restored susceptibility to ceftazidime for most isolates producing ; two isolates, one expressing and the other producing , displayed an MIC of 16 μg/ml for the combination. FOX-4 possessed a/ value of 1,800 ± 100 M · s and an off rate () of 0.0013 ± 0.0003 s Mass spectrometry showed that the FOX-4-avibactam complex did not undergo chemical modification for 24 h. Analysis of the crystal structure of FOX-4 with avibactam at a 1.5-Å resolution revealed a unique characteristic of this AmpC β-lactamase. Unlike in the -derived cephalosporinase 1 (PDC-1)-avibactam crystal structure, interactions (e.g., hydrogen bonding) between avibactam and position I346 in FOX-4 are not evident. Furthermore, another residue is not observed to be close enough to compensate for the loss of these critical hydrogen-bonding interactions. This observation supports findings from the inhibition analysis of FOX-4; FOX-4 possessed the highest (dissociation constant) value (1,600 nM) for avibactam compared to other AmpCs (7 to 660 nM). Medicinal chemists must consider the properties of extended-spectrum AmpCs, such as the FOX β-lactamases, for the design of future diazabicyclooctanes.

Survival of the human pathogen Streptococcus pneumoniae requires a functional mevalonate pathway, which produces isopentenyl diphosphate, the essential building block of isoprenoids. Flux through this pathway appears to be regulated at the mevalonate kinase (MK) step, which is strongly feedback-inhibited by diphosphomevalonate (DPM), the penultimate compound in the pathway. The human mevalonate pathway is not regulated by DPM, making the bacterial pathway an attractive antibiotic target. Since DPM has poor drug characteristics, being highly charged, we propose to use unphosphorylated, cell-permeable prodrugs based on mevalonate that will be phosphorylated in turn by MK and phosphomevalonate kinase (PMK) to generate the active compound in situ. To test the limits of this approach, we synthesized a series of C 3 -substituted mevalonate analogues to probe the steric and electronic requirements of the MK and PMK active sites. MK and PMK accepted substrates with up to two additional carbons, showing a preference for small substitutents. This result establishes the feasibility of using a prodrug strategy for DPM-based antibiotics in S. pneumoniae and identified several analogues to be tested as inhibitors of MK. Among the substrates accepted by both enzymes were cyclopropyl, vinyl, and ethynyl mevalonate analogues that, when diphosphorylated, might be mechanism-based inactivators of the next enzyme in the pathway, diphosphomevalonate decarboxylase.