Oxazolidinones mechanism of action: inhibition of the first peptide bond formation.

Oxazolidinones are potent inhibitors of bacterial protein biosynthesis. Previous studies have demonstrated that this new class of antimicrobial agent blocks translation by inhibiting initiation complex formation, while post-initiation translation by polysomes and poly(U)-dependent translation is not a target for these compounds. We found that oxazolidinones inhibit translation of natural mRNA templates but have no significant effect on poly(A)-dependent translation. Here we show that various oxazolidinones inhibit ribosomal peptidyltransferase activity in the simple reaction of 70 S ribosomes using initiator-tRNA or N-protected CCA-Phe as a P-site substrate and puromycin as an A-site substrate. Steady-state kinetic analysis shows that oxazolidinones display a competitive inhibition pattern with respect to both the P-site and A-site substrates. This is consistent with a rapid equilibrium, ordered mechanism of the peptidyltransferase reaction, wherein binding of the A-site substrate can occur only after complex formation between peptidyltransferase and the P-site substrate. We propose that oxazolidinones inhibit bacterial protein biosynthesis by interfering with the binding of initiator fMet-tRNA(i)(Met) to the ribosomal peptidyltransferase P-site, which is vacant only prior to the formation of the first peptide bond.

Inhibition of IMP-1 metallo-beta-lactamase and sensitization of IMP-1-producing bacteria by thioester derivatives.

IMP-1 metallo-beta-lactamase is a transferable carbapenem-hydrolyzing enzyme found in some clinical isolates of Pseudomonas aeruginosa, Serratia marcescens and Klebsiella pneumoniae. Bacteria that express IMP-1 show significantly reduced sensitivity to carbapenems and other beta-lactam antibiotics. A series of thioester derivatives has been shown to competitively inhibit purified IMP-1. As substrates for IMP-1, the thioesters yielded thiol hydrolysis products which themselves were reversible competitive inhibitors. The thioesters also increased sensitivity to the carbapenem L-742,728 in an IMP-1-producing laboratory stain of Escherichia coli, but will need further modification to improve their activity in less permeable organisms such as Pseudomonas and Serratia. Nonetheless, the thioester IMP-1 inhibitors offer an encouraging start to overcoming metallo-beta-lactamase-mediated resistance in bacteria.

Synthesis and SAR of thioester and thiol inhibitors of IMP-1 metallo-beta-lactamase.

Potent thioester and thiol inhibitors of IMP-1 metallo-beta-lactamase have been synthesized employing a solid-phase Mitsunobu reaction as the key step.

N-Arylalkyl pseudopeptide inhibitors of farnesyltransferase.

Inhibitors of Ras protein farnesyltransferase are described which are reduced pseudopeptides related to the C-terminal tetrapeptide of the Ras protein that signals farnesylation. Reduction of the carbonyl groups linking the first three residues of the tetrapeptide leads to active inhibitors which are chemically unstable. Stability can be restored by alkylating the central amine of the tetrapeptide. Studies of the SAR of these alkylated pseudopeptides with concomitant modification of the side chain of the third residue led to 2(S)-(2(S)-¿[2(S)-(2(R)-amino-3-mercaptopropylamino)-3(S)- methylpentyl]naphthalen-1-ylmethylamino¿acetylamino)-4 -methylsulfany lbutyric acid (11), a subnanomolar inhibitor. The methyl ester (10) of this compound exhibited submicromolar activity in the processing assay and selectively inhibited anchorage-independent growth of Rat1 cells transformed by v-ras at 2.5-5 microM.

Antibiotic sensitization using biphenyl tetrazoles as potent inhibitors of Bacteroides fragilis metallo-beta-lactamase.

BACKGROUND: High level resistance to carbapenem antibiotics in gram negative bacteria such as Bacteroides fragilis is caused, in part, by expression of a wide-spectrum metallo-beta-lactamase that hydrolyzes the drug to an inactive form. Co-administration of metallo-beta-lactamase inhibitors to resistant bacteria is expected to restore the antibacterial activity of carbapenems. RESULTS: Biphenyl tetrazoles (BPTs) are a structural class of potent competitive inhibitors of metallo-beta-lactamase identified through screening and predicted using molecular modeling of the enzyme structure. The X-ray crystal structure of the enzyme bound to the BPT L-159,061 shows that the tetrazole moiety of the inhibitor interacts directly with one of the two zinc atoms in the active site, replacing a metal-bound water molecule. Inhibition of metallo-beta-lactamase by BPTs in vitro correlates well with antibiotic sensitization of resistant B. fragilis. CONCLUSIONS: BPT inhibitors can sensitize a resistant B. fragilis clinical isolate expressing metallo-beta-lactamase to the antibiotics imipenem or penicillin G but not to rifampicin.

Cloning and expression of Staphylococcus aureus and Treptococcus pyogenes murD genes encoding uridine diphosphate N-acetylmuramoyl-L-alanine:D-glutamate ligases.

Bacterial UDP-N-acetylmuramyl-L-alanine:D-glutamate ligase (MurD), a cytoplasmic peptidoglycan biosynthetic enzyme, catalyzes the ATP-dependent addition of D-glutamate to an alanyl residue of the UDP-N-acetylmuramyl-L-alanine precursor, generating the dipeptide. The murD gene was cloned from both Staphylococcus aureus and Streptococcus pyogenes. Sequence analysis of the S. aureus murD gene revealed an open reading frame of 449 amino acids. The deduced aa sequence of S. aureus MurD is highly homologous to MurD from Escherichia coli, Haemophilus influenzae, Bacillus subtilis and St. pyogenes. Recombinant MurD protein from both S. aureus and St. pyogenes was separately overproduced in E. coli and purified as His-tagged fusion. Both recombinant enzymes catalyzed the ATP-dependent addition of D-glutamate to the precursor sugar peptide.

Soluble penicillin-binding protein 2a: beta-lactam binding and inhibition by non-beta-lactams using a 96-well format.

High level methicillin resistance in Staphylococcus aureus is dependent upon the acquisition of the mecA gene encoding penicillin-binding protein 2a (PBP2a). PBP2a is a member of a family of peptidoglycan biosynthetic enzymes involved in assembly of the cell wall in bacteria and is poorly inactivated by beta-lactam antibiotics. We describe a 96-well-filter binding assay using recombinant, soluble PBP2a which allows for kinetic measurement of penicillin binding. The deacylation rate constant for the PBP2a-penicillin G covalent complex was found to be 5.7 +/- 1.0 x 10(-5) s-1 at 30 degrees C (half-life of approximately 200 min). For the PBP2a acylation reaction, the value of K(m) (penicillin G) = 0.5 +/- 0.1 mM and kcat = 1 x 10(-3) s-1, which yields a second-order rate constant (kcat/K(m)) for inactivation of 2.0 M-1 s-1. Using this assay, several non-beta-lactam inhibitors including Cibacron blue have been found which exhibit IC50 values between 10 and 30 microM. The binding affinities of several carbapenems and beta-lactams correlated well between the filter binding assay described in this report and an electrophoretic assay for PBP2a using membranes prepared form methicillin-resistant S. aureus.

Potent, non-thiol inhibitors of farnesyltransferase.

The structure-activity relationship of a series of non-thiol CaaX analogs, which are inhibitors of farnesyltransferase, is described. These inhibitors contain a substituted phenyl group at the N terminus, which may occupy a novel binding domain on the Ras protein.

Conditionally lethal Escherichia coli murein mutants contain point defects that map to regions conserved among murein and folyl poly-gamma-glutamate ligases: identification of a ligase superfamily.

Bacterial peptidoglycan biosynthesis includes four enzymatic reactions in which successive amino acid residues are ligated to uridine diphospho-N-acetylmuramic acid (UDP-MurNAc). By comparing the amino acid sequences of MurC, -D, -E, and -F proteins from various bacterial genera, four regions of homology were identified. A profile search of Swissprot for related sequences revealed that these regional similarities were present in the folyl-gamma-polyglutamate ligases. These sequence homologies appear to track with catalytic function: both enzyme families proceed through an ordered kinetic mechanism and form product via an acyl phosphate intermediate. Two highly conserved residues in region II were examined through site-directed mutagenesis of the murein D-alanyl-D-alanine-adding enzyme from Escherichia coli (murF; E158 and H188). All mutations were highly detrimental to activity with enzyme specific activity reductions of 200-4500-fold, validating the critical nature of these residues. DNA sequence analysis from three E. coli mutants harboring the murC3 (G344D), murE1 (G344K, A495S), and murF2 (A288T) mutations revealed the presence of point mutation(s) closely associated with the fourth of these aligned regions. The murF2 allele, expressed and purified as a glutathione S-transferase::MurF2 fusion, was 181-fold less catalytically active at 30 degrees C and was further reduced at the nonpermissive temperature (42 degrees C). Thus the murF2 temperature-sensitive phenotype arises from a point mutation within a highly conserved region within this protein family. These data argue that these proteins comprise a superfamily of three substrate amide ligases that share significant structural and catalytic homologies.

High-yield expression, purification, and characterization of active, soluble Bacteroides fragilis metallo-beta-lactamase, CcrA.

The gene from Bacteroides fragilis encoding a metallo-beta-lactamase, ccrA, was expressed in Escherichia coli BL21(DE3) containing the wild-type disulfide bond-catalyzing system dsb as an active, soluble enzyme in quantities exceeding 100 mg/liter using both rich and minimal media. Both the nonfusion and a glutathione S-transferase fusion enzyme lacking the periplasmic signal sequence were purified to homogeneity. Characteristics of the purified nonfusion enzyme are shown to be similar to those of the renatured enzyme previously reported. Thermal denaturation studies using circular dichroism and fluorescence spectroscopy show that CcrA undergoes a transition at approximately 50 degrees C which corresponds to the transition temperature of catalytic activity. The secondary structure of the protein and the catalytic apparatus are thus intimately linked.

Kinetic mechanism of the Escherichia coli UDPMurNAc-tripeptide D-alanyl-D-alanine-adding enzyme: use of a glutathione S-transferase fusion.

The D-alanyl-D-alanine-adding enzyme encoded by the murF gene catalyzes the ATP-dependent formation of UDP-N-acetylmuramyl-L-gamma-D-Glu-meso-diaminopimelyl-D-Ala-D-Ala (UDP-MurNAc-tripeptide). MurF has been cloned from Escherichia coli and expressed as a glutathione S-transferase (GST) fusion using the tac promoter-based pGEX-KT vector. From induced, broken cell preparations, highly active fusion was recovered and purified in one step by affinity chromatography. The purified fusion protein was strongly inhibited by substrate UDPMurNAc-tripeptide, a response unaltered by changes in assay pH or by cleavage from the fusion partner. However, this effect was suppressed by the addition of 0.5 M NaCl. Initial velocity and dead-end inhibitor studies with the fusion enzyme were most consistent with a sequential ordered kinetic mechanism for the forward reaction in which ATP binds to free enzyme, followed by tripeptide and D-Ala-D-Ala in sequence prior to product release. Reported homologies between the MurF protein and the three preceding steps of cytoplasmic murein biosynthesis, MurC, -D, and -E, [Ikeda et al. (1990) J. Gen. Appl. Microbiol. 36, 179-187], raise the prospect that all of these enzymes will be found to proceed via this mechanism.

Pseudodipeptide inhibitors of protein farnesyltransferase.

A series of pseudodipeptide amides are described that inhibit Ras protein farnesyltransferase (PFTase). These inhibitors are truncated versions of the C-terminal tetrapeptide (CAAX motif) of Ras that serves as the signal sequence for PFTase-catalyzed protein farnesylation. In contrast to CAAX peptidomimetics previously reported, these inhibitors do not have a C-terminal carboxyl moiety, yet they inhibit farnesylation in vitro at < 100 nM. Despite the absence of the X residue in the CAAX motif, which normally directs prenylation specificity, these pseudodipeptides are greater than 100-fold selective for PFTase over type 1 protein geranylgeranyltransferase.

src-homology 2 (SH2) domain ligation as an allosteric regulator: modulation of phosphoinositide-specific phospholipase C gamma 1 structure and activity.

Phosphoinositide-specific phospholipase C gamma 1 (PI-PLC gamma 1) catalyses the hydrolysis of PtdIns(4,5)P2 to generate the second messengers diacylglycerol and Ins(1,4,5)P3. PI-PLC gamma 1, an src-homology 2/3 (SH2/SH3)-domain-containing enzyme, is activated in response to growth-factor-induced tyrosine phosphorylation, and, in vivo, is translocated from the cytosol to the particulate cell fraction. Here we report the bacterial expression of rat brain PI-PLC gamma 1 under the control of the T7 promoter. Production of the active enzyme in amounts suitable for structure-function analysis depended on coupling the translation of PLC gamma 1 to the expression of the phage-phi 10 coat protein. Purification of the enzyme was facilitated by the presence of a three-amino-acid C-terminal antibody epitope tag (Glu-Glu-Phe) engineered into the cloned PLC gamma 1. Examination of the specific activity, pH-rate profile, [Ca2+]-dependence and substrate specificity of bacterially expressed PLC gamma indicated that it had kinetic properties similar to those of PLC gamma isolated from bovine brain. The substrate specificity was dependent on [Ca2+]: at low [Ca2+] (1-10 microM) PtdIns(4,5)P2 was a better substrate than PtdIns. Addition of phosphotyrosine-containing peptides (12-mers) with the cognate sequence of the high-affinity binding site for PLC gamma 1 on the activated epidermal-growth-factor (EGF) receptor (Tyr-992) increased enzyme activity (up to 85%) in vitro. Cognate non-phosphorylated peptides had no effect on activity. When c.d. spectroscopy was used to monitor the effect of added phosphotyrosine-containing peptide on the structure of recombinant PLC gamma 1, significant spectral shifts, indicative of a conformational change, were observed upon complexation with the EGF-receptor phosphotyrosine-containing 12-residue peptide (Tyr*-992). How SH2 domains from PLC gamma 1 can mediate structural rearrangements and modulate enzymic activity on their ligation by growth-factor receptors is discussed.

Synthesis and biological activity of ras farnesyl protein transferase inhibitors. Tetrapeptide analogs with amino methyl and carbon linkages.

Replacement of the central amino methylene linkage of C[psi CH2NH]A[psi CH2NH]AX tetrapeptide inhibitors with carbon tethers led to compounds with potency in the nanomolar range. Some of the more potent olefinic compounds inhibit Ras processing in intact v-ras transformed NIH 3T3 cells with IC50 values in the 0.1 to 1 microM range, and inhibit selectively the anchorage-independent growth of H-ras transformed Rat1 cells at 10 microM.

Pseudopeptide inhibitors of Ras farnesyl-protein transferase.

Inhibitors of Ras farnesyl-protein transferase are described. These are reduced pseudopeptides related to the C-terminal tetrapeptide of the Ras protein that signals farnesylation. Deletion of the carbonyl groups between the first two residues of the tetrapeptides either preserves or improves activity, depending on the peptide sequence. The most potent in vitro enzyme inhibitor described (IC50 = 5 nM) is Cys [psi CH2NH]Ile[psi CH2NH]Phe-Met (3). To obtain compounds able to suppress Ras farnesylation in cell culture, further structural modification to include a homoserine lactone prodrug was required. Compound 18 (Cys[psi CH2NH]Ile[psi CH2NH]Ile-homoserine lactone) reduced the extent of Ras farnesylation by 50% in NIH3T3 fibroblasts in culture at a concentration of 50 microM. Structure-activity studies also led to 12 (Cys[psi CH2NH]Val-Ile-Leu), a potent and selective inhibitor of a related enzyme, the type-I geranylgeranyl protein transferase.

Isoprenoid diphosphate utilization by recombinant human farnesyl:protein transferase: interactive binding between substrates and a preferred kinetic pathway.

The catalytic utilization of dimethylallyl, geranyl, farnesyl, and geranylgeranyl diphosphates in the reaction catalyzed by recombinant human farnesyl:protein transferase (hFPTase) has been examined in the presence of three different protein substrates, Ras-CVLS, Ras-CVIM, and Ras-CAIL. hFPTase catalyzed both farnesylation and geranylation of Ras-CVLS and of Ras-CVIM but not of Ras-CAIL. Geranylgeranylation was observed, but only when Ras-CVIM was the acceptor substrate. Steady-state initial velocity and dead-end inhibitor studies indicate that hFPTase-catalyzed geranylation, like bovine FPTase-catalyzed farnesylation, proceeds through a random order, sequential mechanism. Surprisingly, however, Michaelis constants for a given protein acceptor substrate varied depending upon which isoprenoid diphosphate was used as the donor substrate, showing that these substrates do not bind independently to the enzyme (under catalytic conditions). In addition, at very high concentrations of Ras-CVIM, substrate inhibition was observed in the presence of both FPP and GPP. Isotope partitioning studies showed that, at high concentrations of Ras-CVIM, more than 80% of the bound farnesyl diphosphate (FPP) can be trapped as product, suggesting that the binary complex is catalytically competent and that the ternary complex proceeds to product faster than it releases FPP. The release rate of FPP from the binary complex was calculated to be 0.05 s-1, which is only about eight times greater than kcat. Thus, the binding of FPP to the enzyme in the presence of the protein substrate is not an equilibrium situation.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective inhibition of ras-dependent transformation by a farnesyltransferase inhibitor.

To acquire transforming potential, the precursor of the Ras oncoprotein must undergo farnesylation of the cysteine residue located in a carboxyl-terminal tetrapeptide. Inhibitors of the enzyme that catalyzes this modification, farnesyl protein transferase (FPTase), have therefore been suggested as anticancer agents for tumors in which Ras contributes to transformation. The tetrapeptide analog L-731,735 is a potent and selective inhibitor of FPTase in vitro. A prodrug of this compound, L-731,734, inhibited Ras processing in cells transformed with v-ras. L-731,734 decreased the ability of v-ras-transformed cells to form colonies in soft agar but had no effect on the efficiency of colony formation of cells transformed by either the v-raf or v-mos oncogenes. The results demonstrate selective inhibition of ras-dependent cell transformation with a synthetic organic inhibitor of FPTase.

Selective inhibition of farnesyl-protein transferase blocks ras processing in vivo.

The ras oncogene product, Ras, is synthesized in vivo as a precursor protein that requires post-translational processing to become biologically active and to be capable of transforming mammalian cells. Farnesylation appears to be a critical modification of Ras, and thus inhibitors of the farnesyl-protein transferase (FPTase) that catalyzes this reaction may block ras-dependent tumorigenesis. Three structural classes of FPTase inhibitors were identified: (alpha-hydroxyfarnesyl)phosphonic acid, chaetomellic acids, and zaragozic acids. By comparison, these compounds were weaker inhibitors of geranylgeranyl-protein transferases. Each of these inhibitors was competitive with respect to farnesyl diphosphate in the FPTase reaction. All compounds were assayed for inhibition of Ras processing in Ha-ras-transformed NIH3T3 fibroblasts. Ras processing was inhibited by 1 microM (alpha-hydroxyfarnesyl)phosphonic acid. Neither chaetomellic acid nor zaragozic acid were active in this assay. These results are the first demonstration that a small organic chemical selected for inhibition of FPTase can inhibit Ras processing in vivo.