Antibacterial agents that inhibit lipid A biosynthesis

MsbA-dependent Translocation of Lipids across the Inner Membrane of Escherichia coli

MsbA is an essential ABC transporter in Escherichia coli required for exporting newly synthesized lipids from the inner to the outer membrane. It remains uncertain whether or not MsbA catalyzes trans-bilayer lipid movement (i.e. flip-flop) within the inner membrane. We now show that newly synthesized lipid A accumulates on the cytoplasmic side of the inner membrane after shifting an E. coli msbA missense mutant to the non-permissive temperature. This conclusion is based on the selective inhibition of periplasmic, but not cytoplasmic, covalent modifications of lipid A that occur in polymyxin-resistant strains of E. coli. The accessibility of newly synthesized phosphatidylethanolamine to membrane impermeable reagents, like 2,4,6-trinitrobenzene sulfonic acid, is also reduced severalfold. Our data showed that MsbA facilitates the rapid translocation of some lipids from the cytoplasmic to the periplasmic side of the inner membrane in living cells.

Seeing is believing: the impact of structural genomics on antimicrobial drug discovery

Over the past decade, the availability of complete microbial genome sequences has led to changes in the strategies that are used to search for novel anti-infectives. However, despite the identification of many new potential drug targets, novel antimicrobial agents have been slow to emerge from these efforts. In part, this reflects the long discovery and development times that are needed to bring new drugs to market and the bottlenecks at the stages of identifying good lead compounds and optimizing these leads into drug candidates. Structural genomics will hopefully provide opportunities to overcome these bottlenecks and populate the antimicrobial pipeline.

Enzymatic synthesis of lipid A molecules with four amide-linked acyl chains. LpxA acyltransferases selective for an analog of UDP-N-acetylglucosamine in which an amine replaces the 3"-hydroxyl group

LpxA of Escherichia coli catalyzes the acylation of the glucosamine 3-OH group of UDP-GlcNAc, using R-3-hydroxymyristoyl-acyl carrier protein (ACP) as the donor substrate. We now demonstrate that LpxA in cell extracts of Mesorhizobium loti and Leptospira interrogans, which synthesize lipid A molecules containing 2,3-diamino-2,3-dideoxy-d-glucopyranose (GlcN3N) units in place of glucosamine, do not acylate UDP-GlcNAc. Instead, these LpxA acyltransferases require a UDP-Glc-NAc derivative (designated UDP 2-acetamido-3-amino-2,3-dideoxy-alpha-d-glucopyranose or UDP-GlcNAc3N), characterized in the preceding paper, in which an amine replaces the glucosamine 3-OH group. L. interrogans LpxA furthermore displays absolute selectivity for 3-hydroxylauroyl-ACP as the donor, whereas M. loti LpxA functions almost equally well with 10-, 12-, and 14-carbon 3-hydroxyacyl-ACPs. The substrate selectivity of L. interrogans LpxA is consistent with the structure of L. interrogans lipid A. The mechanism of L. interrogans LpxA appears to be similar to that of E. coli LpxA, given that the essential His(125) residue of E. coli LpxA is conserved and is also required for acyltransferase activity in L. interrogans. Acidithiobacillus ferrooxidans (an organism that makes lipid A molecules containing both GlcN and GlcN3N) has an ortholog of LpxA that is selective for UDP-GlcNAc3N, but the enzyme also catalyzes the acylation of UDP-GlcNAc at a slow rate. E. coli LpxA acylates UDP-GlcNAc and UDP-GlcNAc3N at comparable rates in vitro. However, UDP-GlcNAc3N is not synthesized in vivo, because E. coli lacks gnnA and gnnB. When the latter are supplied together with A. ferrooxidans lpxA, E. coli incorporates a significant amount of GlcN3N into its lipid A.

A methylated phosphate group and four amide-linked acyl chains in leptospira interrogans lipid A. The membrane anchor of an unusual lipopolysaccharide that activates TLR2

Leptospira interrogans differs from other spirochetes in that it contains homologs of all the Escherichia coli lpx genes required for the biosynthesis of the lipid A anchor of lipopolysaccharide (LPS). LPS from L. interrogans cells is unusual in that it activates TLR2 rather than TLR4. The structure of L. interrogans lipid A has now been determined by a combination of matrix-assisted laser desorption ionization time-of-flight mass spectrometry, NMR spectroscopy, and biochemical studies. Lipid A was released from LPS of L. interrogans serovar Pomona by 100 degrees C hydrolysis at pH 4.5 in the presence of SDS. Following purification by anion exchange and thin layer chromatography, the major component was shown to have a molecular weight of 1727. Mild hydrolysis with dilute NaOH reduced this to 1338, consistent with the presence of four N-linked and two O-linked acyl chains. The lipid A molecules of both the virulent and nonvirulent forms of L. interrogans serovar Icterohaemorrhagiae (strain Verdun) were identical to those of L. interrogans Pomona by the above criteria. Given the selectivity of L. interrogans LpxA for 3-hydroxylaurate, we propose that L. interrogans lipid A is acylated with R-3-hydroxylaurate at positions 3 and 3' and with R-3-hydroxypalmitate at positions 2 and 2'. The hydroxyacyl chain composition was validated by gas chromatography and mass spectrometry of fatty acid methyl esters. Intact hexa-acylated lipid A of L. interrogans Pomona was also analyzed by NMR, confirming the presence a beta-1',6-linked disaccharide of 2,3-diamino-2,3-dideoxy-d-glucopyranose units. Two secondary unsaturated acyl chains are attached to the distal residue. The 1-position of the disaccharide is derivatized with an axial phosphate moiety, but the 4'-OH is unsubstituted. (1)H and (31)P NMR analyses revealed that the 1-phosphate group is methylated. Purified L. interrogans lipid A is inactive against human THP-1 cells but does stimulate tumor necrosis factor production by mouse RAW264.7 cells.

Comparative genomics of Rickettsia prowazekii Madrid E and Breinl strains

Rickettsia prowazekii, the causative agent of epidemic typhus, has been responsible for millions of human deaths. Madrid E is an attenuated strain of R. prowazekii, while Breinl is a virulent strain. The genomic DNA sequence of Madrid E has recently been published. To study the genomic variations between Madrid E (reference) and Breinl (test) DNAs, cohybridization experiments were performed on a DNA microarray containing all 834 protein-coding genes of Madrid E. Of the 834 genes assessed, 24 genes showed 1.5- to 2.0-fold increases in hybridization signals in Breinl DNA compared to Madrid E DNA, indicating the presence of genomic variations in approximately 3% of the total genes. Eighteen of these 24 genes are predicted to be involved in different functions. Southern blot analysis of five genes, virB4, ftsK, rfbE, lpxA, and rpoH, suggested the presence of an additional paralog(s) in Breinl, which might be related to the observed increase in hybridization signals. Studies by real-time reverse transcription-PCR revealed an increase in expression of the above-mentioned five genes and five other genes. In addition to the elevated hybridization signals of 24 genes observed in the Breinl strain, one gene (rp084) showed only 1/10 the hybridization signal of Madrid E. Further analysis of this gene by PCR and sequencing revealed a large deletion flanking the whole rp084 gene and part of the rp083 gene in the virulent Breinl strain. The results of this first rickettsial DNA microarray may provide some important information for the elucidation of pathogenic mechanisms of R. prowazekii.

Discovery of a small molecule that inhibits cell division by blocking FtsZ, a novel therapeutic target of antibiotics

The emergence of bacterial resistance to antibiotics is a major health problem and, therefore, it is critical to develop new antibiotics with novel modes of action. FtsZ, a tubulin-like GTPase, plays an essential role in bacterial cell division, and its homologs are present in almost all eubacteria and archaea. During cell division, FtsZ forms polymers in the presence of GTP that recruit other division proteins to make the cell division apparatus. Therefore, inhibition of FtsZ polymerization will prevent cells from dividing, leading to cell death. Using a fluorescent FtsZ polymerization assay, the screening of >100,000 extracts of microbial fermentation broths and plants followed by fractionation led to the identification of viriditoxin, which blocked FtsZ polymerization with an IC50 of 8.2 microg/ml and concomitant GTPase inhibition with an IC50 of 7.0 microg/ml. That the mode of antibacterial action of viriditoxin is via inhibition of FtsZ was confirmed by the observation of its effects on cell morphology, macromolecular synthesis, DNA-damage response, and increased minimum inhibitory concentration as a result of an increase in the expression of the FtsZ protein. Viriditoxin exhibited broad-spectrum antibacterial activity against clinically relevant Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococci, without affecting the viability of eukaryotic cells.

Hydroxamic acids: proton donor and acceptor strength for use in drug design

Hydroxamic acids, the naturally occurring and synthetic products, generally have low toxicities and are of interest for many therapeutic applications. The present investigation describes the measurement of hydrogen bond donor (HBD) strength of ten hydroxamic acids by measuring their log P(O/W) values. Hydroxamic acid functional group contains two oxygen and one nitrogen atom as the acceptor sites. Thus, HBA strength of these reagents is also computed. A knowledge of these parameters is valuable in the field of toxicology, pharmacology and environmental sciences.

Expression cloning and biochemical characterization of a Rhizobium leguminosarum lipid A 1-phosphatase

Lipid A of Rhizobium leguminosarum, a nitrogen-fixing plant endosymbiont, displays several significant structural differences when compared with Escherichia coli. An especially striking feature of R. leguminosarum lipid A is that it lacks both the 1- and 4'-phosphate groups. Distinct lipid A phosphatases that attack either the 1 or the 4' positions have previously been identified in extracts of R. leguminosarum and Rhizobium etli but not Sinorhizobium meliloti or E. coli. Here we describe the identification of a hybrid cosmid (pMJK-1) containing a 25-kb R. leguminosarum 3841 DNA insert that directs the overexpression of the lipid A 1-phosphatase. Transfer of pMJK-1 into S. meliloti 1021 results in heterologous expression of 1-phosphatase activity, which is normally absent in extracts of strain 1021, and confers resistance to polymyxin. Sequencing of a 7-kb DNA fragment derived from the insert of pMJK-1 revealed the presence of a lipid phosphatase ortholog (designated LpxE). Expression of lpxE in E. coli behind the T7lac promoter results in the appearance of robust 1-phosphatase activity, which is normally absent in E. coli membranes. Matrix-assisted laser-desorption/time of flight and radiochemical analysis of the product generated in vitro from the model substrate lipid IVA confirms the selective removal of the 1-phosphate group. These findings show that lpxE is the structural gene for the 1-phosphatase. The availability of lpxE may facilitate the re-engineering of lipid A structures in diverse Gram-negative bacteria and allow assessment of the role of the 1-phosphatase in R. leguminosarum symbiosis with plants. Possible orthologs of LpxE are present in some intracellular human pathogens, including Francisella tularensis, Brucella melitensis, and Legionella pneumophila.

Protease inhibitors: Synthesis of L-alanine hydroxamate sulfonylated derivatives as inhibitors of clostridium histolyticum collagenase

L-alanine hydroxamate derivatives were obtained by reaction of alkyl/arylsulfonyl halides with L-alanine, followed by treatment with benzyl chloride, and conversion of the COOH moiety to the CONHOH group with hydroxylamine in the presence of carbodiimides. Other derivatives were obtained by reaction of N-benzyl-alanine with aryl isocyanates, arylsulfonyl isocyanates or benzoyl isothiocyanate, followed by a similar conversion of the COOH to the CONHOH moiety. The obtained compounds were assayed as inhibitors of Clostridium histolyticum collagenase, ChC (EC 3.4.24.3), a zinc enzyme which degrades triple helical collagen. The hydroxamate derivatives were generally 100-500 times more active than the corresponding carboxylates. In the series of synthesized derivatives, substitution patterns leading to the most potent ChC inhibitors were those involving perfluoroalkylsulfonyl- and substituted-arylsulfonyl moieties, such as pentafluorophenylsulfonyl, 3- and 4-protected-aminophenylsulfonyl-, 3- and 4-carboxy-phenylsulfonyl-, 3-trifluoromethyl-phenylsulfonyl-, or 1- and 2-naphthylsulfonyl among others. Similarly to the matrix metalloproteinase (MMP) hydroxamate inhibitors, ChC inhibitors of the type reported here must incorporate hydrophobic moieties at the P(2') and P(3') sites, in order to achieve tight binding to the enzyme.

Structure of the LpxC deacetylase with a bound substrate-analog inhibitor

The zinc-dependent UDP-3-O-acyl-N-acetylglucosamine deacetylase (LpxC) catalyzes the first committed step in the biosynthesis of lipid A, the hydrophobic anchor of lipopolysaccharide (LPS) that constitutes the outermost monolayer of Gram-negative bacteria. As LpxC is crucial for the survival of Gram-negative organisms and has no sequence homology to known mammalian deacetylases or amidases, it is an excellent target for the design of new antibiotics. The solution structure of LpxC from Aquifex aeolicus in complex with a substrate-analog inhibitor, TU-514, reveals a novel alpha/beta fold, a unique zinc-binding motif and a hydrophobic passage that captures the acyl chain of the inhibitor. On the basis of biochemical and structural studies, we propose a catalytic mechanism for LpxC, suggest a model for substrate binding and provide evidence that mobility and dynamics in structural motifs close to the active site have key roles in the capture of the substrate.

Crystal structure of LpxC, a zinc-dependent deacetylase essential for endotoxin biosynthesis

The outer leaflet of the outer membrane of the Gram-negative bacterium serves as a permeability barrier and is composed of lipopolysaccharide, also known as endotoxin. The membrane anchor of lipopolysaccharide is lipid A, the biosynthesis of which is essential for cell viability. The first committed step in lipid A biosynthesis is catalyzed by UDP-(3-O-(R-3-hydroxymyristoyl))-N-acetylglucosamine deacetylase (LpxC), a zinc-dependent deacetylase. Here we report the crystal structure of LpxC from Aquifex aeolicus, which reveals a new alpha+beta fold reflecting primordial gene duplication and fusion, as well as a new zinc-binding motif. The catalytic zinc ion resides at the base of an active-site cleft and adjacent to a hydrophobic tunnel occupied by a fatty acid. This tunnel accounts for the specificity of LpxC toward substrates and inhibitors bearing appropriately positioned 3-O-fatty acid substituents. Notably, simple inhibitors designed to target interactions in the hydrophobic tunnel bind with micromolar affinity, thereby representing a step toward the structure-based design of a potent, broad-spectrum antibacterial drug.

Novel approaches to the treatment of pneumonia

As the prevalence of resistance to multiple antibiotics increases it is progressively more difficult to treat pneumonia in hospitalized patients. Therefore, anti-infectious agents that have new modes of action are needed urgently. Recent advances in DNA sequencing technology make it possible to elucidate the sequences of the entire genomes of pathogenic bacteria. This allows many novel, non-traditional targets for therapeutic intervention to be identified, such as those involved in disease pathogenesis, and in adaptation and growth at sites of infection. In the past few years, inhibitors of new bacterial targets have been developed, including inhibitors of genes that are required for either virulence or pathogenesis. The challenge is to optimize and develop these agents to provide novel approaches to the treatment of pneumonia in hospitalized patients.

The Omp85 protein of Neisseria meningitidis is required for lipid export to the outer membrane

In Gram-negative bacteria, lipopolysaccharide and phospholipid biosynthesis takes place at the inner membrane. How the completed lipid molecules are subsequently transported to the outer membrane remains unknown. Omp85 of Neisseria meningitidis is representative for a family of outer membrane proteins conserved among Gram-negative bacteria. We first demonstrated that the omp85 gene is co-transcribed with genes involved in lipid biosynthesis, suggesting an involvement in lipid assembly. A meningococcal strain was constructed in which Omp85 expression could be switched on or off through a tac promoter-controlled omp85 gene. We demonstrated that the presence of Omp85 is essential for viability. Depletion of Omp85 leads to accumulation of electron-dense amorphous material and vesicular structures in the periplasm. We demonstrated, by fractionation of inner and outer membranes, that lipopolysaccharide and phospholipids mostly disappeared from the outer membrane and instead accumulated in the inner membrane, upon depletion of Omp85. Omp85 depletion did not affect localization of integral outer membrane proteins PorA and Opa. These results provide compelling evidence for a role for Omp85 in lipid transport to the outer membrane.

Synthesis of a carbohydrate-derived hydroxamic acid inhibitor of the bacterial enzyme (LpxC) involved in lipid A biosynthesis

[reaction: see text] The enzyme LpxC (UDP-3-O-[(R)-3-hydroxymyristoyl]-GlcNAc deacetylase) catalyzes the second step of lipid A biosynthesis and is essential for bacterial growth. A GlcNAc-derived hydroxamic acid inhibitor 8 of this enzyme was synthesized using two different routes. Compound 8 exhibits activity toward LpxC enzymes from a wider spectrum of bacterial species than any of the previously reported hydroxamic acid inhibitors.

High-throughput catch-and-release synthesis of oxazoline hydroxamates. Structure-activity relationships in novel inhibitors of Escherichia coli LpxC: in vitro enzyme inhibition and antibacterial properties

LpxC is a zinc amidase that catalyses the second step of lipid A biosynthesis in Gram-negative bacteria. Oxazolines incorporating a hydroxamic acid, which is believed to coordinate to the single essential zinc ion, at the 4-position are known inhibitors of this enzyme. Some of these enzyme inhibitors exhibit antibacterial activity through their inhibition of LpxC. We recently developed a method for the synthesis of oxazolines using resin capture and ring-forming release that eliminates traditional purification steps and can be used in high-throughput synthesis. Using our method, oxazoline hydroxamates with diverse 2-substituents were prepared in library form as candidate inhibitors for LpxC. Two conventional methods for oxazoline synthesis were also applied to generate more than 70 compounds. The groups at the 2-position included a wide variety of substituted aromatic rings and a limited selection of alkyl groups. These compounds were screened against wild-type and LpxC inhibitor-sensitive strains of Escherichia coli, as well as wild-type Pseudomonas aeruginosa. Inhibition of the E. coli LpxC enzyme was also investigated. A broad correlation between enzyme inhibitory and antibacterial activity was observed, and novel compounds were discovered that exhibit antibacterial activity but fall outside earlier-known structural classes.

Endotoxin as a drug target

OBJECTIVE

To review the preclinical and clinical evidence that antiendotoxin therapeutic strategies are potentially useful in the prevention and treatment of septic shock.

STUDY DESIGN

A critical review of the literature over the past 30 yrs relating basic and clinical research on the therapeutic value of endotoxin as a target for the prevention and treatment of severe sepsis and septic shock.

MAIN RESULTS

Bacterial endotoxin is a potent and predominant microbial mediator that induces an intense inflammatory and procoagulant response by elements of the innate immune response. This macromolecule is capable of inducing lethal septic shock in experimental animals, and a large number of preclinical studies consistently demonstrate the survival advantage of endotoxin inhibition in experimental models of sepsis. Clinical studies indicate that endotoxin may be found in the systemic circulation in the majority of humans with septic shock. Endotoxemia is largely independent of the nature of the infecting microorganism despite the fact that this molecule is specifically found in the outer membrane of Gram-negative bacteria only. Antiendotoxin strategies studied thus far have not provided reproducible survival benefits in clinical trials in septic patients.

CONCLUSIONS

Despite compelling evidence of the critical importance of endotoxin in the pathogenesis of Gram-negative bacterial sepsis in preclinical investigations and numerous clinical interventional trials, the utility of antiendotoxin approaches to significantly reduce the mortality rate in human septic shock remains unproven. Ongoing clinical trials with specific endotoxin inhibitors should determine the potential value of this therapeutic approach to the management of septic shock.

Exploring relationships between mimic configuration, peptide conformation and biological activity in indolizidin-2-one amino acid analogs of gramicidin S

Indolizidin-2-one amino acids (I2aas, 6S- and 6R-1) possessing 6S- and 6R-ring-fusion stereochemistry were introduced into the antimicrobial peptide gramicidin S (GS) to explore the relationships between configuration, peptide conformation and biological activity. Solution-phase and solid-phase techniques were used to synthesize three analogs with I2aa residues in place of the d-Phe-Pro residues at the turn regions of GS: [(6S)-I2aa4-5,4'-5']GS (2), [Lys2,2',(6S)-I2aa4-5,4'-5']GS (3) and [(6R)-I2aa4-5,4'-5']GS (4). Although conformational analysis of [I2aa4-5,4'-5']GS analogs 2-4 indicated that both ring-fusion stereoisomers of I2aa gave peptides with CD and NMR spectral data characteristic of GS, the (6S)-I2aa analogs 2 and 3 exhibited more intense CD curve shapes, as well as greater numbers of nonsequential NOE between opposing Val and Leu residues, relative to the (6R)-I2aa analog 4, suggesting a greater propensity for the (6S)-diastereomer to adopt the beta-turn/antiparallel beta-pleated sheet conformation. In measurements of antibacterial and antifungal activity, the (6S)-I2aa analog 2 exhibited significantly better potency than the (6R)-I2aa diastereomer 4. Relative to GS, [(6S)-I2aa4-5,4'-5']GS (2) exhibited usually 1/2 to 1/4 antimicrobial activity as well as 1/4 hemolytic activity. In certain cases, antimicrobial and hemolytic activities of GS were shown to be dissociated through modification at the peptide turn regions with the (6S)-I2aa diastereomer. The synthesis and evaluation of GS analogs 2-4 has furnished new insight into the importance of ring-fusion stereochemistry for turn mimicry by indolizidin-2-one amino acids as well as novel antimicrobial peptides.

ATPase activity of the MsbA lipid flippase of Escherichia coli

Escherichia coli MsbA, the proposed inner membrane lipid flippase, is an essential ATP-binding cassette transporter protein with homology to mammalian multidrug resistance proteins. Depletion or loss of function of MsbA results in the accumulation of lipopolysaccharide and phospholipids in the inner membrane of E. coli. MsbA modified with an N-terminal hexahistidine tag was overexpressed, solubilized with a nonionic detergent, and purified by nickel affinity chromatography to approximately 95% purity. The ATPase activity of the purified protein was stimulated by phospholipids. When reconstituted into liposomes prepared from E. coli phospholipids, MsbA displayed an apparent K(m) of 878 microm and a V(max) of 37 nmol/min/mg for ATP hydrolysis in the presence of 10 mm Mg(2+). Preincubation of MsbA-containing liposomes with 3-deoxy-d-mannooctulosonic acid (Kdo)(2)-lipid A increased the ATPase activity 4-5-fold, with half-maximal stimulation seen at 21 microm Kdo(2)-lipid A. Addition of Kdo(2)-lipid A increased the V(max) to 154 nmol/min/mg and decreased the K(m) to 379 microm. Stimulation was only seen with hexaacylated lipid A species and not with precursors, such as diacylated lipid X or tetraacylated lipid IV(A). MsbA containing the A270T substitution, which renders cells temperature-sensitive for growth and lipid export, displayed ATPase activity similar to that of the wild type protein at 30 degrees C but was significantly reduced at 42 degrees C. These results provide the first in vitro evidence that MsbA is a lipid-activated ATPase and that hexaacylated lipid A is an especially potent activator.

Inhibition of the antibacterial target UDP-(3-O-acyl)-N-acetylglucosamine deacetylase (LpxC): isoxazoline zinc amidase inhibitors bearing diverse metal binding groups

UDP-3-O-[R-3-hydroxymyristoyl]-GlcNAc deacetylase (LpxC) is a zinc amidase that catalyzes the second step of lipid A biosynthesis in Gram negative bacteria. Known inhibitors of this enzyme are oxazolines incorporating a hydroxamic acid at the 4-position, which is believed to coordinate to the single essential zinc ion. A new structural class of inhibitors was designed to incorporate a more stable and more synthetically versatile isoxazoline core. The synthetic versatility of the isoxazoline allowed for a broad study of metal binding groups. Nine of 17 isoxazolines, each incorporating a different potential metal binding functional group, were found to exhibit enzyme inhibitory activity, including one that is more active than the corresponding hydroxamic acid. Additionally, a designed affinity label inhibits LpxC in a time-dependent manner.

The Escherichia coli gene encoding the UDP-2,3-diacylglucosamine pyrophosphatase of lipid A biosynthesis

UDP-2,3-diacylglucosamine hydrolase is believed to catalyze the fourth step of lipid A biosynthesis in Escherichia coli. This reaction involves pyrophosphate bond hydrolysis of the precursor UDP-2,3-diacylglucosamine to yield 2,3-diacylglucosamine 1-phosphate and UMP. To identify the gene encoding this hydrolase, E. coli lysates generated with individual lambda clones of the ordered Kohara library were assayed for overexpression of the enzyme. The sequence of lambda clone 157[6E7], promoting overproduction of hydrolase activity, was examined for genes encoding hypothetical proteins of unknown function. The amino acid sequence of one such open reading frame, ybbF, is 50.5% identical to a Haemophilus influenzae hypothetical protein and is also conserved in most other Gram-negative organisms, but is absent in Gram-positives. Cell extracts prepared from cells overexpressing ybbF behind the T7lac promoter have approximately 540 times more hydrolase activity than cells with vector alone. YbbF was purified to approximately 60% homogeneity, and its catalytic properties were examined. Enzymatic activity is maximal at pH 8 and is inhibited by 0.01% (or more) Triton X-100. The apparent K(m) for UDP-2,3-diacylglucosamine is 62 microm. YbbF requires a diacylated substrate and does not cleave CDP-diacylglycerol. (31)P NMR studies of the UMP product generated from UDP-2,3-diacylglucosamine in the presence of 40% H(2)180 show that the enzyme attacks the alpha-phosphate group of the UDP moiety. Because ybbF encodes the specific UDP-2,3-diacylglucosamine hydrolase involved in lipid A biosynthesis, it is now designated lpxH.

Potent, novel in vitro inhibitors of the Pseudomonas aeruginosa deacetylase LpxC

Deacetylation of uridyldiphospho-3-O-(R-hydroxydecanoyl)-N-acetylglucosamine by LpxC is the first committed step in the Pseudomonas aeruginosa biosynthetic pathway to lipid A; homologous enzymes are found widely among Gram-negative bacteria. As an essential enzyme for which no inhibitors have yet been reported, the P. aeruginosa LpxC represents a highly attractive target for a novel antibacterial drug. We synthesized several focused small-molecule libraries, each composed of a variable aromatic ring, one of four heterocyclic/spacer moieties, and a hydroxamic acid and evaluated the LpxC inhibition of these compounds against purified P. aeruginosa enzyme. To ensure that the in vitro assay would be as physiologically relevant as possible, we synthesized a tritiated form of the specific P. aeruginosa glycolipid substrate and measured directly the enzymatically released acetate. Several of our novel compounds, predominantly those having fluorinated substituents on the aromatic ring and an oxazoline as the heterocyclic moiety, demonstrated in vitro IC(50) values less than 1 microM. We now report the synthesis and in vitro evaluation of these P. aeruginosa LpxC inhibitors.

Antibacterial activities and characterization of novel inhibitors of LpxC

Lipid A is the hydrophobic anchor of lipopolysaccharide (LPS) and forms the major lipid component of the outer monolayer of the outer membrane of gram-negative bacteria. Lipid A is required for bacterial growth and virulence, and inhibition of its biosynthesis is lethal to bacteria. UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC) is a metalloenzyme that catalyzes the second step in the biosynthesis of lipid A. Inhibitors of LpxC have previously been shown to have antibiotic activities. We have screened a metalloenzyme inhibitor library for antibacterial activities against an Escherichia coli strain with reduced LpxC activity. From this screen, a series of sulfonamide derivatives of the alpha-(R)-amino hydroxamic acids, exemplified by BB-78484 and BB-78485, have been identified as having potent inhibitory activities against LpxC in an in vitro assay. Leads from this series showed gram-negative selective activities against members of the Enterobacteriaceae, Serratia marcescens, Morganella morganii, Haemophilus influenzae, Moraxella catarrhalis, and Burkholderia cepacia. BB-78484 was bactericidal against E. coli, achieving 3-log killing in 4 h at a concentration 4 times above the MIC, as would be predicted for an inhibitor of lipid A biosynthesis. E. coli mutants with decreased susceptibility to BB-78484 were selected. Analysis of these mutants revealed that resistance arose as a consequence of mutations in the fabZ or lpxC genes. These data confirm the antibacterial target of BB-78484 and BB-78485 and validate LpxC as a target for gram-negative selective antibacterials.

Relaxed acyl chain specificity of Bordetella UDP-N-acetylglucosamine acyltransferases

Lipid A (endotoxin) is a major structural component of Gram-negative outer membranes. It also serves as the hydrophobic anchor of lipopolysaccharide and is a potent activator of the innate immune response. Lipid A molecules from the genus Bordetella are reported to exhibit unusual structural asymmetry with respect to the acyl chains at the 3- and 3'-positions. These acyl chains are attached by UDP-N-acetylglucosamine acyltransferase (LpxA). To determine the origin of the acyl variability, the single lpxA ortholog present in each of the genomes of Bordetella bronchiseptica (lpxA(Br)), Bordetella parapertussis (lpxA(Pa)), and Bordetella pertussis (lpxA(Pe)) was cloned and expressed in Escherichia coli. In contrast to all LpxA proteins studied to date, LpxA(Br) and LpxA(Pe) display relaxed acyl chain length specificity in vitro, utilizing C(10)OH-ACP, C(12)OH-ACP, and C(14)OH-ACP at similar rates. Furthermore, hybrid lipid A molecules synthesized at 42 degrees C by an E. coli lpxA mutant complemented with lpxA(Pe) contain C(10)OH, C(12)OH, and C(14)OH at both the 3- and 3'-positions, as determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. In contrast, LpxA from B. parapertussis did not display relaxed specificity but was selective for C(10)OH-ACP. This study provides an enzymatic explanation for some of the unusual acyl chain variations found in Bordetella lipid A.

Helicobacter pylori 3-deoxy-D-manno-octulosonate-8-phosphate (KDO-8-P) synthase is a zinc-metalloenzyme

3-Deoxy-D-manno-2-octulosonate-8-phosphate (KDO-8-P) synthase catalyzes the aldol-type condensation of phosphoenolpyruvate and D-arabinose-5-phosphate (A-5-P) to produce KDO-8-P and inorganic phosphate. All KDO-8-P synthases, as exemplified by the enzyme from Escherichia coli, were believed not to require a metal cofactor for catalytic activity. However, recent studies have demonstrated that the KDO-8-P synthase from Aquifex aeolicus is a metalloenzyme. Moreover, sequence alignments and phylogenetic analysis of KDO-8-P synthase protein sequences strongly suggested that there is a whole subfamily of KDO-8-P synthases that are also metalloenzymes. One of these putative metalloenzymes is the ortholog from the human pathogen Helicobacter pylori. In order to test this model, we have cloned the kdsa gene encoding H. pylori KDO-8-P synthase, and overexpressed and purified the protein. This enzyme was found to bind one mol Zn/mol monomer, and the removal of this metal by treatment with 2,6-pyridine dicarboxylic acid abolished enzymatic activity. The Zn(2+) in the enzyme could be quantitatively replaced by Cd(2+), which increased the observed k(cat) by approximately 2-fold, and decreased the apparent K(m)(A-5-P) by approximately 6.5-fold. Furthermore, removal of the Zn(2+) from the enzyme did not greatly perturb its circular dichroism spectra. Thus, the divalent metal most likely serves as cofactor directly involved in catalysis.

Combinatorial chemistry in anti-infectives research

The ever-increasing resistance to current anti-infective drugs has become a major concern to the medical community. As a result, research efforts have been stepped up with the ultimate goal to provide new, more effective and safer antimicrobial treatments that will overcome the resistance problem. In this context, advances in molecular biology, automation and combinatorial chemistry will play a crucial role in the timely introduction of these products onto the market. How the application of combinatorial techniques can impact anti-infectives research will be reviewed using illustrative examples.

Lipopolysaccharide endotoxins

Bacterial lipopolysaccharides (LPS) typically consist of a hydrophobic domain known as lipid A (or endotoxin), a nonrepeating "core" oligosaccharide, and a distal polysaccharide (or O-antigen). Recent genomic data have facilitated study of LPS assembly in diverse Gram-negative bacteria, many of which are human or plant pathogens, and have established the importance of lateral gene transfer in generating structural diversity of O-antigens. Many enzymes of lipid A biosynthesis like LpxC have been validated as targets for development of new antibiotics. Key genes for lipid A biosynthesis have unexpectedly also been found in higher plants, indicating that eukaryotic lipid A-like molecules may exist. Most significant has been the identification of the plasma membrane protein TLR4 as the lipid A signaling receptor of animal cells. TLR4 belongs to a family of innate immunity receptors that possess a large extracellular domain of leucine-rich repeats, a single trans-membrane segment, and a smaller cytoplasmic signaling region that engages the adaptor protein MyD88. The expanding knowledge of TLR4 specificity and its downstream signaling pathways should provide new opportunities for blocking inflammation associated with infection.

Lipid biosynthesis as a target for antibacterial agents

Fatty acid biosynthesis, the first stage in membrane lipid biogenesis, is catalyzed in most bacteria by a series of small, soluble proteins that are each encoded by a discrete gene (Fig. 1; Table 1). This arrangement is termed the type II fatty acid synthase (FAS) system and contrasts sharply with the type I FAS of eukaryotes which is a dimer of a single large, multifunctional polypeptide. Thus, the bacterial pathway offers several unique sites for selective inhibition by chemotherapeutic agents. The site of action of isoniazid, used in the treatment of tuberculosis for 50 years, and the consumer antimicrobial agent triclosan were revealed recently to be the enoyl-ACP reductase of the type II FAS. The fungal metabolites, cerulenin and thiolactomycin, target the condensing enzymes of the bacterial pathway while the dehydratase/isomerase is inhibited by a synthetic acetylenic substrate analogue. Transfer of fatty acids to the membrane has also been inhibited via interference with the first acyltransferase step, while a new class of drugs targets lipid A synthesis. This review will summarize the data generated on these inhibitors to date, and examine where additional efforts will be required to develop new chemotherapeutics to help combat microbial infections.

A Chlamydia trachomatis UDP-N-acetylglucosamine acyltransferase selective for myristoyl-acyl carrier protein. Expression in Escherichia coli and formation of hybrid lipid A species

Chlamydia trachomatis lipid A is unusual in that it is acylated with myristoyl chains at the glucosamine 3 and 3' positions. We have cloned and expressed the gene encoding UDP-N-acetylglucosamine 3-O-acyltransferase of C. trachomatis (CtlpxA), the first enzyme of lipid A biosynthesis. C. trachomatis LpxA displays approximately 20-fold selectivity for myristoyl-ACP over R/S-3-hydroxymyristoyl-ACP under standard assay conditions, consistent with the proposed structure of C. trachomatis lipid A. CtLpxA is the first reported UDP-N-acetylglucosamine acyltransferase that prefers a non-hydroxylated acyl-ACP to a hydroxyacyl-ACP. When CtlpxA was expressed in RO138, a temperature-sensitive lpxA mutant of Escherichia coli, five new hybrid lipid A species were made in vivo after 2 h at 42 degrees C, in place of Escherichia coli lipid A. These compounds were purified and analyzed by matrix-assisted laser desorption ionization/time of flight mass spectrometry. In each case, a myristoyl chain replaced one or both of the ester linked 3-hydroxymyristoyl residues of E. coli lipid A. With prolonged growth at 42 degrees C, all the ester-linked 3-hydroxymyristoyl residues were replaced with myristate chains. Re-engineering the structure of E. coli lipid A should facilitate the microbiological production of novel agonists or antagonists of the innate immunity receptor TLR-4, with possible uses as adjuvants or anti-inflammatory agents.

An Escherichia coli mutant defective in lipid export

Escherichia coli phospholipids and lipopolysaccharide, made on the inner surface of the inner membrane, are rapidly transported to the outer membrane by mechanisms that are not well characterized. We now report a temperature-sensitive mutant (WD2) with an A270T substitution in a trans-membrane region of the ABC transporter MsbA. As shown by (32)P(i) and (14)C-acetate labeling, export of all major lipids to the outer membrane is inhibited by approximately 90% in WD2 after 30 min at 44 degrees C. Transport of newly synthesized proteins is not impaired. Electron microscopy shows reduplicated inner membranes in WD2 at 44 degrees C, consistent with a key role for MsbA in lipid trafficking.

The cell wall and cell division gene cluster in the Mra operon of Pseudomonas aeruginosa: cloning, production, and purification of active enzymes

We have cloned the Pseudomonas aeruginosa cell wall biosynthesis and cell division gene cluster that corresponds to the mra operon in the 2-min region of the Escherichia coli chromosome. The organization of the two chromosomal regions in P. aeruginosa and E. coli is remarkably similar with the following gene order: pbp3/pbpB, murE, murF, mraY, murD, ftsW, murG, murC, ddlB, ftsQ, ftsA, ftsZ, and envA/LpxC. All of the above P. aeruginosa genes are transcribed from the same strand of DNA with very small, if any, intragenic regions, indicating that these genes may constitute a single operon. All five amino acid ligases, MurC, MurD, MurE, MurF, and DdlB, in addition to MurG and MraY were cloned in expression vectors. The four recombinant P. aeruginosa Mur ligases, MurC, MurD, MurE, and MurF were overproduced in E. coli and purified as active enzymes.

A fluorescence-based homogeneous assay for measuring activity of UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase

UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC) is one of the key enzymes of bacterial lipid A biosynthesis, catalyzing the removal of the N-acetyl group of UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine. The lpxC gene is essential in Gram-negative bacteria but absent from mammalian genomes, making it an attractive target for antibacterial drug discovery. Current assay methods for LpxC are not suitable for high throughput screening, since they require multiple product separation steps and the use of radioactively labeled material that is difficult to prepare. A homogeneous fluorescence-based assay was developed that uses UDP-3-O-(N-hexyl-propionamide)-N-acetylglucosamine as a surrogate substrate. This surrogate can be prepared from commercially available UDP-GlcNAc by enzymatic conversion to UDP-MurNAc, which is then chemically coupled to n-hexylamine. Following the LpxC reaction, the free amine of the deacetylation product can be derivatized by fluorescamine, thus generating a fluorescent signal. This surrogate substrate has a K(m) of 367 microM and k(cat) of 0.36 s(-1), compared to 2 microM and 1.5 s(-1) for the natural substrate. Since no separation is needed, the assay is easily adaptable to high throughput screening. IC(50)s of LpxC inhibitors determined using this assay method is similar to those measured by traditional method with the natural substrate.

New trends in antimicrobial development

So far, two strategies have been applied to develop new anti-infective agents: (a) the synthesis of analogs of classical antibiotics with enhanced activity against resistant pathogens and (b) the screening of naturally occurring substances and libraries of synthetic compounds for antimicrobial activity in whole-cell assays. Today, the same principles are being used; however, the search for antimicrobial compounds with novel modes of action is based on targeting specific resistance and virulence factors. Novel targets for anti-infective agents are currently being discovered as a consequence of a better understanding of cell biology, the molecular basis of bacterial resistance, the gene-pathogenicity relationship and the mechanism of the infection process.

3-Deoxy-D-manno-oct-2-ulosonic acid (Kdo) transferase (WaaA) and kdo kinase (KdkA) of Haemophilus influenzae are both required to complement a waaA knockout mutation of Escherichia coli

The lipopolysaccharide (LPS) of the deep rough mutant Haemophilus influenzae I69 consists of lipid A and a single 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) residue substituted with one phosphate at position 4 or 5 (Helander, I. M., Lindner, B., Brade, H., Altmann, K., Lindberg, A. A., Rietschel, E. T., and Zähringer, U. (1988) Eur. J. Biochem. 177, 483-492). The waaA gene encoding the essential LPS-specific Kdo transferase was cloned from this strain, and its nucleotide sequence was identical to H. influenzae DSM11121. The gene was expressed in the Gram-positive host Corynebacterium glutamicum and characterized in vitro to encode a monofunctional Kdo transferase. waaA of H. influenzae could not complement a knockout mutation in the corresponding gene of an Re-type Escherichia coli strain. However, complementation was possible by coexpressing the recombinant waaA together with the LPS-specific Kdo kinase gene (kdkA) of H. influenzae DSM11121 or I69, respectively. The sequences of both kdkA genes were determined and differed in 25 nucleotides, giving rise to six amino acid exchanges between the deduced proteins. Both E. coli strains which expressed waaA and kdkA from H. influenzae synthesized an LPS containing a single Kdo residue that was exclusively phosphorylated at position 4. The structure was determined by nuclear magnetic resonance spectroscopy of deacylated LPS. Therefore, the reaction products of both cloned Kdo kinases represent only one of the two chemical structures synthesized by H. influenzae I69.

What are the microbial components implicated in the pathogenesis of sepsis? Report on a symposium

Despite considerable efforts in the past quarter century to improve therapy for sepsis, mortality rates remain unacceptably high. Microbe-derived constituents can induce the host to produce many mediators that can contribute to immune dysregulation, tissue damage, and death. Although endotoxin-mediated events are clearly important in gram-negative infections, gram-positive bacteria can also play a dominant role. Understanding the interplay of microbial constituents and host immune or inflammatory responses prompted a meeting at Rockefeller University in May 1998. Participants discussed the relative merits of a "2-hit" hypothesis to explain the course of lethal septic shock and a "multihit" synergistic threshold hypothesis. Recommendations include the following: (1) developing animal models that closely mimic human sepsis; (2) further investigating antibiotic effects on bacteria; (3) assessing the relationships between endotoxin, prokaryotic DNA, and peptidoglycan (i.e., independent, additive, or synergistic) in inducing host responses; and (4) developing new strategies to improve outcomes. Studies are needed to better define which and how different microbial constituents lead to sepsis and to provide critical leads for therapeutic intervention.

Purification and mass spectrometry of six lipid A species from the bacterial endosymbiont Rhizobium etli. Demonstration of a conserved distal unit and a variable proximal portion

Lipid A of Rhizobium etli CE3 differs dramatically from that of other Gram-negative bacteria. Key features include the presence of an unusual C28 acyl chain, a galacturonic acid moiety at position 4', and an acylated aminogluconate unit in place of the proximal glucosamine. In addition, R. etli lipid A is reported to lack phosphate and acyloxyacyl residues. Most of these remarkable structural claims are consistent with our recent enzymatic studies. However, the proposed R. etli lipid A structure is inconsistent with the ability of the precursor (3-deoxy-D-manno-octulosonic acid)(2)-4'-(32)P-lipid IV(A) to accept a C28 chain in vitro (Brozek, K. A., Carlson, R. W., and Raetz, C. R. H. (1996) J. Biol. Chem. 271, 32126-32136). To re-evaluate the structure, CE3 lipid A was isolated by new chromatographic procedures. CE3 lipid A is now resolved into six related components. Aminogluconate is present in D-1, D-2, and E, whereas B and C contain the typical glucosamine disaccharide seen in lipid A of most other bacteria. All the components possess a peculiar acyloxyacyl moiety at position 2', which includes the ester-linked C28 chain. As judged by mass spectrometry, the distal glucosamine units of A through E are the same, but the proximal units are variable. As described in the accompanying article (Que, N. L. S., Ribeiro, A. A., and Raetz, C. R. H. (2000) J. Biol. Chem. 275, 28017-28027), the discovery of component B suggests a plausible enzymatic pathway for the biosynthesis of the aminogluconate residue found in species D-1, D-2, and E of R. etli lipid A. We suggest that the unusual lipid A species of R. etli might be essential during symbiosis with leguminous host plants.

Comparative analyses of secondary gene products of 3-deoxy-D-manno-oct-2-ulosonic acid transferases from Chlamydiaceae in Escherichia coli K-12

The waaA gene encoding the essential, lipopolysaccharide (LPS)-specific 3-deoxy-Dmanno-oct-2-ulosonic acid (Kdo) transferase was inactivated in the chromosome of a heptosyltransferase I and II deficient Escherichia coli K-12 strain by insertion of gene expression cassettes encoding the waaA genes of Chlamydia trachomatis, Chlamydophila pneumoniae or Chlamydophila psittaci. The three chlamydial Kdo transferases were able to complement the knockout mutation without changing the growth or multiplication behaviour. The LPS of the mutants were serologically and structurally characterized in comparison to the LPS of the parent strain using compositional analyses, high performance anion exchange chromatography, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and specific monoclonal antibodies. The data show that chlamydial Kdo transferases can replace in E. coli K-12 the host's Kdo transferase and retain the product specificities described in their natural background. In addition, we unequivocally proved that WaaA from C. psittaci transfers predominantly four Kdo residues to lipid A, forming a branched tetrasaccharide with the structure alpha-Kdo-(2-->8)-[alpha-Kdo-(2-->4)]-alpha-Kdo-(2-->4)-alpha-Kdo.

Role of phosphoglucomutase of Bordetella bronchiseptica in lipopolysaccharide biosynthesis and virulence

The phosphoglucomutase (PGM)-encoding gene of Bordetella bronchiseptica is required for lipopolysaccharide (LPS) biosynthesis. An insertion mutant of the wild-type B. bronchiseptica strain BB7865 which disrupted LPS biosynthesis was created and characterized (BB7865pgm). Genetic analysis of the mutated gene showed it shares high identity with PGM genes of various bacterial species and forms part of an operon which also encompasses the gene encoding phosphoglucose isomerase. Functional assays for PGM revealed that enzyme activity is expressed in both bvg-positive and bvg-negative strains of B. bronchiseptica and is substantially reduced in BB7865pgm. Complementation of the mutated PGM gene with that from BB7865 restored the wild-type condition for all phenotypes tested. The ability of the mutant BB7865pgm to survive within J774. A1 cells was significantly reduced at 2 h (40% reduction) and 24 h (56% reduction) postinfection. BB7865pgm was also significantly attenuated in its ability to survive in vivo following intranasal infection of mice, being effectively cleared from the lungs within 4 days, whereas the wild-type strain persisted at least 35 days. The activities of superoxide dismutase, urease, and acid phosphatase were unaffected in the PGM-deficient strain. In contrast, the inability to produce wild-type LPS resulted in a reduced bacterial resistance to oxidative stress and a higher susceptibility to the antimicrobial peptide cecropin P.

Antibacterial agents that target lipid A biosynthesis in gram-negative bacteria. Inhibition of diverse UDP-3-O-(r-3-hydroxymyristoyl)-n-acetylglucosamine deacetylases by substrate analogs containing zinc binding motifs

UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC) catalyzes the second step in the biosynthesis of lipid A, a unique amphiphilic molecule found in the outer membranes of virtually all Gram-negative bacteria. Since lipid A biosynthesis is required for bacterial growth, inhibitors of LpxC have potential utility as antibiotics. The enzymes of lipid A biosynthesis, including LpxC, are encoded by single copy genes in all sequenced Gram-negative genomes. We have now cloned, overexpressed, and purified LpxC from the hyperthermophile Aquifex aeolicus. This heat-stable LpxC variant (the most divergent of all known LpxCs) displays 32% identity and 51% similarity over 277 amino acid residues out of the 305 in Escherichia coli LpxC. Although A. aeolicus LpxC deacetylates the substrate UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine at a rate comparable with E. coli LpxC, a phenyloxazoline-based hydroxamate that inhibits E. coli LpxC with K(i) of approximately 50 nM (Onishi, H. R., Pelak, B. A., Gerckens, L. S., Silver, L. L., Kahan, F. M., Chen, M. H., Patchett, A. A., Galloway, S. M., Hyland, S. A., Anderson, M. S., and Raetz, C. R. H. (1996) Science 274, 980-982) does not inhibit A. aeolicus LpxC. To determine whether or not broad-spectrum deacetylase inhibitors can be found, we have designed a new class of hydroxamate-containing inhibitors of LpxC, starting with the structure of the physiological substrate. Several of these compounds inhibit both E. coli and A. aeolicus LpxC at similar concentrations. We have also identified a phosphinate-containing substrate analog that inhibits both E. coli and A. aeolicus LpxC, suggesting that the LpxC reaction proceeds by a mechanism similar to that described for other zinc metalloamidases, like carboxypeptidase A and thermolysin. The differences between the phenyloxazoline and the substrate-based LpxC inhibitors might be exploited for developing novel antibiotics targeted either against some or all Gram-negative strains. We suggest that LpxC inhibitors with antibacterial activity be termed "deacetylins."

Lipase and esterase-catalyzed acylation of hetero-substituted nitrogen nucleophiles in water and organic solvents

The lipase- and esterase-catalyzed acylations of hydroxylamine and hydrazine derivatives with octanoic acid and ethyl octanoate are described. The influence of solvent and nucleophile on the initial reaction rate was investigated for a number of free and immobilized enzymes. Initial rates were highest in water, but the overall productivity was optimal in dioxane. Octanoic acid (250 g/L) was converted for 93% into the hydroxamic acid in 36 h with only 1% (w/w) Candida antarctica lipase B (Novozym 435) in dioxane at 40 degrees C. This translates to a catalyst productivity of 68.5 g. g(-1). day(-1) and a space time yield of 149 g. L(-1). day(-1), unprecedented figures in the direct reaction of an acid with a nitrogen nucleophile in an organic solvent.

A bacterial model system for understanding multi-drug resistance

Mankind stands at the crossroads, recognizing the need for a radical change in bacterial disease management. The development of several antimicrobial agents in the 1940s and 1950s allowed man to gain the upper hand in controlling these diseases. However, the horizon is now clouded by the activation in bacteria of cryptic multi-drug resistance (MDR) genes and the spread of plasmid- and integron-born MDR genes through bacterial populations. Unless remedial measures are taken, nearly all currently available antimicrobial agents are likely to soon lose their efficacies. We briefly review the bacterial MDR phenomenon and focus on a recently emerging family of small multi-drug resistance (SMR) pumps which may provide an ideal model system for understanding the MDR phenomenon in general.

Protease inhibitors - part 5. Alkyl/arylsulfonyl- and arylsulfonylureido-/arylureido- glycine hydroxamate inhibitors of Clostridium histolyticum collagenase

Reaction of alkyl/arylsulfonyl halides with glycine afforded a series of derivatives which were first N-benzylated by treatment with benzyl chloride, and then converted to the corresponding hydroxamic acids with hydroxylamine in the presence of carbodiimide derivatives. Other derivatives were obtained by reaction of N-benzyl-glycine with aryl isocyanates, arylsulfonyl isocyanates or benzoyl isothiocyanate, followed by conversion of their COOH group into the CONHOH moiety, as mentioned above. The 90 new compounds reported here were assayed as inhibitors of the Clostridium histolyticum collagenase (EC 3.4.24.3), a zinc enzyme which degrades triple helical regions of native collagen. The prepared hydroxamate derivatives were generally 100-500 times more active than the corresponding carboxylates. In the series of synthesized hydroxamates, substitution patterns leading to the best inhibitors were those involving perfluoroalkylsulfonyl- and substituted-arylsulfonyl moieties, such as pentafluorophenylsulfonyl, 3- and 4-carboxyphenylsulfonyl-, 3-trifluoromethyl-phenylsulfonyl or 1- and 2-naphthyl among others. Thus, it seems that similarly to the matrix metalloproteinase (MMP) hydroxamate inhibitors, Clostridium histolyticum collagenase inhibitors should incorporate hydrophobic moieties at the P(1') and P(2') sites, whereas the alpha-carbon substituent may be a small and compact moiety (such as H, for the Gly derivatives reported here). Such compounds might lead to the design of collagenase inhibitor-based drugs useful as anti-cancer, anti-arthritis or anti-bacterial agents for the treatment of corneal keratitis.

Protease inhibitors. Part 7. Inhibition of Clostridium histolyticum collagenase with sulfonylated derivatives of L-valine hydroxamate

Sulfonylated L-valine hydroxamate derivatives were obtained by reaction of alkyl/arylsulfonyl halides with the title amino acid, followed by treatment with benzyl chloride, and conversion of the COOH moiety to the CONHOH group. Other derivatives were obtained by reaction of N-benzyl-L-valine with arylisocyanates, arylsulfonylisocyanates or benzoylisothiocyanate, followed by the similar conversion of the COOH into the CONHOH moiety, with hydroxylamine in the presence of carbodiimides. The obtained compounds were assayed as inhibitors of the Clostridium histolyticum collagenase, ChC (EC 3.4.24.3), a zinc enzyme which degrades triple helical collagen. The hydroxamate derivatives were generally 100-500 times more active than the corresponding carboxylates. In the series of synthesized derivatives, substitution patterns leading to best ChC inhibitors were those involving perfluoroalkylsulfonyl- and substituted-arylsulfonyl moieties, such as pentafluorophenylsulfonyl; 3- and 4-protected-aminophenylsulfonyl-; 3- and 4-carboxyphenylsulfonyl-; 3-trifluoromethylphenylsulfonyl; or 1- and 2-naphthyl among others. Similarly to the matrix metalloproteinase hydroxamate inhibitors, ChC inhibitors of the type reported here must incorporate hydrophobic moieties at the P(2') and P(3') subsites, in order to achieve tight binding to the enzyme. Such compounds might lead to drugs useful in the treatment corneal bacterial keratitis.

Protease inhibitors. Part 8: synthesis of potent Clostridium histolyticum collagenase inhibitors incorporating sulfonylated L-alanine hydroxamate moieties

A series of hydroxamates was prepared by reaction of alkyl/arylsulfonyl halides with N-2-chlorobenzyl-L-alanine, followed by conversion of the COOH moiety to the CONHOH group, with hydroxylamine in the presence of carbodiimides. Other structurally related compounds were obtained by reaction of N-2-chlorobenzyl-L-alanine with aryl isocyanates, arylsulfonyl isocyanates or benzoyl isothiocyanate, followed by the similar conversion of the COOH into the CONHOH moiety. The new compounds were assayed as inhibitors of the Clostridium histolyticum collagenase, ChC (EC 3.4.24.3), a bacterial zinc metallo-peptidase which degrades triple helical collagen as well as a large number of synthetic peptides. The prepared hydroxamate derivatives proved to be 100-500 times more active collagenase inhibitors than the corresponding carboxylates. Substitution patterns leading to best ChC inhibitors (both for carboxylates as well as for the hydroxamates) were those involving perfluoroalkylsulfonyl- and substituted-arylsulfonyl moieties, such as pentafluorophenylsulfonyl; 3- and 4-protected-aminophenylsulfonyl-; 3- and 4-carboxyphenylsulfonyl-; 3-trifluoromethyl-phenylsulfonyl; as well as 1- and 2-naphthyl-, quinoline-8-yl- or substituted-arylsulfonylamidocarboxyl moieties among others. Similarly to the matrix metalloproteinase (MMP) hydroxamate inhibitors, ChC inhibitors of the type reported here must incorporate hydrophobic moieties at the P2' and P3' sites, in order to achieve tight binding to the enzyme. This study also proves that the 2-chlorobenzyl moiety, investigated here for the first time, is an efficient P2' anchoring moiety for obtaining potent ChC inhibitors.