Inhibition Of Peptidoglycan Biosynthesis Research Articles

Potential Antibacterial of Leaf Sirih Merah Against Enterococcus Faecalis ATCC 29212 Bacteria.

Dental root canal failure is a disease caused by gram-positive bacteria, Enterococcus faecalis. The disease is caused by the bacterial cell wall consisting of a peptidoglycan layer that protects the bacteria from internal osmotic pressure. Peptidoglycan biosynthesis includes many enzymes, such as MurA, Penicillin-binding protein (PBP), and SrtA. Herbal plants are a source of bioactive compounds, including antibacterial agents. There is information that red betel leaves, also known as Piper crocatum, contain active substances such as flavonoids, terpenoids, and steroids. However, there is no additional information on the antibacterial properties of P. crocatum and the molecular mechanisms that affect the cell wall of E. faecalis ATCC 29212 bacteria. This study aims to determine the antibacterial activity of the extract in vitro, screen and study the antibacterial compounds of red betel leaves against oral pathogenic bacteria, namely E.faecalis ATCC 29212 through molecular docking. The n-hexan:ea (9:1) fraction of P. crocatum extract was tested for inhibition zones against E. faecalis ATCC 29212 bacteria, fractions that had positive results were then identified using the LC-MS method. The LC-MS resulting compounds were tested using In Silico. Antibacterial in the n-hexane: ethyl acetate (9:1) fraction of Red Betel Leaf has the best concentration of 10% with a moderate inhibition zone category. LC-MS test results identified compounds including Longicamphenylone, m/z 207, Nootkatone m/z 219, and Tridecanal m/z 221. Molecular interactions between these compounds with target proteins, namely MurA, PBP, and SrtA, show lower binding affinity values than natural ligands and positive controls for each protein. Nootkatone compounds demonstrated potential as MurA and PBP inhibitors, while Longicamphenylone compounds showed potential as SrtA inhibitors. Both compounds have the potential to inhibit peptidoglycan biosynthesis and bacterial cell wall formation through docking simulations.

The mechanism of the phage-encoded protein antibiotic from ΦX174.

The historically important phage ΦX174 kills its host bacteria by encoding a 91-residue protein antibiotic called protein E. Using single-particle electron cryo-microscopy, we demonstrate that protein E bridges two bacterial proteins to form the transmembrane YES complex [MraY, protein E, sensitivity to lysis D (SlyD)]. Protein E inhibits peptidoglycan biosynthesis by obstructing the MraY active site leading to loss of lipid I production. We experimentally validate this result for two different viral species, providing a clear model for bacterial lysis and unifying previous experimental data. Additionally, we characterize the Escherichia coli MraY structure-revealing features of this essential enzyme-and the structure of the chaperone SlyD bound to a protein. Our structures provide insights into the mechanism of phage-mediated lysis and for structure-based design of phage therapeutics.

Cyanobacteria: Isolation, Purification and Principles

The primary means of nutrient uptake for cyanobacteria is oxygen-producing photosynthesis. Their ecological variety is astounding; they occupy a very wide range of lighted ecological niches in terrestrial, marine, and freshwater habitats. Despite this apparent metabolic consistency, they exhibit tremendous phylogenetic diversity. The fact that cyanobacteria have certain physiological and metabolic traits that are exclusively seen in prokaryotes significantly broadens this spectrum. The capacity to fix nitrogen in an aerobic manner under light is a special characteristic. Apart from Gonotheca species, all aerobic, nitrogen-fixing cyanobacteria—a structurally diverse group—produce heterocyst, the highly specialised cells that allow them to fix nitrogen efficiently in a fully aerobic environment by preventing the oxygen-sensitive enzyme nitrogenase from being inactivated quickly in vivo (Hazelton, 1978; Stanier and Cohen-Bazire, 1978; Stewart, Haystead, and Pearson, 1969). The same quantities of cyanobacteria could be isolated from freshwater using a technique using nutrient-saturated glass fibre filters, but the quantity of accompanying heterotrophic bacteria was reduced by 2- to 15-fold. a broad-spectrum antibiotic called imipenem. In comparison to some other Plactam antibiotics, the B-lactam antibiotic that inhibits peptidoglycan biosynthesis, was more effective at lowering the levels of heterotrophic bacterial contaminants associated with newly isolated cyanobacteria to a point that made it easier to grow axenic cyanobacterial cultures.

Structural basis for phage single gene lysis protein mechanisms.

The rapid emergence of antibiotic resistant bacterial strains represents a significant threat to public health, making the development of novel therapeutic strategies critical. Bacterial cell wall biosynthesis has been exploited as a major target for antibiotics and historically been quite successful in treating bacterial infections. A major component of the bacterial cell wall is the peptidoglycan (PG) layer, providing structural support to maintain cell shape and protect bacteria from osmotic lysis. One promising strategy to inhibit PG biosynthesis is to leverage the naturally occurring bacteriophage-derived lysis proteins. Small lytic phages of the families Microviridae and Leviviridae disrupt the formation of the PG matrix through a single gene lysis protein (Sgl). Sgls lack enzymatic activity to degrade cell wall directly and instead induce the host cell to undergo autolysis by inhibiting cell wall synthesis. Three Sgls have been identified that target PG biosynthesis enzymes and act as protein antibiotics. The A2 protein of levivirus Qβ, protein E of ΦX174 (EΦX174) and lysis protein of levivirus M (LysM) were shown to inhibit MurA, MraY and MurJ steps of the PG synthesis pathway respectively. To date, the molecular details of how Sgls inhibit PG biosynthesis enzymes are not well understood. Here we present the cryo-EM structure of the EΦX174-MraY inhibition complex which reveals that E inhibits MraY activity by blocking the substrate binding sites. Furthermore, we are currently exploring other peptidoglycan targeting Sgls from various bacteriophages to expand our understanding of cell wall synthesis inhibition by protein antibiotics. Structure elucidation of these Sgl-PG biosynthetic enzyme complexes helps identify structural properties of the inhibitor binding sites, providing insights into rational design strategies for developing novel bacteriophage Sgl-based inhibitors targeting cell wall biosynthesis.

Organoarsenicals inhibit bacterial peptidoglycan biosynthesis by targeting the essential enzyme MurA

Trivalent organoarsenicals such as methylarsenite (MAs(III)) are considerably more toxic than inorganic arsenate (As(V)) or arsenite (As(III)). In microbial communities MAs(III) exhibits significant antimicrobial activity. Although MAs(III) and other organoarsenicals contribute to the global arsenic biogeocycle, how they exert antibiotic-like properties is largely unknown. To identify possible targets of MAs(III), a genomic library of the gram-negative bacterium, Shewanella putrefaciens 200, was expressed in Escherichia coli with selection for MAs(III) resistance. One clone contained the S. putrefaciens murA gene (SpmurA), which catalyzes the first committed step in peptidoglycan biosynthesis. Overexpression of SpmurA conferred MAs(III) resistance to E. coli. Purified SpMurA was inhibited by MAs(III), phenylarsenite (PhAs(III)) or the phosphonate antibiotic fosfomycin but not by inorganic As(III). Fosfomycin inhibits MurA by binding to a conserved residue that corresponds to Cys117 in SpMurA. A C117D mutant was resistant to fosfomycin but remained sensitive to MAs(III), indicating that the two compounds have different mechanisms of action. New inhibitors of peptidoglycan biosynthesis are highly sought after as antimicrobial drugs, and organoarsenicals represent a new area for the development of novel compounds for combating the threat of antibiotic resistance.

Co-substrate addition accelerated amoxicillin degradation and detoxification by up-regulating degradation related enzymes and promoting cell resistance

β-Lactam antibiotics are the most commonly used antibiotics, and are difficult to remove by conventional biological treatments because of their persistent and toxic nature. The addition of co-substrates has been successfully employed to improve the removal of refractory pollutants. So, we hypothesized that the co-substrate strategy would increase antibiotic degradation and benefit microbial survival. In this work, we reported that co-substrate (acetate) addition up-regulated key degrading enzymes and resistance related genes in a model bacteria strain (L. aquatilis) when being treated with 0.055 mM amoxicillin (AMO). β-Lactamase, amidases, transaminase, and amide C-N hydrolase showed increased activation. As a result, AMO removal reached ∼95 %, a ∼60 % increase over the control. Furthermore, the addition of acetate drove the down-stream TCA cycle, which accelerated the detoxification of the intermediates and reduced the microbial inhibition by the antibiotic products to as low as ∼15 %. Besides, the expression levels of genes encoding the efflux pump, penicillin binding proteins, and β-Lactamase were up-regulated, and the inhibition of peptidoglycan biosynthesis was down-regulated. The cell density was enhanced by ∼170 % and showed improved DNA replication. In conclusion, the addition of the co-substrate accelerated AMO degradation and detoxification by up-regulating degrading enzymes and promoting cell resistance.

A Metabolic Dependency for Host Isoprenoids in the Obligate Intracellular Pathogen Rickettsia parkeri Underlies a Sensitivity to the Statin Class of Host-Targeted Therapeutics.

Gram-negative bacteria in the order Rickettsiales have an obligate intracellular growth requirement, and some species cause human diseases such as typhus and spotted fever. The bacteria have evolved a dependence on essential nutrients and metabolites from the host cell as a consequence of extensive genome reduction. However, it remains largely unknown which nutrients they acquire and whether their metabolic dependency can be exploited therapeutically. Here, we describe a genetic rewiring of bacterial isoprenoid biosynthetic pathways in the Rickettsiales that has resulted from reductive genome evolution. Furthermore, we investigated whether the spotted fever group Rickettsia species Rickettsia parkeri scavenges isoprenoid precursors directly from the host. Using targeted mass spectrometry, we found that infection caused decreases in host isoprenoid products and concomitant increases in bacterial isoprenoid metabolites. Additionally, we report that treatment of infected cells with statins, which inhibit host isoprenoid synthesis, prohibited bacterial growth. We show that growth inhibition correlates with changes in bacterial size and shape that mimic those caused by antibiotics that inhibit peptidoglycan biosynthesis, suggesting that statins lead to an inhibition of cell wall synthesis. Altogether, our results describe a potential Achilles' heel of obligate intracellular pathogens that can potentially be exploited with host-targeted therapeutics that interfere with metabolic pathways required for bacterial growth.IMPORTANCE Obligate intracellular pathogens, which include viruses as well as certain bacteria and eukaryotes, are a subset of infectious microbes that are metabolically dependent on and unable to grow outside an infected host cell because they have lost or lack essential biosynthetic pathways. In this study, we describe a metabolic dependency of the bacterial pathogen Rickettsia parkeri on host isoprenoid molecules that are used in the biosynthesis of downstream products, including cholesterol, steroid hormones, and heme. Bacteria make products from isoprenoids, such as an essential lipid carrier for making the bacterial cell wall. We show that bacterial metabolic dependency can represent a potential Achilles' heel and that inhibiting host isoprenoid biosynthesis with the FDA-approved statin class of drugs inhibits bacterial growth by interfering with the integrity of the cell wall. This work supports the potential to treat infections by obligate intracellular pathogens through inhibition of host biosynthetic pathways that are susceptible to parasitism.

Insights into the Mechanism of Action of the Two-Peptide Lantibiotic Lacticin 3147

Lacticin 3147 is a two peptide lantibiotc (LtnA1 and LtnA2) that displays nanomolar activity against many Gram-positive bacteria. Lacticin 3147 may exert its antimicrobial effect by several mechanisms. Isothermal titration calorimetry experiments show that only LtnA1 binds to the peptidoglycan precursor lipid II, which could inhibit peptidoglycan biosynthesis. An experimentally supported model of the resulting complex suggests that the key binding partners are the C-terminus of LtnA1 and pyrophosphate of lipid II. A combination of in vivo and in vitro assays indicates that LtnA1 and LtnA2 can induce rapid membrane lysis without the need for lipid II binding. However, the presence of lipid II substantially increases the activity of lacticin 3147. Furthermore, studies with synthetic LtnA2 analogues containing either desmethyl- or oxa-lanthionine rings confirm that the precise geometry of these rings is essential for this synergistic activity.

Sansanmycin natural product analogues as potent and selective anti-mycobacterials that inhibit lipid I biosynthesis

Tuberculosis (TB) is responsible for enormous global morbidity and mortality, and current treatment regimens rely on the use of drugs that have been in use for more than 40 years. Owing to widespread resistance to these therapies, new drugs are desperately needed to control the TB disease burden. Herein, we describe the rapid synthesis of analogues of the sansanmycin uridylpeptide natural products that represent promising new TB drug leads. The compounds exhibit potent and selective inhibition of Mycobacterium tuberculosis, the etiological agent of TB, both in vitro and intracellularly. The natural product analogues are nanomolar inhibitors of Mtb phospho-MurNAc-pentapeptide translocase, the enzyme responsible for the synthesis of lipid I in mycobacteria. This work lays the foundation for the development of uridylpeptide natural product analogues as new TB drug candidates that operate through the inhibition of peptidoglycan biosynthesis.

The action mechanism of daptomycin

Daptomycin is a lipopeptide antibiotic produced by the soil bacterium Streptomyces roseosporus that is clinically used to treat severe infections with Gram-positive bacteria. In this review, we discuss the mode of action of this important antibiotic. Although daptomycin is structurally related to amphomycin and similar lipopeptides that inhibit peptidoglycan biosynthesis, experimental studies have not produced clear evidence that daptomycin shares their action mechanism. Instead, the best characterized effect of daptomycin is the permeabilization and depolarization of the bacterial cell membrane. This activity, which can account for daptomycin’s bactericidal effect, correlates with the level of phosphatidylglycerol (PG) in the membrane. Accordingly, reduced synthesis of PG or its increased conversion to lysyl-PG promotes bacterial resistance to daptomycin. While other resistance mechanisms suggest that daptomycin may indeed directly interfere with cell wall synthesis or cell division, such effects still await direct experimental confirmation. Daptomycin’s complex structure and biosynthesis have hampered the analysis of its structure activity relationships. Novel methods of total synthesis, including a recent one that is carried out entirely on a solid phase, will enable a more thorough and systematic exploration of the sequence space.

Global Association between Thermophilicity and Vancomycin Susceptibility in Bacteria.

Exploration of the aquatic microbiota of several circum-neutral (6.0–8.5 pH) mid-temperature (55–85°C) springs revealed rich diversities of phylogenetic relatives of mesophilic bacteria, which surpassed the diversity of the truly-thermophilic taxa. To gain insight into the potentially-thermophilic adaptations of the phylogenetic relatives of Gram-negative mesophilic bacteria detected in culture-independent investigations we attempted pure-culture isolation by supplementing the enrichment media with 50 μg ml−1 vancomycin. Surprisingly, this Gram-positive-specific antibiotic eliminated the entire culturable-diversity of chemoorganotrophic and sulfur-chemolithotrophic bacteria present in the tested hot water inocula. Moreover, it also killed all the Gram-negative hot-spring isolates that were obtained in vancomycin-free media. Concurrent literature search for the description of Gram-negative thermophilic bacteria revealed that at least 16 of them were reportedly vancomycin-susceptible. While these data suggested that vancomycin-susceptibility could be a global trait of thermophilic bacteria (irrespective of their taxonomy, biogeography and Gram-character), MALDI Mass Spectroscopy of the peptidoglycans of a few Gram-negative thermophilic bacteria revealed that tandem alanines were present in the fourth and fifth positions of their muropeptide precursors (MPPs). Subsequent phylogenetic analyses revealed a close affinity between the D-alanine-D-alanine ligases (Ddl) of taxonomically-diverse Gram-negative thermophiles and the thermostable Ddl protein of Thermotoga maritima, which is well-known for its high specificity for alanine over other amino acids. The Ddl tree further illustrated a divergence between the homologs of Gram-negative thermophiles and mesophiles, which broadly coincided with vancomycin-susceptibility and vancomycin-resistance respectively. It was thus hypothesized that thermophilic Ddls have been evolutionarily selected to favor a D-ala-D-ala bonding. However, preference for D-ala-D-ala-terminated MPPs does not singlehandedly guarantee vancomycin susceptibility of thermophilic bacteria as the large and relatively-hydrophilic vancomycin molecule has to cross the outer membrane before it can inhibit peptidoglycan biosynthesis. Literature shows that many mesophilic Gram-negative bacteria also have D-ala-D-ala-terminated MPPs, but they still remain resistant to vancomycin due to the relative impermeability of their membranes. But the global vancomycin-susceptibility phenotype of thermophilic bacteria itself testifies that the drug crosses the membrane in all these cases. As a corollary, it seems quite likely that the outer membranes of thermophilic bacteria have some yet-unknown characteristic feature(s) that invariably ensures the entry of vancomycin.

Cold Stress Makes Escherichia coli Susceptible to Glycopeptide Antibiotics by Altering Outer Membrane Integrity

A poor understanding of the mechanisms by which antibiotics traverse the outer membrane remains a considerable obstacle to the development of novel Gram-negative antibiotics. Herein, we demonstrate that the Gram-negative bacterium Escherichia coli becomes susceptible to the narrow-spectrum antibiotic vancomycin during growth at low temperatures. Heterologous expression of an Enterococcus vanHBX vancomycin resistance cluster in E. coli confirmed that the mechanism of action was through inhibition of peptidoglycan biosynthesis. To understand the nature of vancomycin permeability, we screened for strains of E. coli that displayed resistance to vancomycin at low temperature. Surprisingly, we observed that mutations in outer membrane biosynthesis suppressed vancomycin activity. Subsequent chemical analysis of lipopolysaccharide from vancomycin-sensitive and -resistant strains confirmed that suppression was correlated with truncations in the core oligosaccharide of lipopolysaccharide. These unexpected observations challenge the current understanding of outer membrane permeability, and provide new chemical insights into the susceptibility of E. coli to glycopeptide antibiotics.

Distinct roles of major peptidoglycan recycling enzymes in β-Lactamase production in Shewanella oneidensis.

β-Lactam antibiotics were the earliest discovered and are the most widely used group of antibiotics that work by inactivating penicillin-binding proteins to inhibit peptidoglycan biosynthesis. As one of the most efficient defense strategies, many bacteria produce β-lactam-degrading enzymes, β-lactamases, whose biochemical functions and regulation have been extensively studied. A signal transduction pathway for β-lactamase induction by β-lactam antibiotics, consisting of the major peptidoglycan recycling enzymes and the LysR-type transcriptional regulator, AmpR, has been recently unveiled in some bacteria. Because inactivation of some of these proteins, especially the permease AmpG and the β-hexosaminidase NagZ, results in substantially elevated susceptibility to the antibiotics, these have been recognized as potential therapeutic targets. Here, we show a contrasting scenario in Shewanella oneidensis, in which the homologue of AmpR is absent. Loss of AmpG or NagZ enhances β-lactam resistance drastically, whereas other identified major peptidoglycan recycling enzymes are dispensable. Moreover, our data indicate that there exists a parallel signal transduction pathway for β-lactamase induction, which is independent of either AmpG or NagZ.

Purification, crystallization and preliminary X-ray crystallographic analysis of the UDP-N-acetylmuramoyl-tripeptide-D-alanyl-D-alanine ligase (MurF) from Acinetobacter baumannii.

The emergence and global spread of multidrug-resistant Acinetobacter baumannii strains are major threats to public health. Inhibition of peptidoglycan biosynthesis is an effective strategy for the development of antibiotics. The ATP-dependent UDP-N-acetylmuramoyl-tripeptide-D-alanyl-D-alanine ligase (MurF) that is responsible for the last step of peptidoglycan biosynthesis is a validated target for the development of antibiotics. Crystals of A. baumannii MurF in complex with ATP were grown by the microbatch crystallization method at 295 K. The crystals belonged to space group P322₁, with unit-cell parameters a=b=85.42, c=129.86 Å. Assuming the presence of one molecule in the asymmetric unit, the solvent content was estimated to be about 54.32%.

The Lantibiotic NAI-107 Binds to Bactoprenol-bound Cell Wall Precursors and Impairs Membrane Functions

The lantibiotic NAI-107 is active against Gram-positive bacteria including vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus. To identify the molecular basis of its potency, we studied the mode of action in a series of whole cell and in vitro assays and analyzed structural features by nuclear magnetic resonance (NMR). The lantibiotic efficiently interfered with late stages of cell wall biosynthesis and induced accumulation of the soluble peptidoglycan precursor UDP-N-acetylmuramic acid-pentapeptide (UDP-MurNAc-pentapeptide) in the cytoplasm. Using membrane preparations and a complete cascade of purified, recombinant late stage peptidoglycan biosynthetic enzymes (MraY, MurG, FemX, PBP2) and their respective purified substrates, we showed that NAI-107 forms complexes with bactoprenol-pyrophosphate-coupled precursors of the bacterial cell wall. Titration experiments indicate that first a 1:1 stoichiometric complex occurs, which then transforms into a 2:1 (peptide: lipid II) complex, when excess peptide is added. Furthermore, lipid II and related molecules obviously could not serve as anchor molecules for the formation of defined and stable nisin-like pores, however, slow membrane depolarization was observed after NAI-107 treatment, which could contribute to killing of the bacterial cell.

Activity‐Based Probes for Penicillin‐Binding Protein Imaging

We aim to investigate the timing and localization of penicillin‐binding proteins (PBPs) during peptidoglycan (PG) biosynthesis using a combination of fluorescence microscopy, gel electrophoresis, and liquid chromatography‐mass spectrometry (LC‐MS) techniques facilitated by the use of a β‐lactam‐based probe. β‐lactam‐containing compounds are the most widely used class of antibiotics that inhibit PG biosynthesis by binding PBPs. We designed and synthesized fluorescent and biotin derivatives of the antibiotic cephalosporin to selectively label high molecular weight (HMW) PBPs in B. subtilis PY79 in vivo. Our microscopy results indicate that the probe facilitated specific labeling of active HMW PBPs that are localized at newly synthesized septa. The MS analysis identified the labeled proteins as PBP 1a and b, PBP 2a, PBP 2b and PBP 4. Both microscopy and gel‐based analysis indicated that our probe gives cleaner staining and is more selective than BOCILLIN‐FL, a commercially available penicillin analog, which labels both high and low molecular PBPs. Our cephalosporin probe is currently being applied to time‐course studies to visualize the roles of HMW PBPs in B. subtilis.

A rapid in situ procedure for determination of bacterial susceptibility or resistance to antibiotics that inhibit peptidoglycan biosynthesis

BackgroundAntibiotics which inhibit bacterial peptidoglycan biosynthesis are the most widely used in current clinical practice. Nevertheless, resistant strains increase dramatically, with serious economic impact and effects on public health, and are responsible for thousands of deaths each year. Critical clinical situations should benefit from a rapid procedure to evaluate the sensitivity or resistance to antibiotics that act at the cell wall. We have adapted a kit for rapid determination of bacterial DNA fragmentation, to assess cell wall integrity.ResultsCells incubated with the antibiotic were embedded in an agarose microgel on a slide, incubated in an adapted lysis buffer, stained with a DNA fluorochrome, SYBR Gold and observed under fluorescence microscopy. The lysis affects the cells differentially, depending on the integrity of the wall. If the bacterium is susceptible to the antibiotic, the weakened cell wall is affected by the lysing solution so the nucleoid of DNA contained inside the bacterium is released and spread. Alternatively, if the bacterium is resistant to the antibiotic, it is practically unaffected by the lysis solution and does not liberate the nucleoid, retaining its normal morphological appearance. In an initial approach, the procedure accurately discriminates susceptible, intermediate and resistant strains of Escherichia coli to amoxicillin/clavulanic acid. When the bacteria came from an exponentially growing liquid culture, the effect on the cell wall of the β-lactam was evident much earlier that when they came from an agar plate. A dose-response experiment with an E. coli strain susceptible to ampicillin demonstrated a weak effect before the MIC dose. The cell wall damage was not homogenous among the different cells, but the level of damage increased as dose increased with a predominant degree of effect for each dose. A microgranular-fibrilar extracellular background was evident in gram-negative susceptible strains after β-lactam treatment. This material was digested by DNase I, hybridised with a specific whole genome probe, and so recognized as DNA fragments released by the bacteria. Finally, 46 clinical strains from eight gram-negative and four gram-positive species were evaluated blind for susceptibility or resistance to one of four different β-lactams and vancomycin, confirming the applicability of the methodology.ConclusionThe technique to assess cell wall integrity appears to be a rapid and simple procedure to identify resistant and susceptible strains to antibiotics that interfere with peptidoglycan biosynthesis.

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Insight into Invertebrate Defensin Mechanism of Action: OYSTER DEFENSINS INHIBIT PEPTIDOGLYCAN BIOSYNTHESIS BY BINDING TO LIPID II

Three oyster defensin variants (Cg-Defh1, Cg-Defh2, and Cg-Defm) were produced as recombinant peptides and characterized in terms of activities and mechanism of action. In agreement with their spectrum of activity almost specifically directed against Gram-positive bacteria, oyster defensins were shown here to be specific inhibitors of a bacterial biosynthesis pathway rather than mere membrane-active agents. Indeed, at lethal concentrations, the three defensins did not compromise Staphylococcus aureus membrane integrity but inhibited the cell wall biosynthesis as indicated by the accumulation of the UDP-N-acetylmuramyl-pentapeptide cell wall precursor. In addition, a combination of antagonization assays, thin layer chromatography, and surface plasmon resonance measurements showed that oyster defensins bind almost irreversibly to the lipid II peptidoglycan precursor, thereby inhibiting the cell wall biosynthesis. To our knowledge, this is the first detailed analysis of the mechanism of action of antibacterial defensins produced by invertebrates. Interestingly, the three defensins, which were chosen as representative of the oyster defensin molecular diversity, bound differentially to lipid II. This correlated with their differential antibacterial activities. From our experimental data and the analysis of oyster defensin sequence diversity, we propose that oyster defensin activity results from selective forces that have conserved residues involved in lipid II binding and diversified residues at the surface of oyster defensins that could improve electrostatic interactions with the bacterial membranes.

Functional and Structural Analysis of a Key Region of the Cell Wall Inhibitor Moenomycin

Moenomycin A (MmA) belongs to a family of natural products that inhibit peptidoglycan biosynthesis by binding to the peptidoglycan glycosyltransferases, the enzymes that make the glycan chains of peptidoglycan. MmA is remarkably potent, but its clinical utility has been hampered by poor physicochemical properties. Moenomycin contains three structurally distinct regions: a pentasaccharide, a phosphoglycerate, and a C25 isoprenyl (moenocinyl) lipid tail that gives the molecule its name. The phosphoglycerate moiety links the pentasaccharide to the moenocinyl chain. This moiety contains two negatively charged groups, a phosphoryl group and a carboxylate. Both the phosphoryl group and the carboxylate have previously been implicated in target binding but the role of the carboxylate has not been explored in detail. Here we report the synthesis of six MmA analogues designed to probe the importance of the phosphoglycerate. These analogues were evaluated for antibacterial and enzyme inhibitory activity; the specific contacts between the phosphoglycerate and the protein target were assessed by X-ray crystallography in conjunction with molecular modeling. Both the phosphoryl group and the carboxylate of the phosphoglycerate chain play roles in target binding. The negative charge of the carboxylate, and not its specific structure, appears to be the critical feature in binding since replacing it with a negatively charged acylsulfonamide group produces a more active compound than replacing it with the isosteric amide. Analysis of the ligand-protein contacts suggests that the carboxylate makes a critical contact with an invariant lysine in the active site. The reported work provides information and validated computational methods critical for the design of analogues based on moenomycin scaffolds.