Identification of the first known inhibitors of O-acetylpeptidoglycan esterase: a potential new antibacterial target

From structure to function: Decoding peptidoglycan O-acetylation in pathogenic bacteria

Numerous pathogenic and non-pathogenic bacteria modulate the structure of their cell wall to escape the action of lytic enzymes that target it, threatening cell integrity. Of these modifications, the most taxonomically widespread is the addition of an acetyl to the C6 hydroxyl group of muramyl residues within the essential cell-wall heteropolymer peptidoglycan. This modification is found in many clinically important pathogens, including the WHO priority pathogens Neisseria gonorrhoeae, Staphylococcus aureus, Enterococcus faecium, and Streptococcus pneumoniae. In this review, we summarize the last 60 years of discoveries about the genetics, biochemistry, structural biology, and cellular metabolism that underlie this enigmatic bacterial self-defence mechanism.

Purpurin suppresses Salmonella invasion of host cells by reducing the secretion of T3SS-1 effector proteins

Salmonella Typhimurium (S. Typhimurium, ST) is a food-borne pathogen that can be transmitted from animals to humans and causes symptoms such as diarrhea, fever, and vomiting. While antibiotics are commonly used to treat clinical infections, the increase in drug resistance has limited their effectiveness. Antivirulence drugs offer a new approach to treating bacterial infections by targeting specific virulence factors without affecting bacterial growth, thus helping to combat infection without exerting selective pressure on bacteria or inducing resistance. Salmonella pathogenicity island 1 (SPI-1), encoding type three secretion system 1 (T3SS-1), serves as a crucial virulence factor for the invasion of ST into host cells, making it an ideal target for screening anti-Salmonella virulence drugs. This project involved screening of ST invasion inhibitors through a gentamicin protection assay and identified purpurin (PPR) as capable of inhibiting the ST invasion of HeLa cells. Subsequent studies revealed that PPR had no effect on the natural growth of bacteria and was not cytotoxic to host cells. A mechanistic study revealed that PPR effectively inhibits the secretion of T3SS-1 in ST. The results from animal experiments indicated that PPR exhibited significant efficacy in a mouse enteritis model caused by ST infection, increasing the survival rate of mice infected with a lethal dose by 50%, reducing spleen colonization in infected mice, and alleviating tissue damage resulting from ST infection. Therefore, PPR represents a promising antivirulence drug that targets the T3SS of ST and may serve as a hit compound for the development of novel antivirulence drugs for the treatment of ST.

When the Host Encounters the Cell Wall and Vice Versa

Peptidoglycan (PGN) and associated surface structures such as secondary polymers and capsules have a central role in the physiology of bacteria. The exoskeletal PGN heteropolymer is the major determinant of cell shape and allows bacteria to withstand cytoplasmic turgor pressure. Thus, its assembly, expansion, and remodeling during cell growth and division need to be highly regulated to avoid compromising cell survival. Similarly, regulation of the assembly impacts bacterial cell shape; distinct shapes enhance fitness in different ecological niches, such as the host. Because bacterial cell wall components, in particular PGN, are exposed to the environment and unique to bacteria, these have been coopted during evolution by eukaryotes to detect bacteria. Furthermore, the essential role of the cell wall in bacterial survival has made PGN an important signaling molecule in the dialog between host and microbes and a target of many host responses. Millions of years of coevolution have resulted in a pivotal role for PGN fragments in shaping host physiology and in establishing a long-lasting symbiosis between microbes and the host. Thus, perturbations of this dialog can lead to pathologies such as chronic inflammatory diseases. Similarly, pathogens have devised sophisticated strategies to manipulate the system to enhance their survival and growth.

Antibacterial activities of anthraquinones: structure-activity relationships and action mechanisms

With the increasing prevalence of untreatable infections caused by antibiotic-resistant bacteria, the discovery of new drugs from natural products has become a hot research topic. The antibacterial activity of anthraquinones widely distributed in traditional Chinese medicine has attracted much attention. Herein, the structure and activity relationships (SARs) of anthraquinones as bacteriostatic agents are reviewed and elucidated. The substituents of anthraquinone and its derivatives are closely related to their antibacterial activities. The stronger the polarity of anthraquinone substituents is, the more potent the antibacterial effects appear. The presence of hydroxyl groups is not necessary for the antibacterial activity of hydroxyanthraquinone derivatives. Substitution of di-isopentenyl groups can improve the antibacterial activity of anthraquinone derivatives. The rigid plane structure of anthraquinone lowers its water solubility and results in the reduced activity. Meanwhile, the antibacterial mechanisms of anthraquinone and its analogs are explored, mainly including biofilm formation inhibition, destruction of the cell wall, endotoxin inhibition, inhibition of nucleic acid and protein synthesis, and blockage of energy metabolism and other substances.

Purpurin: A natural anthraquinone with multifaceted pharmacological activities

Purpurin is a naturally occurring anthraquinone isolated from the roots of Rubia cordifolia. Historically, it has been used as a red dye. However, its photosensitizing property and biological effects have deciphered its novel application. Purpurin shows antigenotoxic, anticancer, neuromodulatory, and antimicrobial potential associated with antioxidant action in in vivo and in vitro experiments. A robust antioxidant nature of purpurin is responsible for the majority of its pharmacological effects. It produces anti-inflammatory activity by reducing oxidative stress, which is a fundamental property to target diseases involving endoplasmic reticulum and mitochondrial stress. It can cross the blood-brain barrier and produce neuroprotective effects, including antidepressant and anti-Alzheimer action. It shows antimutagenic property via inhibiting essential CYP-450 enzymes. Interestingly, it gets photosensitized by UV-light and produces target-specific ROS-dependent apoptosis in cancer cells. Therefore, it owns cell killing and cell survival potential subject to the influence of external conditions. Hitherto, limited research studies are performed with purpurin to understand its therapeutic potential. Hence, this review describes and discusses different in vivo, in vitro, and in silico studies performed using purpurin. It also covers physicochemical, pharmacokinetics, and toxicology aspects of purpurin. Moreover, in the end, the prospect of purpurin in the management of cancer has also been proposed.

Defective lytic transglycosylase disrupts cell morphogenesis by hindering cell wall de-O-acetylation in Neisseria meningitidis

Lytic transglycosylases (LT) are enzymes involved in peptidoglycan (PG) remodeling. However, their contribution to cell-wall-modifying complexes and their potential as antimicrobial drug targets remains unclear. Here, we determined a high-resolution structure of the LT, an outer membrane lipoprotein from Neisseria species with a disordered active site helix (alpha helix 30). We show that deletion of the conserved alpha-helix 30 interferes with the integrity of the cell wall, disrupts cell division, cell separation, and impairs the fitness of the human pathogen Neisseria meningitidis during infection. Additionally, deletion of alpha-helix 30 results in hyperacetylated PG, suggesting this LtgA variant affects the function of the PG de-O-acetylase (Ape 1). Our study revealed that Ape 1 requires LtgA for optimal function, demonstrating that LTs can modulate the activity of their protein-binding partner. We show that targeting specific domains in LTs can be lethal, which opens the possibility that LTs are useful drug-targets.

Assays for the Enzymes Catalyzing the O-Acetylation of Bacterial Cell Wall Polysaccharides

The polysaccharides that comprise bacterial cell walls are commonly O-acetylated. This modification confers resistance to hydrolases of innate immune systems and/or controls endogenous autolytic activity. Herein, we present protocols for the compositional analysis of bacterial cell wall O-acetylation, and assays for monitoring O-acetyltransferases and O-acetylesterases. The assays are amenable for the development of high-throughput screens in search of inhibitors of the respective enzymes.

Development of a High Throughput Screen for the Identification of Inhibitors of Peptidoglycan O-Acetyltransferases, New Potential Antibacterial Targets

The O-acetylation of peptidoglycan occurs in many Gram-negative and most Gram-positive pathogens and this modification to the essential wall polymer controls the lytic activity of the autolysins, particularly the lytic transglycosylases, and inhibits that of the lysozymes of innate immunity systems. As such, the peptidoglycan O-acetyltransferases PatA/B and OatA are recognized as virulence factors. In this study, we present the high throughput screening of small compound libraries to identify the first known inhibitors of these enzymes. The fluorometric screening assay developed involved monitoring the respective O-acetyltransferases as esterases using 4-methylumbelliferylacetate as substrate. Pilot screens of 3921 compounds validated the usefulness of the HTS protocol. A number of potential inhibitors were identified amongst a total of 145,000 low molecular-weight compounds, some of which were common to both enzymes, while others were unique to each. After eliminating a number of false positives in secondary screens, dose response curves confirmed the apparent specificity of a benzothiazolyl-pyrazolo-pyridine as an inhibitor of Neisseria gonorrhoeae PatB, and several coumarin-based compounds as inhibitors of both this PatB and OatA from Staphylococcus aureus. The benzothiazolyl-pyrazolo-pyridine was determined to be a non-competitive inhibitor of PatB with a Ki of 126 µM. At 177 µg/mL and close to its solubility limit, this compound caused a 90% reduction in growth of N. gonorrhoeae, while growth of Escherichia coli, a bacterium that lacks PatB and, hence, does not produce O-acetylated peptidoglycan, was unaffected. These data provide preliminary proof of concept that peptidoglycan O-acetyltransferases would serve as useful antibacterial targets.

Mechanistic Pathways for Peptidoglycan O-Acetylation and De-O-Acetylation

The post-synthetic O-acetylation of the essential component of bacterial cell walls, peptidoglycan (PG), is performed by many pathogenic bacteria to help them evade the lytic action of innate immunity responses. Occurring at the C-6 hydroxyl of N-acetylmuramoyl residues, this modification to the glycan backbone of PG sterically blocks the activity of lysozymes. As such, the enzyme responsible for this modification in Gram-positive bacteria is recognized as a virulence factor. With Gram-negative bacteria, the O-acetylation of PG provides a means of control of their autolysins at the substrate level. In this review, we discuss the pathways for PG O-acetylation and de-O-acetylation and the structure and function relationship of the O-acetyltransferases and O-acetylesterases that catalyze these reactions. The current understanding of their mechanisms of action is presented and the prospects of targeting these systems for the development of novel therapeutics are explored.

Selective inhibition of monoamine oxidase A by purpurin, an anthraquinone

Monoamine oxidase (MAO) catalyzes the oxidation of monoamines that act as neurotransmitters. During a target-based screening of natural products using two isoforms of recombinant human MAO-A and MAO-B, purpurin (an anthraquinone derivative) was found to potently and selectively inhibit MAO-A, with an IC50 value of 2.50μM, and not to inhibit MAO-B. Alizarin (also an anthraquinone) inhibited MAO-A less potently with an IC50 value of 30.1μM. Furthermore, purpurin was a reversible and competitive inhibitor of MAO-A with a Ki value of 0.422μM. A comparison of their chemical structures suggested the 4-hydroxy group of purpurin might play an important role in its inhibition of MAO-A. Molecular docking simulation showed that the binding affinity of purpurin for MAO-A (-40.0kcal/mol) was higher than its affinity for MAO-B (-33.9kcal/mol), and that Ile 207 and Gly 443 of MAO-A were key residues for hydrogen bonding with purpurin. The findings of this study suggest purpurin is a potent, selective, reversible inhibitor of MAO-A, and that it be considered a new potential lead compound for development of novel reversible inhibitors of MAO-A (RIMAs).

Accumulation of Peptidoglycan O-Acetylation Leads to Altered Cell Wall Biochemistry and Negatively Impacts Pathogenesis Factors of Campylobacter jejuni

Campylobacter jejuni is a leading cause of bacterial gastroenteritis in the developed world. Despite its prevalence, its mechanisms of pathogenesis are poorly understood. Peptidoglycan (PG) is important for helical shape, colonization, and host-pathogen interactions in C. jejuni Therefore, changes in PG greatly impact the physiology of this organism. O-acetylation of peptidoglycan (OAP) is a bacterial phenomenon proposed to be important for proper cell growth, characterized by acetylation of the C6 hydroxyl group of N-acetylmuramic acid in the PG glycan backbone. The OAP gene cluster consists of a PG O-acetyltransferase A (patA) for translocation of acetate into the periplasm, a PG O-acetyltransferase B (patB) for O-acetylation, and an O-acetylpeptidoglycan esterase (ape1) for de-O-acetylation. In this study, reduced OAP in ΔpatA and ΔpatB had minimal impact on C. jejuni growth and fitness under the conditions tested. However, accumulation of OAP in Δape1 resulted in marked differences in PG biochemistry, including O-acetylation, anhydromuropeptide levels, and changes not expected to result directly from Ape1 activity. This suggests that OAP may be a form of substrate level regulation in PG biosynthesis. Ape1 acetylesterase activity was confirmed in vitro using p-nitrophenyl acetate and O-acetylated PG as substrates. In addition, Δape1 exhibited defects in pathogenesis-associated phenotypes, including cell shape, motility, biofilm formation, cell surface hydrophobicity, and sodium deoxycholate sensitivity. Δape1 was also impaired for chick colonization and adhesion, invasion, intracellular survival, and induction of IL-8 production in INT407 cells in vitro The importance of Ape1 in C. jejuni biology makes it a good candidate as an antimicrobial target.

The intrinsic cephalosporin resistome of Listeria monocytogenes in the context of stress response, gene regulation, pathogenesis and therapeutics

Intrinsic resistance to antibiotics is a serious therapeutic problem in the case of many bacterial species. The Gram-positive human pathogen Listeria monocytogenes is intrinsically resistant to broad spectrum cephalosporin antibiotics, which are commonly used in therapy of bacterial infections. Besides three penicillin-binding proteins the intrinsic cephalosporin resistome of L. monocytogenes includes multidrug resistance transporter transporters, proteins involved in peptidoglycan biosynthesis and modification, cell envelope proteins with structural or general detoxification function, cytoplasmic proteins with unknown function and regulatory proteins. Analysis of the regulation of the expression of genes involved in the intrinsic resistance of L. monocytogenes to cephalosporins highlights the high complexity of control of the intrinsic resistance phenotype. The regulation of the transcription of the intrinsic resistome determinants involves the activity of eight regulators, namely LisR, CesR, LiaR, VirR, σ(B) , σ(H) , σ(L) and PrfA, of which the most prominent role play LisR, CesR and σ(B) . Furthermore, the vast majority of the intrinsic resistome determinants contribute to the tolerance of different stress conditions and virulence. A study indicates that O-acetyltransferase OatA is the most promising candidate for co-drug development since an agent targeting OatA should sensitize L. monocytogenes to certain antibiotics, therefore improving the efficacy of listeriosis treatment as well as food preservation measures.

Substrate specificity and kinetic characterization of peptidoglycan O-acetyltransferase B from Neisseria gonorrhoeae

The O-acetylation of the essential cell wall polymer peptidoglycan is a major virulence factor identified in many bacteria, both Gram-positive and Gram-negative, including Staphylococcus aureus, Bacillus anthracis, Neisseria gonorrhoeae, and Neisseria meningitidis. With Gram-negative bacteria, the translocation of acetyl groups from the cytoplasm is performed by an integral membrane protein, PatA, for its transfer to peptidoglycan by O-acetyltransferase PatB, whereas a single bimodal membrane protein, OatA, appears to catalyze both reactions of the process in Gram-positive bacteria. Only phenotypic evidence existed in support of these pathways because no in vitro biochemical assay was available for their analysis, which reflected the complexities of investigating integral membrane proteins that act on a totally insoluble and heterogeneous substrate, such as peptidoglycan. In this study, we present the first biochemical and kinetic analysis of a peptidoglycan O-acetyltransferase using PatB from N. gonorrhoeae as the model system. The enzyme has specificity for muropeptides that possess tri- and tetrapeptide stems on muramyl residues. With chitooligosaccharides as substrates, rates of reaction increase with increasing degrees of polymerization to 5/6. This information will be valuable for the identification and development of peptidoglycan O-acetyltransferase inhibitors that could represent potential leads to novel classes of antibiotics.

Assay for peptidoglycan O-acetyltransferase: a potential new antibacterial target

The O-acetylation of peptidoglycan occurs at the C-6 hydroxyl group of muramoyl residues in many human pathogens, both gram positive and gram negative, such as Staphylococcus aureus and species of Campylobacter, Helicobacter, Neisseria, and Bacillus, including Bacillus anthracis. The process is a maturation event being catalyzed either by integral membrane O-acetylpeptidoglycan transferase (Oat) of gram-positive bacteria or by a two-component peptidoglycan O-acetyltransferase system (PatA/PatB) in gram-negative cells. Here, we describe the development of the first in vitro assay for any peptidoglycan O-acetyltransferase using PatB from Neisseria gonorrhoeae as the model enzyme. This assay is based on the use of chromogenic p-nitrophenyl acetate as the donor substrate and chitooligosaccharides as model acceptor substrates in place of peptidoglycan. The identity of the O-acetylated chitooligosaccharides was confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Rates of transacetylations were determined spectrophotometrically by monitoring p-nitrophenol release after accounting for both spontaneous and enzyme-catalyzed hydrolysis of the acetate donor. Conditions were established for use of the assay in microtiter plate format, and its applicability was demonstrated by determining the first Michaelis-Menten kinetic parameters for PatB. The assay is readily amenable for application in the high-throughput screening for potential inhibitors of peptidoglycan O-acetyltransferases that may prove to be leads for novel classes of antibiotics.

Mechanism of action of Neisseria gonorrhoeae O-acetylpeptidoglycan esterase, an SGNH serine esterase

O-Acetylpeptidoglycan esterase from Neisseria gonorrhoeae functions to release O-acetyl groups from the C-6 position of muramoyl residues in O-acetylated peptidoglycan, thereby permitting the continued metabolism of this essential cell wall heteropolymer. It has been demonstrated to be a serine esterase with sequence similarity to the family CE-3 carbohydrate esterases of the CAZy classification system. In the absence of a three-dimensional structure for any Ape, further knowledge of its structure and function relationship is dependent on modeling and kinetic studies. In this study, we predicted Neisseria gonorrhoeae Ape1a to be an SGNH hydrolase with an adopted α/β-hydrolase fold containing a central twisted four-stranded parallel β-sheet flanked by six α-helices with the putative catalytic triad, Asp-366, His-369, and Ser-80 appropriately aligned within a pocket. The role of eight invariant and highly conserved residues localized to the active site was investigated by site-directed replacements coupled with kinetic characterization and binding studies of the resultant engineered enzymes. Based on these data and theoretical considerations, Gly-236 and Asn-268 were identified as participating at the oxyanion hole to stabilize the tetrahedral species in the reaction mechanism, whereas Gly-78, Asp-79, His-81, Asn-235, Thr-267, and Val-368 are proposed to position appropriately the catalytic residues and participate in substrate binding.

Bacterial cell-wall recycling

Many Gram-negative and Gram-positive bacteria recycle a significant proportion of the peptidoglycan components of their cell walls during their growth and septation. In many--and quite possibly all--bacteria, the peptidoglycan fragments are recovered and recycled. Although cell-wall recycling is beneficial for the recovery of resources, it also serves as a mechanism to detect cell-wall-targeting antibiotics and to regulate resistance mechanisms. In several Gram-negative pathogens, anhydro-MurNAc-peptide cell-wall fragments regulate AmpC β-lactamase induction. In some Gram-positive organisms, short peptides derived from the cell wall regulate the induction of both β-lactamase and β-lactam-resistant penicillin-binding proteins. The involvement of peptidoglycan recycling with resistance regulation suggests that inhibitors of the enzymes involved in the recycling might synergize with cell-wall-targeted antibiotics. Indeed, such inhibitors improve the potency of β-lactams in vitro against inducible AmpC β-lactamase-producing bacteria. We describe the key steps of cell-wall remodeling and recycling, the regulation of resistance mechanisms by cell-wall recycling, and recent advances toward the discovery of cell-wall-recycling inhibitors.