Crystal structure of mycothiol synthase (Rv0819) from Mycobacterium tuberculosis shows structural homology to the GNAT family of N-acetyltransferases

Bacterial protein acetylation: mechanisms, functions, and methods for study

Lysine acetylation is an evolutionarily conserved protein modification that changes protein functions and plays an essential role in many cellular processes, such as central metabolism, transcriptional regulation, chemotaxis, and pathogen virulence. It can alter DNA binding, enzymatic activity, protein-protein interactions, protein stability, or protein localization. In prokaryotes, lysine acetylation occurs non-enzymatically and by the action of lysine acetyltransferases (KAT). In enzymatic acetylation, KAT transfers the acetyl group from acetyl-CoA (AcCoA) to the lysine side chain. In contrast, acetyl phosphate (AcP) is the acetyl donor of chemical acetylation. Regardless of the acetylation type, the removal of acetyl groups from acetyl lysines occurs only enzymatically by lysine deacetylases (KDAC). KATs are grouped into three main superfamilies based on their catalytic domain sequences and biochemical characteristics of catalysis. Specifically, members of the GNAT are found in eukaryotes and prokaryotes and have a core structural domain architecture. These enzymes can acetylate small molecules, metabolites, peptides, and proteins. This review presents current knowledge of acetylation mechanisms and functional implications in bacterial metabolism, pathogenicity, stress response, translation, and the emerging topic of protein acetylation in the gut microbiome. Additionally, the methods used to elucidate the biological significance of acetylation in bacteria, such as relative quantification and stoichiometry quantification, and the genetic code expansion tool (CGE), are reviewed.

Actionable mechanisms of drug tolerance and resistance in Mycobacterium tuberculosis

The emergence of antimicrobial resistance (AMR) across bacterial pathogens presents a serious threat to global health. This threat is further exacerbated in tuberculosis (TB), mainly due to a protracted treatment regimen involving a combination of drugs. A diversity of factors contributes to the emergence of drug resistance in TB, which is caused by the pathogen Mycobacterium tuberculosis (Mtb). While the traditional genetic mutation-driven drug resistance mechanisms operate in Mtb, there are also several additional unique features of drug resistance in this pathogen. Research in the past decade has enriched our understanding of such unconventional factors as efflux pumps, bacterial heterogeneity, metabolic states, and host microenvironment. Given that the discovery of new antibiotics is outpaced by the emergence of drug resistance patterns displayed by the pathogen, newer strategies for combating drug resistance are desperately needed. In the context of TB, such approaches include targeting the efflux capability of the pathogen, modulating the host environment to prevent bacterial drug tolerance, and activating the host anti-mycobacterial pathways. In this review, we discuss the traditional mechanisms of drug resistance in Mtb, newer understandings and the shaping of a set of unconventional approaches to target both the emergence and treatment of drug resistance in TB.

An acetyltransferase effector conserved across Legionella species targets the eukaryotic eIF3 complex to modulate protein translation

The survival of Legionella spp. as intracellular pathogens relies on the combined action of protein effectors delivered inside their eukaryotic hosts by the Dot/Icm (defective in organelle trafficking/intracellular multiplication) type IVb secretion system. The specific repertoire of effector arsenals varies dramatically across over 60 known species of this genera with Legionella pneumophila responsible for most cases of Legionnaires' disease in humans encoding over 360 Dot/Icm effectors. However, a small subset of "core" effectors appears to be conserved across all Legionella species raising an intriguing question of their role in these bacteria's pathogenic strategy, which for most of these effectors remains unknown. L. pneumophila Lpg0103 effector, also known as VipF, represents one of the core effector families that features a tandem of Gcn5-related N-acetyltransferase (GNAT) domains. Here, we present the crystal structure of the Lha0223, the VipF representative from Legionella hackeliae in complex with acetyl-coenzyme A determined to 1.75 Å resolution. Our structural analysis suggested that this effector family shares a common fold with the two GNAT domains forming a deep groove occupied by residues conserved across VipF homologs. Further analysis suggested that only the C-terminal GNAT domain of VipF effectors retains the active site composition compatible with catalysis, whereas the N-terminal GNAT domain binds the ligand in a non-catalytical mode. We confirmed this by in vitro enzymatic assays which revealed VipF activity not only against generic small molecule substrates, such as chloramphenicol, but also against poly-L-lysine and histone-derived peptides. We identified the human eukaryotic translation initiation factor 3 (eIF3) complex co-precipitating with Lpg0103 and demonstrated the direct interaction between the several representatives of the VipF family, including Lpg0103 and Lha0223 with the K subunit of eIF3. According to our data, these interactions involve primarily the C-terminal tail of eIF3-K containing two lysine residues that are acetylated by VipF. VipF catalytic activity results in the suppression of eukaryotic protein translation in vitro, revealing the potential function of VipF "core" effectors in Legionella's pathogenic strategy.IMPORTANCEBy translocating effectors inside the eukaryotic host cell, bacteria can modulate host cellular processes in their favor. Legionella species, which includes the pneumonia-causing Legionella pneumophila, encode a widely diverse set of effectors with only a small subset that is conserved across this genus. Here, we demonstrate that one of these conserved effector families, represented by L. pneumophila VipF (Lpg0103), is a tandem Gcn5-related N-acetyltransferase interacting with the K subunit of human eukaryotic initiation factor 3 complex. VipF catalyzes the acetylation of lysine residues on the C-terminal tail of the K subunit, resulting in the suppression of eukaryotic translation initiation factor 3-mediated protein translation in vitro. These new data provide the first insight into the molecular function of this pathogenic factor family common across Legionellae.

Structural basis for the acetylation mechanism of the Legionella effector VipF

The pathogen Legionella pneumophila, which is the causative agent of Legionnaires' disease, secrets hundreds of effectors into host cells via its Dot/Icm secretion system to subvert host-cell pathways during pathogenesis. VipF, a conserved core effector among Legionella species, is a putative acetyltransferase, but its structure and catalytic mechanism remain unknown. Here, three crystal structures of VipF in complex with its cofactor acetyl-CoA and/or a substrate are reported. The two GNAT-like domains of VipF are connected as two wings by two β-strands to form a U-shape. Both domains bind acetyl-CoA or CoA, but only in the C-terminal domain does the molecule extend to the bottom of the U-shaped groove as required for an active transferase reaction; the molecule in the N-terminal domain folds back on itself. Interestingly, when chloramphenicol, a putative substrate, binds in the pocket of the central U-shaped groove adjacent to the N-terminal domain, VipF remains in an open conformation. Moreover, mutations in the central U-shaped groove, including Glu129 and Asp251, largely impaired the acetyltransferase activity of VipF, suggesting a unique enzymatic mechanism for the Legionella effector VipF.

Molecular basis of antibiotic self-resistance in a bee larvae pathogen

Paenibacillus larvae, the causative agent of the devastating honey-bee disease American Foulbrood, produces the cationic polyketide-peptide hybrid paenilamicin that displays antibacterial and antifungal activity. Its biosynthetic gene cluster contains a gene coding for the N-acetyltransferase PamZ. We show that PamZ acts as self-resistance factor in Paenibacillus larvae by deactivation of paenilamicin. Using tandem mass spectrometry, nuclear magnetic resonance spectroscopy and synthetic diastereomers, we identified the N-terminal amino group of the agmatinamic acid as the N-acetylation site. These findings highlight the pharmacophore region of paenilamicin, which we very recently identified as a ribosome inhibitor. Here, we further determined the crystal structure of PamZ:acetyl-CoA complex at 1.34 Å resolution. An unusual tandem-domain architecture provides a well-defined substrate-binding groove decorated with negatively-charged residues to specifically attract the cationic paenilamicin. Our results will help to understand the mode of action of paenilamicin and its role in pathogenicity of Paenibacillus larvae to fight American Foulbrood.

The authors show that the N-acetyltransferase PamZ acts as a self-resistance factor disabling the antibacterial paenilamicin that is produced by the honey bee larvae pathogen Paenibacillus larvae.

Structural characterization of a GNAT family acetyltransferase from Elizabethkingia anophelis bound to acetyl-CoA reveals a new dimeric interface

General control non-repressible 5 (GCN5)-related N-acetyltransferases (GNATs) catalyse the acetylation of a diverse range of substrates, thereby orchestrating a variety of biological processes within prokaryotes and eukaryotes. GNAT enzymes can catalyze the transfer of an acetyl group from acetyl coenzyme A to substrates such as aminoglycoside antibiotics, amino acids, polyamines, peptides, vitamins, catecholamines, and large macromolecules including proteins. Although GNATs generally exhibit low to moderate sequence identity, they share a conserved catalytic fold and conserved structural motifs. In this current study we characterize the high-resolution X-ray crystallographic structure of a GNAT enzyme bound with acetyl-CoA from Elizabethkingia anophelis, an important multi-drug resistant bacterium. The tertiary structure is comprised of six α-helices and nine β-strands, and is similar with other GNATs. We identify a new and uncharacterized GNAT dimer interface, which is conserved in at least two other unpublished GNAT structures. This suggests that GNAT enzymes can form at least five different types of dimers, in addition to a range of other oligomers including trimer, tetramer, hexamer, and dodecamer assemblies. The high-resolution structure presented in this study is suitable for future in-silico docking and structure–activity relationship studies.

Small-Molecule Acetylation by GCN5-Related N-Acetyltransferases in Bacteria

Acetylation is a conserved modification used to regulate a variety of cellular pathways, such as gene expression, protein synthesis, detoxification, and virulence. Acetyltransferase enzymes transfer an acetyl moiety, usually from acetyl coenzyme A (AcCoA), onto a target substrate, thereby modulating activity or stability. Members of the GCN5- N -acetyltransferase (GNAT) protein superfamily are found in all domains of life and are characterized by a core structural domain architecture. These enzymes can modify primary amines of small molecules or of lysyl residues of proteins. From the initial discovery of antibiotic acetylation, GNATs have been shown to modify a myriad of small-molecule substrates, including tRNAs, polyamines, cell wall components, and other toxins. This review focuses on the literature on small-molecule substrates of GNATs in bacteria, including structural examples, to understand ligand binding and catalysis. Understanding the plethora and versatility of substrates helps frame the role of acetylation within the larger context of bacterial cellular physiology.

Mycothiol acetyltransferase (Rv0819) of Mycobacterium tuberculosis is a potential biomarker for direct diagnosis of tuberculosis using patient serum specimens

Mycobacterium tuberculosis infection constitutes a global threat that results in significant morbidity and mortality worldwide. Efficient and early diagnosis of tuberculosis (TB) is of paramount importance for successful treatment. The aim of the current study is to investigate the mycobacterial mycothiol acetyltransferase Rv0819 as a potential novel biomarker for the diagnosis of active TB infection. The gene encoding Rv0819 was cloned and successfully expressed in Escherichia coli. The recombinant Rv0819 was purified using metal affinity chromatography and was used to raise murine polyclonal antibodies against Rv0819. The raised antibodies were employed for direct detection of Rv0819 in patient serum samples using dot blot assay and competitive enzyme-linked immunosorbent assay (ELISA). Serum samples were obtained from 68 confirmed new TB patients and 35 healthy volunteers as negative controls. The dot blot assay showed sensitivity of 64·7% and specificity of 100%, whereas the competitive ELISA assay showed lower sensitivity (54·4%) and specificity (88·57%). The overall sensitivity of the combined results of the two tests was found to be 89·7%. Overall, the mycobacterial Rv0819 is a potential TB serum biomarker that can be exploited, in combination with other TB biomarkers, for efficient and reliable diagnosis of active TB infection.

SIGNIFICANCE AND IMPACT OF THE STUDY

The early and accurate diagnosis of tuberculosis infection is of paramount importance for initiating treatment and avoiding clinical complications. Most current diagnostic tests have poor sensitivity and/or specificity and in many cases they are too expensive for routine diagnostic testing in resource-limited settings. In the current study, we examined a novel mycobacterial serum biomarker, namely mycothiol acetyltransferase Rv0819. The antigen was detectable in serum specimens of a significant number of tuberculosis patients. This article proves the importance of Rv0819 and paves the way towards its future use as a useful diagnostic marker for tuberculosis infection.

Homocysteine Editing, Thioester Chemistry, Coenzyme A, and the Origin of Coded Peptide Synthesis †

Aminoacyl-tRNA synthetases (AARSs) have evolved “quality control” mechanisms which prevent tRNA aminoacylation with non-protein amino acids, such as homocysteine, homoserine, and ornithine, and thus their access to the Genetic Code. Of the ten AARSs that possess editing function, five edit homocysteine: Class I MetRS, ValRS, IleRS, LeuRS, and Class II LysRS. Studies of their editing function reveal that catalytic modules of these AARSs have a thiol-binding site that confers the ability to catalyze the aminoacylation of coenzyme A, pantetheine, and other thiols. Other AARSs also catalyze aminoacyl-thioester synthesis. Amino acid selectivity of AARSs in the aminoacyl thioesters formation reaction is relaxed, characteristic of primitive amino acid activation systems that may have originated in the Thioester World. With homocysteine and cysteine as thiol substrates, AARSs support peptide bond synthesis. Evolutionary origin of these activities is revealed by genomic comparisons, which show that AARSs are structurally related to proteins involved in coenzyme A/sulfur metabolism and non-coded peptide bond synthesis. These findings suggest that the extant AARSs descended from ancestral forms that were involved in non-coded Thioester-dependent peptide synthesis, functionally similar to the present-day non-ribosomal peptide synthetases.

Biochemical studies of inositol N-acetylglucosaminyltransferase involved in mycothiol biosynthesis in Corynebacterium diphtheria

First-time expression, isolation, biochemical characterization, and mutagenesis studies of a MshA from Corynebacterium diphtheria involved in its mycothiol biosynthesis.

Structural Studies on a Glucosamine/Glucosaminide N-Acetyltransferase

Glucosamine/glucosaminide N-acetyltransferase or GlmA catalyzes the transfer of an acetyl group from acetyl CoA to the primary amino group of glucosamine. The enzyme from Clostridium acetobutylicum is thought to be involved in cell wall rescue. In addition to glucosamine, GlmA has been shown to function on di- and trisaccharides of glucosamine as well. Here we present a structural and kinetic analysis of the enzyme. For this investigation, eight structures were determined to resolutions of 2.0 Å or better. The overall three-dimensional fold of GlmA places it into the tandem GNAT superfamily. Each subunit of the dimer folds into two distinct domains which exhibit high three-dimensional structural similarity. Whereas both domains bind acetyl CoA, it is the C-terminal domain that is catalytically competent. On the basis of the various structures determined in this investigation, two amino acid residues were targeted for further study: Asp 287 and Tyr 297. Although their positions in the active site suggested that they may play key roles in catalysis by functioning as active site bases and acids, respectively, this was not borne out by characterization of the D287N and Y297F variants. The kinetic properties revealed that both residues were important for substrate binding but had no critical roles as acid/base catalysts. Kinetic analyses also indicated that GlmA follows an ordered mechanism with acetyl CoA binding first followed by glucosamine. The product N-acetylglucosamine is then released prior to CoA. The investigation described herein provides significantly new information on enzymes belonging to the tandem GNAT superfamily.

Characterization of the structure and catalytic activity of Legionella pneumophila VipF

The pathogenic bacteria Legionella pneumophila is known to cause Legionnaires' Disease, a severe pneumonia that can be fatal to immunocompromised individuals and the elderly. Shohdy et al. identified the L. pneumophila vacuole sorting inhibitory protein VipF as a putative N-acetyltransferase based on sequence homology. We have characterized the basic structural and functional properties of VipF to confirm this original functional assignment. Sequence conservation analysis indicates two putative CoA-binding regions within VipF. Homology modeling and small angle X-ray scattering suggest a monomeric, dual-domain structure joined by a flexible linker. Each domain contains the characteristic beta-splay motif found in many acetyltransferases, suggesting that VipF may contain two active sites. Docking experiments suggest reasonable acetyl-CoA binding locations within each beta-splay motif. Broad substrate screening indicated that VipF is capable of acetylating chloramphenicol and both domains are catalytically active. Given that chloramphenicol is not known to be N-acetylated, this is a surprising finding suggesting that VipF is capable of O-acetyltransferase activity. Proteins 2016; 84:1422-1430. © 2016 Wiley Periodicals, Inc.

Structure and Functional Diversity of GCN5-Related N-Acetyltransferases (GNAT)

General control non-repressible 5 (GCN5)-related N-acetyltransferases (GNAT) catalyze the transfer of an acyl moiety from acyl coenzyme A (acyl-CoA) to a diverse group of substrates and are widely distributed in all domains of life. This review of the currently available data acquired on GNAT enzymes by a combination of structural, mutagenesis and kinetic methods summarizes the key similarities and differences between several distinctly different families within the GNAT superfamily, with an emphasis on the mechanistic insights obtained from the analysis of the complexes with substrates or inhibitors. It discusses the structural basis for the common acetyltransferase mechanism, outlines the factors important for the substrate recognition, and describes the mechanism of action of inhibitors of these enzymes. It is anticipated that understanding of the structural basis behind the reaction and substrate specificity of the enzymes from this superfamily can be exploited in the development of novel therapeutics to treat human diseases and combat emerging multidrug-resistant microbial infections.

Aminoacyl-tRNA synthetases and the evolution of coded peptide synthesis: the Thioester World

Coded peptide synthesis must have been preceded by a prebiotic stage, in which thioesters played key roles. Fossils of the Thioester World are found in extant aminoacyl-tRNA synthetases (AARSs). Indeed, studies of the editing function reveal that AARSs have a thiol-binding site in their catalytic modules. The thiol-binding site confers the ability to catalyze aminoacyl~coenzyme A thioester synthesis and peptide bond formation. Genomic comparisons show that AARSs are structurally related to proteins involved in sulfur and coenzyme A metabolisms and peptide bond synthesis. These findings point to the origin of the amino acid activation and peptide bond synthesis functions in the Thioester World and suggest that the present-day AARSs had originated from ancestral forms that were involved in noncoded thioester-dependent peptide synthesis.

Bacterial GCN5-Related N-Acetyltransferases: From Resistance to Regulation

The GCN5-related N-acetyltransferases family (GNAT) is an important family of proteins that includes more than 100000 members among eukaryotes and prokaryotes. Acetylation appears as a major regulatory post-translational modification and is as widespread as phosphorylation. N-Acetyltransferases transfer an acetyl group from acetyl-CoA to a large array of substrates, from small molecules such as aminoglycoside antibiotics to macromolecules. Acetylation of proteins can occur at two different positions, either at the amino-terminal end (αN-acetylation) or at the ε-amino group (εN-acetylation) of an internal lysine residue. GNAT members have been classified into different groups on the basis of their substrate specificity, and in spite of a very low primary sequence identity, GNAT proteins display a common and conserved fold. This Current Topic reviews the different classes of bacterial GNAT proteins, their functions, their structural characteristics, and their mechanism of action.

A unique GCN5-related glucosamine N-acetyltransferase region exist in the fungal multi-domain glycoside hydrolase family 3 β-N-acetylglucosaminidase

Glycoside hydrolase (GH) family 3 β-N-acetylglucosaminidases widely exist in the filamentous fungi, which may play a key role in chitin metabolism of fungi. A multi-domain GH family 3 β-N-acetylglucosaminidase from Rhizomucor miehei (RmNag), exhibiting a potential N-acetyltransferase region, has been recently reported to show great potential in industrial applications. In this study, the crystal structure of RmNag was determined at 2.80 Å resolution. The three-dimensional structure of RmNag showed four distinctive domains, which belong to two distinguishable functional regions — a GH family 3 β-N-acetylglucosaminidase region (N-terminal) and a N-acetyltransferase region (C-terminal). From structural and functional analysis, the C-terminal region of RmNag was identified as a unique tandem array linking general control non-derepressible 5 (GCN5)-related N-acetyltransferase (GNAT), which displayed glucosamine N-acetyltransferase activity. Structural analysis of this glucosamine N-acetyltransferase region revealed that a unique glucosamine binding pocket is located in the pantetheine arm binding terminal region of the conserved CoA binding pocket, which is different from all known GNAT members. This is the first structural report of a glucosamine N-acetyltransferase, which provides novel structural information about substrate specificity of GNATs. The structural and functional features of this multi-domain β-N-acetylglucosaminidase could be useful in studying the catalytic mechanism of GH family 3 proteins.

The Physiology and Genetics of Oxidative Stress in Mycobacteria

During infection, Mycobacterium tuberculosis is exposed to a diverse array of microenvironments in the human host, each with its own unique set of redox conditions. Imbalances in the redox environment of the bacillus or the host environment serve as stimuli, which could regulate virulence. The ability of M. tuberculosis to evade the host immune response and cause disease is largely owing to the capacity of the mycobacterium to sense changes in its environment, such as host-generated gases, carbon sources, and pathological conditions, and alter its metabolism and redox balance accordingly for survival. In this article we discuss the redox sensors that are, to date, known to be present in M. tuberculosis , such as the Dos dormancy regulon, WhiB family, anti-σ factors, and MosR, in addition to the strategies present in the bacillus to neutralize free radicals, such as superoxide dismutases, catalase-peroxidase, thioredoxins, and methionine sulfoxide reductases, among others. M. tuberculosis is peculiar in that it appears to have a hierarchy of redox buffers, namely, mycothiol and ergothioneine. We discuss the current knowledge of their biosynthesis, function, and regulation. Ergothioneine is still an enigma, although it appears to have distinct and overlapping functions with mycothiol, which enable it to protect against a wide range of toxic metabolites and free radicals generated by the host. Developing approaches to quantify the intracellular redox status of the mycobacterium will enable us to determine how the redox balance is altered in response to signals and environments that mimic those encountered in the host.

Structural and functional analysis of the yeast N-acetyltransferase Mpr1 involved in oxidative stress tolerance via proline metabolism

Mpr1 (sigma1278b gene for proline-analog resistance 1), which was originally isolated as N-acetyltransferase detoxifying the proline analog L-azetidine-2-carboxylate, protects yeast cells from various oxidative stresses. Mpr1 mediates the L-proline and L-arginine metabolism by acetylating L-Δ(1)-pyrroline-5-carboxylate, leading to the L-arginine-dependent production of nitric oxide, which confers oxidative stress tolerance. Mpr1 belongs to the Gcn5-related N-acetyltransferase (GNAT) superfamily, but exhibits poor sequence homology with the GNAT enzymes and unique substrate specificity. Here, we present the X-ray crystal structure of Mpr1 and its complex with the substrate cis-4-hydroxy-L-proline at 1.9 and 2.3 Å resolution, respectively. Mpr1 is folded into α/β-structure with eight-stranded mixed β-sheets and six α-helices. The substrate binds to Asn135 and the backbone amide of Asn172 and Leu173, and the predicted acetyl-CoA-binding site is located near the backbone amide of Phe138 and the side chain of Asn178. Alanine substitution of Asn178, which can interact with the sulfur of acetyl-CoA, caused a large reduction in the apparent kcat value. The replacement of Asn135 led to a remarkable increase in the apparent Km value. These results indicate that Asn178 and Asn135 play an important role in catalysis and substrate recognition, respectively. Such a catalytic mechanism has not been reported in the GNAT proteins. Importantly, the amino acid substitutions in these residues increased the L-Δ(1)-pyrroline-5-carboxylate level in yeast cells exposed to heat stress, indicating that these residues are also crucial for its physiological functions. These studies provide some benefits of Mpr1 applications, such as the breeding of industrial yeasts and the development of antifungal drugs.

New targets and inhibitors of mycobacterial sulfur metabolism

The identification of new antibacterial targets is urgently needed to address multidrug resistant and latent tuberculosis infection. Sulfur metabolic pathways are essential for survival and the expression of virulence in many pathogenic bacteria, including Mycobacterium tuberculosis. In addition, microbial sulfur metabolic pathways are largely absent in humans and therefore, represent unique targets for therapeutic intervention. In this review, we summarize our current understanding of the enzymes associated with the production of sulfated and reduced sulfur-containing metabolites in Mycobacteria. Small molecule inhibitors of these catalysts represent valuable chemical tools that can be used to investigate the role of sulfur metabolism throughout the Mycobacterial lifecycle and may also represent new leads for drug development. In this light, we also summarize recent progress made in the development of inhibitors of sulfur metabolism enzymes.

Glutathione analogs in prokaryotes

BACKGROUND

Oxygen is both essential and toxic to all forms of aerobic life and the chemical versatility and reactivity of thiols play a key role in both aspects. Cysteine thiol groups have key catalytic functions in enzymes but are readily damaged by reactive oxygen species (ROS). Low-molecular-weight thiols provide protective buffers against the hazards of ROS toxicity. Glutathione is the small protective thiol in nearly all eukaryotes but in prokaryotes the situation is far more complex.

SCOPE OF REVIEW

This review provides an introduction to the diversity of low-molecular-weight thiol protective systems in bacteria. The topics covered include the limitations of cysteine as a protector, the multiple origins and distribution of glutathione biosynthesis, mycothiol biosynthesis and function in Actinobacteria, recent discoveries involving bacillithiol found in Firmicutes, new insights on the biosynthesis and distribution of ergothioneine, and the potential protective roles played by coenzyme A and other thiols.

MAJOR CONCLUSIONS

Bacteria have evolved a diverse collection of low-molecular-weight protective thiols to deal with oxygen toxicity and environmental challenges. Our understanding of how many of these thiols are produced and utilized is still at an early stage.

GENERAL SIGNIFICANCE

Extensive diversity existed among prokaryotes prior to evolution of the cyanobacteria and the development of an oxidizing atmosphere. Bacteria that managed to adapt to life under oxygen evolved, or acquired, the ability to produce a variety of small thiols for protection against the hazards of aerobic metabolism. Many pathogenic prokaryotes depend upon novel thiol protection systems that may provide targets for new antibacterial agents. This article is part of a Special Issue entitled Cellular functions of glutathione.

A novel N-acetylglutamate synthase architecture revealed by the crystal structure of the bifunctional enzyme from Maricaulis maris

Novel bifunctional N-acetylglutamate synthase/kinases (NAGS/K) that catalyze the first two steps of arginine biosynthesis and are homologous to vertebrate N-acetylglutamate synthase (NAGS), an essential cofactor-producing enzyme in the urea cycle, were identified in Maricaulis maris and several other bacteria. Arginine is an allosteric inhibitor of NAGS but not NAGK activity. The crystal structure of M. maris NAGS/K (mmNAGS/K) at 2.7 Å resolution indicates that it is a tetramer, in contrast to the hexameric structure of Neisseria gonorrhoeae NAGS. The quaternary structure of crystalline NAGS/K from Xanthomonas campestris (xcNAGS/K) is similar, and cross-linking experiments indicate that both mmNAGS/K and xcNAGS are tetramers in solution. Each subunit has an amino acid kinase (AAK) domain, which is likely responsible for N-acetylglutamate kinase (NAGK) activity and has a putative arginine binding site, and an N-acetyltransferase (NAT) domain that contains the putative NAGS active site. These structures and sequence comparisons suggest that the linker residue 291 may determine whether arginine acts as an allosteric inhibitor or activator in homologous enzymes in microorganisms and vertebrates. In addition, the angle of rotation between AAK and NAT domains varies among crystal forms and subunits within the tetramer. A rotation of 26° is sufficient to close the predicted AcCoA binding site, thus reducing enzymatic activity. Since mmNAGS/K has the highest degree of sequence homology to vertebrate NAGS of NAGS and NAGK enzymes whose structures have been determined, the mmNAGS/K structure was used to develop a structural model of human NAGS that is fully consistent with the functional effects of the 14 missense mutations that were identified in NAGS-deficient patients.

Reversible acetylation and inactivation of Mycobacterium tuberculosis acetyl-CoA synthetase is dependent on cAMP

Recent proteomics studies have revealed that protein acetylation is an abundant and evolutionarily conserved post-translational modification from prokaryotes to eukaryotes. Although an astonishing number of acetylated proteins have been identified in those studies, the acetyltransferases that target these proteins remain largely unknown. Here we characterized MSMEG_5458, one of the GCN5-related N-acetyltransferases (GNAT's) in Mycobacterium smegmatis, and show that it is a protein acetyltransferase (MsPat) that specifically acetylates the ε-amino group of a highly conserved lysine residue in acetyl-CoA synthetase (ACS) with a k(cat)/K(m) of nearly 10(4) M(-1) s(-1). This acetylation results in the inactivation of ACS activity. Lysine acetylation by MsPat is dependent on 3',5'-cyclic adenosine monophosphate (cAMP), an important second messenger, indicating that MsPat is a downstream target of the intracellular cAMP signaling pathway. To the best of our knowledge, this is the first protein acetyltransferase in mycobacteria that both is dependent on cAMP and targets a central metabolic enzyme by a specific post-translational modification. Since cAMP is synthesized by adenylate cyclases (AC's) that sense various environmental signals, we hypothesize that the acetylation and inactivation of ACS is important for mycobacteria to adjust to environmental changes. In addition, we show that Rv1151c, a sirtuin-like deacetylase in Mycobacterium tuberculosis, reactivates acetylated ACS through an NAD(+)-dependent deacetylation. Therefore, Pat and the sirtuin-like deacetylase in mycobacteria constitute a reversible acetylation system that regulates the activity of ACS.

Crystallographic and mass spectrometric analyses of a tandem GNAT protein from the clavulanic acid biosynthesis pathway

(3R,5R)-Clavulanic acid (CA) is a clinically important inhibitor of Class A beta-lactamases. Sequence comparisons suggest that orf14 of the clavulanic acid biosynthesis gene cluster encodes for an acetyl transferase (CBG). Crystallographic studies reveal CBG to be a member of the emerging structural subfamily of tandem Gcn5-related acetyl transferase (GNAT) proteins. Two crystal forms (C2 and P2(1) space groups) of CBG were obtained; in both forms one molecule of acetyl-CoA (AcCoA) was bound to the N-terminal GNAT domain, with the C-terminal domain being unoccupied by a ligand. Mass spectrometric analyzes on CBG demonstrate that, in addition to one strongly bound AcCoA molecule, a second acyl-CoA molecule can bind to CBG. Succinyl-CoA and myristoyl-CoA displayed the strongest binding to the "second" CoA binding site, which is likely in the C-terminal GNAT domain. Analysis of the CBG structures, together with those of other tandem GNAT proteins, suggest that the AcCoA in the N-terminal GNAT domain plays a structural role whereas the C-terminal domain is more likely to be directly involved in acetyl transfer. The available crystallographic and mass spectrometric evidence suggests that binding of the second acyl-CoA occurs preferentially to monomeric rather than dimeric CBG. The N-terminal AcCoA binding site and the proposed C-terminal acyl-CoA binding site of CBG are compared with acyl-CoA binding sites of other tandem and single domain GNAT proteins.

Catalytic mechanism of bleomycin N-acetyltransferase proposed on the basis of its crystal structure

Bleomycin (Bm) N-acetyltransferase, BAT, is a self-resistance determinant in Bm-producing Streptomyces verticillus ATCC15003. In our present study, we crystallized BAT under both a terrestrial and a microgravity environment in the International Space Station. In addition to substrate-free BAT, the crystal structures of BAT in a binary complex with CoA and in a ternary complex with Bm and CoA were determined. BAT forms a dimer structure via interaction of its C-terminal domains in the monomers. However, each N-terminal domain in the dimer is positioned without mutual interaction. The tunnel observed in the N-terminal domain of BAT has two entrances: one that adopts a wide funnel-like structure necessary to accommodate the metal-binding domain of Bm, and another narrow entrance that accommodates acetyl-CoA (AcCoA). A groove formed on the dimer interface of two BAT C-terminal domains accommodates the DNA-binding domain of Bm. In a ternary complex of BAT, BmA(2), and CoA, a thiol group of CoA is positioned near the primary amine of Bm at the midpoint of the tunnel. This proximity ensures efficient transfer of an acetyl group from AcCoA to the primary amine of Bm. Based on the BAT crystal structure and the enzymatic kinetic study, we propose that the catalytic mode of BAT takes an ordered-like mechanism.

Structures and mechanisms of the mycothiol biosynthetic enzymes

In the past decade, the genes encoding all four enzymes responsible for the biosynthesis of mycothiol in Mycobacterium tuberculosis have been identified. Orthologs of each of these have been stably expressed and structurally characterized. The chemical mechanisms of all the four have also been studied. Because of the unique phylogenetic distribution of mycothiol, and the enzymes responsible for its biosynthesis, these enzymes represent interesting potential targets for antimycobacterial agents.

Biosynthesis and functions of mycothiol, the unique protective thiol of Actinobacteria

Mycothiol (MSH; AcCys-GlcN-Ins) is the major thiol found in Actinobacteria and has many of the functions of glutathione, which is the dominant thiol in other bacteria and eukaryotes but is absent in Actinobacteria. MSH functions as a protected reserve of cysteine and in the detoxification of alkylating agents, reactive oxygen and nitrogen species, and antibiotics. MSH also acts as a thiol buffer which is important in maintaining the highly reducing environment within the cell and protecting against disulfide stress. The pathway of MSH biosynthesis involves production of GlcNAc-Ins-P by MSH glycosyltransferase (MshA), dephosphorylation by the MSH phosphatase MshA2 (not yet identified), deacetylation by MshB to produce GlcN-Ins, linkage to Cys by the MSH ligase MshC, and acetylation by MSH synthase (MshD), yielding MSH. Studies of MSH mutants have shown that the MSH glycosyltransferase MshA and the MSH ligase MshC are required for MSH production, whereas mutants in the MSH deacetylase MshB and the acetyltransferase (MSH synthase) MshD produce some MSH and/or a closely related thiol. Current evidence indicates that MSH biosynthesis is controlled by transcriptional regulation mediated by sigma(B) and sigma(R) in Streptomyces coelicolor. Identified enzymes of MSH metabolism include mycothione reductase (disulfide reductase; Mtr), the S-nitrosomycothiol reductase MscR, the MSH S-conjugate amidase Mca, and an MSH-dependent maleylpyruvate isomerase. Mca cleaves MSH S-conjugates to generate mercapturic acids (AcCySR), excreted from the cell, and GlcN-Ins, used for resynthesis of MSH. The phenotypes of MSH-deficient mutants indicate the occurrence of one or more MSH-dependent S-transferases, peroxidases, and mycoredoxins, which are important targets for future studies. Current evidence suggests that several MSH biosynthetic and metabolic enzymes are potential targets for drugs against tuberculosis. The functions of MSH in antibiotic-producing streptomycetes and in bioremediation are areas for future study.

Rv0802c from Mycobacterium tuberculosis: the first structure of a succinyltransferase with the GNAT fold

Gene rv0802c from Mycobacterium tuberculosis encodes a 218-amino-acid protein and is annotated as a hypothetical protein with homology to GCN5-related N-acetyltransferases. The structure of Rv0802c was determined in an unliganded form to 2.0 A resolution utilizing single-wavelength anomalous dispersion from a samarium soak that resulted in a single bound Sm(3+):citrate(2) complex. The structure confirms that Rv0802c exhibits the GCN5-related N-acetyltransferase fold and revealed a tetramer composed of a dimer of dimers with approximate 222 symmetry. In addition, a bound acetate ion indicated that Rv0802c may utilize a unique acyl donor for the family. The subsequent determination of the structure of Rv0802c in complex with succinyl-CoA to 2.3 A resolution suggests that Rv0802c is the first known GCN5-related N-acetyltransferase family member to utilize succinyl-CoA as a substrate.

Mycothiol: synthesis, biosynthesis and biological functions of the major low molecular weight thiol in actinomycetes

Actinomycetes produce mycothiol as their major low molecular weight thiol, which parallels the functions of glutathione found in prokaryotes and most Gram-negative bacteria. This review covers progress that has so far been made in terms of its distribution, biosynthesis and metabolic functions, as well as chemical syntheses of mycothiol and alternative substrates and inhibitors of mycothiol biosynthesis and mycothiol-dependent enzymes. 152 references are cited.

Structural and enzymatic analysis of MshA from Corynebacterium glutamicum: substrate-assisted catalysis

The glycosyltransferase termed MshA catalyzes the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to 1-L-myo-inositol-1-phosphate in the first committed step of mycothiol biosynthesis. The structure of MshA from Corynebacterium glutamicum was determined both in the absence of substrates and in a complex with UDP and 1-L-myo-inositol-1-phosphate. MshA belongs to the GT-B structural family whose members have a two-domain structure with both domains exhibiting a Rossman-type fold. Binding of the donor sugar to the C-terminal domain produces a 97 degrees rotational reorientation of the N-terminal domain relative to the C-terminal domain, clamping down on UDP and generating the binding site for 1-L-myo-inositol-1-phosphate. The structure highlights the residues important in binding of UDP-N-acetylglucosamine and 1-L-myo-inositol-1-phosphate. Molecular models of the ternary complex suggest a mechanism in which the beta-phosphate of the substrate, UDP-N-acetylglucosamine, promotes the nucleophilic attack of the 3-hydroxyl group of 1-L-myo-inositol-1-phosphate while at the same time promoting the cleavage of the sugar nucleotide bond.

Identification of amino acid residues essential for the yeast N-acetyltransferase Mpr1 activity by site-directed mutagenesis

We previously discovered that the budding yeast Saccharomyces cerevisiae Sigma1278b has the MPR1 gene that confers resistance to the proline analogue azetidine-2-carboxylate (AZC). The MPR1-encoded protein (Mpr1) is an N-acetyltransferase that detoxifies AZC and is a novel member of the GCN5-related N-acetyltransferase (GNAT) superfamily. Mpr1 can reduce intracellular oxidation levels and protect yeast cells from oxidative stress, heat shock, freezing, or ethanol treatment. Here, we analyzed the amino acid residues in Mpr1 involved in substrate binding and catalysis by site-directed mutagenesis. The mutated genes were expressed in Escherichia coli, and the recombinant Strep-tagged fusion proteins were analyzed in terms of AZC resistance and acetyltransferase activity. The replacement of Arg145, which is conserved in the GNAT superfamily, by Ala, Asp, Glu, Gly, or Trp led to a growth defect of transformants grown in the presence of AZC. Kinetic studies demonstrated that these mutations caused a large reduction in the affinity for AZC and acetyl-CoA, suggesting that Arg145 interacts with both substrates. Among seven conserved Tyr residues, one of which may be a catalytic residue in the GNAT superfamily, Tyr166Ala- showed no detectable activity and Tyr166Phe-Mpr1, a remarkable decrease of the k(cat)/K(m) value. This result suggests that Tyr166 is critical for the catalysis.

The crystal structure of N-acetyl-L-glutamate synthase from Neisseria gonorrhoeae provides insights into mechanisms of catalysis and regulation

The crystal structures of N-acetylglutamate synthase (NAGS) in the arginine biosynthetic pathway of Neisseria gonorrhoeae complexed with acetyl-CoA and with CoA plus N-acetylglutamate have been determined at 2.5- and 2.6-A resolution, respectively. The monomer consists of two separately folded domains, an amino acid kinase (AAK) domain and an N-acetyltransferase (NAT) domain connected through a 10-A linker. The monomers assemble into a hexameric ring that consists of a trimer of dimers with 32-point symmetry, inner and outer ring diameters of 20 and 100A, respectively, and a height of 110A(.) Each AAK domain interacts with the cognate domains of two adjacent monomers across two 2-fold symmetry axes and with the NAT domain from a second monomer of the adjacent dimer in the ring. The catalytic sites are located within the NAT domains. Three active site residues, Arg316, Arg425, and Ser427, anchor N-acetylglutamate in a position at the active site to form hydrogen bond interactions to the main chain nitrogen atoms of Cys356 and Leu314, and hydrophobic interactions to the side chains of Leu313 and Leu314. The mode of binding of acetyl-CoA and CoA is similar to other NAT family proteins. The AAK domain, although catalytically inactive, appears to bind arginine. This is the first reported crystal structure of any NAGS, and it provides insights into the catalytic function and arginine regulation of NAGS enzymes.

Evolution of peptidoglycan biosynthesis under the selective pressure of antibiotics in Gram-positive bacteria

Acquisition of resistance to the two classes of antibiotics therapeutically used against Gram-positive bacteria, the glycopeptides and the beta-lactams, has revealed an unexpected flexibility in the peptidoglycan assembly pathway. Glycopeptides select for diversification of the fifth position of stem pentapeptides because replacement of D-Ala by D-lactate or D-Ser at this position prevents binding of the drugs to peptidoglycan precursors. The substitution is generally well tolerated by the classical D,D-transpeptidases belonging to the penicillin-binding protein family, except by low-affinity enzymes. Total elimination of the fifth residue by a D,D-carboxypeptidase requires a novel cross-linking enzyme able to process the resulting tetrapeptide stems. This enzyme, an L,D-transpeptidase, confers cross-resistance to beta-lactams and glycopeptides. Diversification of the side chain of the precursors, presumably in response to the selective pressure of peptidoglycan endopeptidases, is controlled by aminoacyl transferases of the Fem family that redirect specific aminoacyl-tRNAs from translation to peptidoglycan synthesis. Diversification of the side chains has been accompanied by a parallel divergent evolution of the substrate specificity of the L,D-transpeptidases, in contrast to the D,D-transpeptidases, which display an unexpected broad specificity. This review focuses on the role of antibiotics in selecting or counter-selecting diversification of the structure of peptidoglycan precursors and their mode of polymerization.

Steady-state and pre-steady-state kinetic analysis of Mycobacterium smegmatis cysteine ligase (MshC)

Mycobacterium tuberculosis and many other members of the Actinomycetes family produce mycothiol, i.e., 1-d-myo-inosityl-2-(N-acetyl-l-cysteinyl)amido-2-deoxy-alpha-d-glucopyranoside (MSH or AcCys-GlcN-Ins), to act against oxidative and antibiotic stress. The biosynthesis of MSH is essential for cell growth and has been proposed to proceed via a biosynthetic pathway involving four key enzymes, MshA-MshD. The MSH biosynthetic enzymes present potential targets for inhibitor design. With this as a long-term goal, we have carried out a kinetic and mechanistic characterization, using steady-state and pre-steady-state approaches, of the recombinant Mycobacterium smegmatis MshC. MshC catalyzes the ATP-dependent condensation of GlcN-Ins and cysteine to form Cys-GlcN-Ins. Initial velocity and inhibition studies show that the steady-state kinetic mechanism of MshC is a Bi Uni Uni Bi Ping Pong mechanism, with ATP binding followed by cysteine binding, release of PPi, binding of GlcN-Ins, followed by the release of Cys-GlcN-Ins and AMP. The steady-state kinetic parameters were determined to be kcat equal to 3.15 s-1, and Km values of 1.8, 0.1, and 0.16 mM for ATP, cysteine, and GlcN-Ins, respectively. A stable bisubstrate analogue, 5'-O-[N-(l-cysteinyl)sulfamonyl]adenosine, exhibits competitive inhibition versus ATP and noncompetitive inhibition versus cysteine, with an inhibition constant of approximately 306 nM versus ATP. Single-turnover reactions of the first and second half reactions were determined using rapid-quench techniques, giving rates of approximately 9.4 and approximately 5.2 s-1, respectively, consistent with the cysteinyl adenylate being a kinetically competent intermediate in the reaction by MshC.

Drug targets in mycobacterial sulfur metabolism

The identification of new antibacterial targets is urgently needed to address multidrug resistant and latent tuberculosis infection. Sulfur metabolic pathways are essential for survival and the expression of virulence in many pathogenic bacteria, including Mycobacterium tuberculosis. In addition, microbial sulfur metabolic pathways are largely absent in humans and therefore, represent unique targets for therapeutic intervention. In this review, we summarize our current understanding of the enzymes associated with the production of sulfated and reduced sulfur-containing metabolites in Mycobacteria. Small molecule inhibitors of these catalysts represent valuable chemical tools that can be used to investigate the role of sulfur metabolism throughout the Mycobacterial lifecycle and may also represent new leads for drug development. In this light, we also summarize recent progress in the development of inhibitors of sulfur metabolism enzymes.

A mycothiol synthase mutant of Mycobacterium tuberculosis has an altered thiol-disulfide content and limited tolerance to stress

Mycothiol (MSH) (acetyl-Cys-GlcN-Ins) is the major low-molecular-mass thiol in Mycobacterium tuberculosis. MSH has antioxidant activity, can detoxify a variety of toxic compounds, and helps to maintain the reducing environment of the cell. The production of MSH provides a potential novel target for tuberculosis treatment. Biosynthesis of MSH requires at least four genes. To determine which of these genes is essential in M. tuberculosis, we have been constructing targeted gene disruptions. Disruption in the mshC gene is lethal to M. tuberculosis, while disruption in the mshB gene results in MSH levels 20 to 100% of those of the wild type. For this study, we have constructed a targeted gene disruption in the mshD gene that encodes mycothiol synthase, the final enzyme in MSH biosynthesis. The mshD mutant produced approximately 1% of normal MSH levels but high levels of the MshD substrate Cys-GlcN-Ins and the novel thiol N-formyl-Cys-GlcN-Ins. Although N-formyl-Cys-GlcN-Ins was maintained in a highly reduced state, Cys-GlcN-Ins was substantially oxidized. In both the wild type and the mshD mutant, cysteine was predominantly oxidized. The M. tuberculosis mshD mutant grew poorly on agar plates lacking catalase and oleic acid and in low-pH media and had heightened sensitivity to hydrogen peroxide. The inability of the mshD mutant to survive and grow in macrophages may be associated with its altered thiol-disulfide status. It appears that N-formyl-Cys-GlcN-Ins serves as a weak surrogate for MSH but is not sufficient to support normal growth of M. tuberculosis under stress conditions such as those found within the macrophage.

Biochemistry of the initial steps of mycothiol biosynthesis

Mycothiol is the major thiol produced by mycobacteria and is required for growth of Mycobacterium tuberculosis. The final three steps in the biosynthesis of mycothiol have been fully elucidated but the initial steps have been unclear. A glycosyltransferase, MshA, is required for production of the mycothiol precursor, 1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol, but its substrates and immediate products were unknown. In this study, we show that the N-acetylglucosamine donor is UDP-N-acetylglucosamine and that the N-acetylglucosamine acceptor is 1L-myo-inositol 1-phosphate. The reaction generates UDP and 1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol 3-phosphate. Using cell-free extracts of M. smegmatis mc(2)155, little activity was obtained with myo-inositol, 1D-myo-inositol 1-phosphate, or myo-inositol 2-phosphate as the N-acetylglucosamine acceptor. A phosphatase, designated MshA2, is required to dephosphorylate 1-O-(2-acetamido-2-deoxy-alpha-glucopyranosyl)-D-myo-inositol 3-phosphate to produce 1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol. The latter is deacetylated, ligated with cysteine, and the cysteinyl amino group acetylated by acetyl-CoA to complete the mycothiol biosynthesis pathway. Uptake and concentration of myo-[14C]inositol is rapid in Mycobacterium smegmatis and leads to production of radiolabeled inositol 1-phosphate and mycothiol. This demonstrates the presence of a myo-inositol transporter and a kinase that generates 1L-myo-inositol 1-phosphate. The biochemical pathway of mycothiol biosynthesis is now fully elucidated.

Crystal structure of TDP-fucosamine acetyltransferase (WecD) from Escherichia coli, an enzyme required for enterobacterial common antigen synthesis

Enterobacterial common antigen (ECA) is a polysaccharide found on the outer membrane of virtually all gram-negative enteric bacteria and consists of three sugars, N-acetyl-d-glucosamine, N-acetyl-d-mannosaminuronic acid, and 4-acetamido-4,6-dideoxy-d-galactose, organized into trisaccharide repeating units having the sequence -->3)-alpha-d-Fuc4NAc-(1-->4)-beta-d-ManNAcA-(1-->4)-alpha-d-GlcNAc-(1-->. While the precise function of ECA is unknown, it has been linked to the resistance of Shiga-toxin-producing Escherichia coli (STEC) O157:H7 to organic acids and the resistance of Salmonella enterica to bile salts. The final step in the synthesis of 4-acetamido-4,6-dideoxy-d-galactose, the acetyl-coenzyme A (CoA)-dependent acetylation of the 4-amino group, is carried out by TDP-fucosamine acetyltransferase (WecD). We have determined the crystal structure of WecD in apo form at a 1.95-Angstrom resolution and bound to acetyl-CoA at a 1.66-Angstrom resolution. WecD is a dimeric enzyme, with each monomer adopting the GNAT N-acetyltransferase fold, common to a number of enzymes involved in acetylation of histones, aminoglycoside antibiotics, serotonin, and sugars. The crystal structure of WecD, however, represents the first structure of a GNAT family member that acts on nucleotide sugars. Based on this cocrystal structure, we have used flexible docking to generate a WecD-bound model of the acetyl-CoA-TDP-fucosamine tetrahedral intermediate, representing the structure during acetyl transfer. Our structural data show that WecD does not possess a residue that directly functions as a catalytic base, although Tyr208 is well positioned to function as a general acid by protonating the thiolate anion of coenzyme A.

Rv0802c acetyltransferase from Mycobacterium tuberculosis H37Rv

Rv0802c acetyltransferase is a mycobacterial RNase E-associated protein. 6His and FLAG-tagged acetyltransferase was cloned from Mycobacterium tuberculosis H37Rv, expressed in Escherichia coli and partially purified. It is a 25 kDa protein showing a modest sequence homology with other acetyltransferases. The R-X-X-G-X-G sequence for acetyl-coenzyme A recognition and binding can be found in the molecule.

Cloning and preliminary characterization of lh3 gene encoding a putative acetyltransferase from a rifamycin SV-producing strain Amycolatopsis mediterranei

An ORF located immediately downstream of glnR gene was cloned from Amycolatopsis mediterranei U32 and was named lh3. Sequence analysis revealed that lh3 encodes a putative acetyltransferase, which shows high amino acid sequence similarities to the mycothiol synthase (MshD) from other actinomycetes. For functional analysis, mutation in lh3 gene was generated by gene replacement with an apramycin resistance gene through homologous recombination. Compared with the wild type strain, the resulting mutant was more sensitive to H2O2, apramycin and erythromycin by two- to three-fold. These results suggest that the lh3 gene plays an important role in the course of detoxification in A. mediterranei U32.

The substrate-induced conformational change of Mycobacterium tuberculosis mycothiol synthase

The structure of the ternary complex of mycothiol synthase from Mycobacterium tuberculosis with bound desacetylmycothiol and CoA was determined to 1.8 A resolution. The structure of the acetyl-CoA-binary complex had shown an active site groove that was several times larger than its substrate. The structure of the ternary complex reveals that mycothiol synthase undergoes a large conformational change in which the two acetyltransferase domains are brought together through shared interactions with the functional groups of desacetylmycothiol, thereby decreasing the size of this large central groove. A comparison of the binary and ternary structures illustrates many of the features that promote catalysis. Desacetylmycothiol is positioned with its primary amine in close proximity and in the proper orientation for direct nucleophilic attack on the si-face of the acetyl group of acetyl-CoA. Glu-234 and Tyr-294 are positioned to act as a general base and general acid to promote acetyl transfer. In addition, this structure provides further evidence that the N-terminal acetyltransferase domain no longer has enzymatic activity and is vestigial in nature.

A mycothiol synthase mutant of Mycobacterium smegmatis produces novel thiols and has an altered thiol redox status

Mycobacteria and other actinomycetes do not produce glutathione but make mycothiol (MSH; AcCys-GlcN-Ins) that has functions similar to those of glutathione and is essential for growth of Mycobacterium tuberculosis. Mycothiol synthase (MshD) catalyzes N acetylation of Cys-GlcN-Ins to produce MSH in Mycobacterium smegmatis mc2155, and Cys-GlcN-Ins is maintained at a low level. The mycothiol synthase mutant, the mshD::Tn5 mutant, produces high levels of Cys-GlcN-Ins along with two novel thiols, N-formyl-Cys-GlcN-Ins and N-succinyl-Cys-GlcN-Ins, and a small amount of MSH. The nonenzymatic reaction of acyl-coenzyme A (CoA) with Cys-GlcN-Ins to produce acyl-Cys-GlcN-Ins is a facile reaction under physiologic conditions, with succinyl-CoA being an order of magnitude more reactive than acetyl-CoA. The uncatalyzed reaction rates are adequate to account for the observed production of N-succinyl-Cys-GlcN-Ins and MSH under physiologic conditions. It was shown that the N-acyl-Cys-GlcN-Ins compounds are maintained in a substantially reduced state in the mutant but that Cys-GlcN-Ins exists in disulfide forms at 5 to 40% at different stages of growth. MSH was able to facilitate reduction of N-succinyl-Cys-GlcN-Ins disulfide through thiol-disulfide exchange, but N-formyl-Cys-GlcN-Ins was ineffective. The oxidized state of Cys-GlcN-Ins in cells appears to result from a high susceptibility to autoxidation and a low capacity of the cell to reduce its disulfide forms. The mutant exhibited no enhanced sensitivity to hydrogen peroxide, tert-butyl hydroperoxide, or cumene hydroperoxide relative to the parent strain, suggesting that the most abundant thiol, N-formyl-Cys-GlcN-Ins, functions as a substitute for MSH.

Investigation of the catalytic triad of arylamine N-acetyltransferases: essential residues required for acetyl transfer to arylamines

The NATs (arylamine N-acetyltransferases) are a well documented family of enzymes found in both prokaryotes and eukaryotes. NATs are responsible for the acetylation of a range of arylamine, arylhydrazine and hydrazine compounds. We present here an investigation into the catalytic triad of residues (Cys-His-Asp) and other structural features of NATs using a variety of methods, including site-directed mutagenesis, X-ray crystallography and bioinformatics analysis, in order to investigate whether each of the residues of the catalytic triad is essential for catalytic activity. The catalytic triad of residues, Cys-His-Asp, is a well defined motif present in several families of enzymes. We mutated each of the catalytic residues in turn to investigate the role they play in catalysis. We also mutated a key residue, Gly126, implicated in acetyl-CoA binding, to examine the effects on acetylation activity. In addition, we have solved the structure of a C70Q mutant of Mycobacterium smegmatis NAT to a resolution of 1.45 A (where 1 A=0.1 nm). This structure confirms that the mutated protein is correctly folded, and provides a structural model for an acetylated NAT intermediate. Our bioinformatics investigation analysed the extent of sequence conservation between all eukaryotic and prokaryotic NAT enzymes for which sequence data are available. This revealed several new sequences, not yet reported, of NAT paralogues. Together, these studies have provided insight into the fundamental core of NAT enzymes, and the regions where sequence differences account for the functional diversity of this family. We have confirmed that each of the three residues of the triad is essential for acetylation activity.

Cloning, purification, crystallization and preliminary crystallographic analysis of a hypothetical acetyltransferase from Pyrococcus furiosus

The GCN5-related N-acetyltransferase (GNAT) superfamily has a primordial role in cellular processes such as transcription initiation and regulation by histone acetylation, aminoglycoside resistance and melatonin metabolism. To date, no acetyltransferase from the archaeal domain of life has been studied. This paper describes the cloning, expression, purification and crystallization of a Pyrococcus furiosus hypothetical acetyltransferase PfGNAT (MW = 22 007 Da). The crystals belong to space group P622, with one molecule in the asymmetric unit and unit-cell parameters a = b = 82.6, c = 105.92 A, alpha = beta = 90, gamma = 120 degrees. Crystals diffract X-rays to 3.0 A resolution on a synchrotron-radiation source. Determination of this structure will provide new insights into the substrate-specificity of this acetyltransferase and the thermal stability of the N-acetyltransferase domain.

Biological chemistry of naturally occurring thiols of microbial and marine origin

The presence of thiols in living systems is critical for the maintenance of cellular redox potentials and protein thiol-disulfide ratios, as well as for the protection of cells from reactive oxygen species. In addition to the well-studied tripeptide glutathione (gamma-Glu-Cys-Gly), a number of compounds have been identified that contribute to these essential cellular roles. This review provides a survey of the chemistry and biochemistry of several critically important and naturally occurring intracellular thiols such as coenzyme M, trypanothione, mycothiol, ergothioneine, and the ovothiols. Coenzyme M is a key thiol required for methane production in methogenic bacteria. Trypanothione and mycothiol are very important to the biochemistry of a number of human pathogens, and the enzymes utilizing these thiols have been recognized as important novel drug targets. Ergothioneine, although synthesized by fungi and the Actinomycetales bacteria, is present at significant physiological levels in humans and may contribute to single electron redox reactions in cells. The ovothiols appear to function as important modulators of reactive oxygen toxicity and appear to serve as small molecule mimics of glutathione peroxidase, a key enzyme in the detoxification of reactive oxygen species.