Structural basis of CTP-dependent peptide bond formation in coenzyme A biosynthesis catalyzed by Escherichia coli PPC synthetase

Functions of Coenzyme A and Acyl-CoA in Post-Translational Modification and Human Disease

Coenzyme A (CoA) is synthesized from pantothenate, L-cysteine and adenosine triphosphate (ATP), and plays a vital role in diverse physiological processes. Protein acylation is a common post-translational modification (PTM) that modifies protein structure, function and interactions. It occurs via the transfer of acyl groups from acyl-CoAs to various amino acids by acyltransferase. The characteristics and effects of acylation vary according to the origin, structure, and location of the acyl group. Acetyl-CoA, formyl-CoA, lactoyl-CoA, and malonyl-CoA are typical acyl group donors. The major acyl donor, acyl-CoA, enables modifications that impart distinct biological functions to both histone and non-histone proteins. These modifications are crucial for regulating gene expression, organizing chromatin, managing metabolism, and modulating the immune response. Moreover, CoA and acyl-CoA play significant roles in the development and progression of neurodegenerative diseases, cancer, cardiovascular diseases, and other health conditions. The goal of this review was to systematically describe the types of commonly utilized acyl-CoAs, their functions in protein PTM, and their roles in the progression of human diseases.

Association of Radical Chemistry with LanD Flavoprotein Activity for C-Terminal Macrocyclization of a Ribosomal Peptide by Formation of an Unsaturated Thioether Residue

LanD flavoproteins catalyze oxidative decarboxylation of the C-terminal Cys residue of a peptide to produce an enethiol. This enethiol is highly reactive and can be coupled with an upstream dehydroamino acid through Michael addition to form S-[2-aminovinyl](3-methyl)cysteine, an unsaturated thioether residue known to be characteristic of an array of C-terminally macrocyclized, ribosomally synthesized and posttranslationally modified peptides (RiPPs). Based on a two-stage bioinformatics mining of posttranslational modifications (PTMs) related to C-terminal Cys processing, we report herein that LanD activity can couple with radical S-adenosylmethionine chemistry to provide a new unsaturated thioether residue, S-[2-aminovinyl]-3-carbamoylcysteine, by conjugating the resultant enethiol with Cβ of the Asn residue in the C-terminal NxxC motif of a peptide for macrocyclization. This study furthers our understanding of the variety of PTMs involved in creating the structure diversity of macrocyclic RiPPs.

Structure prediction and discovery of inhibitors against phosphopantothenoyl cysteine synthetase of Acinetobacter baumannii

Acinetobacter baumannii is an extremely dangerous multidrug-resistant (MDR) gram-negative pathogen which poses a serious life-threatening risk in immunocompromised patients. Phosphopantothenoyl cysteine synthetase (PPCS) catalyzes the formation of an amide bond between L-cysteine and phosphopantothenic acid (PPA) to form 4'- Phosphopantothenoylcysteine during Coenzyme A (CoA) biosynthesis. CoA is a crucial cofactor for cellular survival and inhibiting its synthesis will result in cell death. Bacterial PPCS differs from eukaryotic PPCS in a number of ways like it exists as a C-terminal domain of a PPCDC/PPCS fusion protein whereas eukaryotic PPCS exists as an independent protein. This difference makes it an attractive drug target. For which a conventional iterative approach of SBDD (structure-based drug design) was used, which began with three-dimensional structure prediction of AbPPCS using PHYRE 2.0. A database of FDA-approved compounds (Drug Bank) was then screened against the target of interest by means of docking score and glide energy, leading to the identification of 6 prominent drug candidates. The shortlisted 6 molecules were further subjected to all-atom MD simulation studies in explicit-solvent conditions (using AMBER force field). The MD simulation studies revealed that the ligands DB65103, DB449108 and DB443210, maintained several H-bonds with intense van der Waals contacts at the active site of the protein with high binding free energies: -11.42 kcal/mol, -10.49 kcal/mol and -10.98 kcal/mol, respectively, calculated via MM-PBSA method. Overall, binding of these compounds at the active site was found to be the most stable and robust highlighting the potential of these compounds to serve as antibacterials.Communicated by Ramaswamy H. Sarma.

The Coenzyme A Level Modulator Hopantenate (HoPan) Inhibits Phosphopantotenoylcysteine Synthetase Activity

The pantothenate analogue hopantenate (HoPan) is widely used as a modulator of coenzyme A (CoA) levels in cell biology and disease models─especially for pantothenate kinase associated neurodegeneration (PKAN), a genetic disease rooted in impaired CoA metabolism. This use of HoPan was based on reports that it inhibits pantothenate kinase (PanK), the first enzyme of CoA biosynthesis. Using a combination of in vitro enzyme kinetic studies, crystal structure analysis, and experiments in a typical PKAN cell biology model, we demonstrate that instead of inhibiting PanK, HoPan relies on it for metabolic activation. Once phosphorylated, HoPan inhibits the next enzyme in the CoA pathway─phosphopantothenoylcysteine synthetase (PPCS)─through formation of a nonproductive substrate complex. Moreover, the obtained structure of the human PPCS in complex with the inhibitor and activating nucleotide analogue provides new insights into the catalytic mechanism of PPCS enzymes─including the elusive binding mode for cysteine─and reveals the functional implications of mutations in the human PPCS that have been linked to severe dilated cardiomyopathy. Taken together, this study demonstrates that the molecular mechanism of action of HoPan is more complex than previously thought, suggesting that the results of studies in which it is used as a tool compound must be interpreted with care. Moreover, our findings provide a clear framework for evaluating the various factors that contribute to the potency of CoA-directed inhibitors, one that will prove useful in the future rational development of potential therapies of both human genetic and infectious diseases.

Targeting Mycobacterium tuberculosis CoaBC through Chemical Inhibition of 4'-Phosphopantothenoyl-l-cysteine Synthetase (CoaB) Activity

Coenzyme A (CoA) is a ubiquitous cofactor present in all living cells and estimated to be required for up to 9% of intracellular enzymatic reactions. Mycobacterium tuberculosis (Mtb) relies on its own ability to biosynthesize CoA to meet the needs of the myriad enzymatic reactions that depend on this cofactor for activity. As such, the pathway to CoA biosynthesis is recognized as a potential source of novel tuberculosis drug targets. In prior work, we genetically validated CoaBC as a bactericidal drug target in Mtb in vitro and in vivo. Here, we describe the identification of compound 1f, a small molecule inhibitor of the 4'-phosphopantothenoyl-l-cysteine synthetase (PPCS; CoaB) domain of the bifunctional Mtb CoaBC, and show that this compound displays on-target activity in Mtb. Compound 1f was found to inhibit CoaBC uncompetitively with respect to 4'-phosphopantothenate, the substrate for the CoaB-catalyzed reaction. Furthermore, metabolomic profiling of wild-type Mtb H37Rv following exposure to compound 1f produced a signature consistent with perturbations in pantothenate and CoA biosynthesis. As the first report of a direct small molecule inhibitor of Mtb CoaBC displaying target-selective whole-cell activity, this study confirms the druggability of CoaBC and chemically validates this target.

Inhibiting Mycobacterium tuberculosis CoaBC by targeting an allosteric site

Coenzyme A (CoA) is a fundamental co-factor for all life, involved in numerous metabolic pathways and cellular processes, and its biosynthetic pathway has raised substantial interest as a drug target against multiple pathogens including Mycobacterium tuberculosis. The biosynthesis of CoA is performed in five steps, with the second and third steps being catalysed in the vast majority of prokaryotes, including M. tuberculosis, by a single bifunctional protein, CoaBC. Depletion of CoaBC was found to be bactericidal in M. tuberculosis. Here we report the first structure of a full-length CoaBC, from the model organism Mycobacterium smegmatis, describe how it is organised as a dodecamer and regulated by CoA thioesters. A high-throughput biochemical screen focusing on CoaB identified two inhibitors with different chemical scaffolds. Hit expansion led to the discovery of potent and selective inhibitors of M. tuberculosis CoaB, which we show to bind to a cryptic allosteric site within CoaB.

Vitamin in the Crosshairs: Targeting Pantothenate and Coenzyme A Biosynthesis for New Antituberculosis Agents

Despite decades of dedicated research, there remains a dire need for new drugs against tuberculosis (TB). Current therapies are generations old and problematic. Resistance to these existing therapies results in an ever-increasing burden of patients with disease that is difficult or impossible to treat. Novel chemical entities with new mechanisms of action are therefore earnestly required. The biosynthesis of coenzyme A (CoA) has long been known to be essential in Mycobacterium tuberculosis (Mtb), the causative agent of TB. The pathway has been genetically validated by seminal studies in vitro and in vivo. In Mtb, the CoA biosynthetic pathway is comprised of nine enzymes: four to synthesize pantothenate (Pan) from l-aspartate and α-ketoisovalerate; five to synthesize CoA from Pan and pantetheine (PantSH). This review gathers literature reports on the structure/mechanism, inhibitors, and vulnerability of each enzyme in the CoA pathway. In addition to traditional inhibition of a single enzyme, the CoA pathway offers an antimetabolite strategy as a promising alternative. In this review, we provide our assessment of what appear to be the best targets, and, thus, which CoA pathway enzymes present the best opportunities for antitubercular drug discovery moving forward.

Structural insights into Escherichia coli phosphopantothenoylcysteine synthetase by native ion mobility-mass spectrometry

CoaBC, part of the vital coenzyme A biosynthetic pathway in bacteria, has recently been validated as a promising antimicrobial target. In this work, we employed native ion mobility-mass spectrometry to gain structural insights into the phosphopantothenoylcysteine synthetase domain of E. coli CoaBC. Moreover, native mass spectrometry was validated as a screening tool to identify novel inhibitors of this enzyme, highlighting the utility and versatility of this technique both for structural biology and for drug discovery.

Crystallographic Analysis of the Catalytic Mechanism of Phosphopantothenoylcysteine Synthetase from Saccharomyces cerevisiae

Phosphopantothenoylcysteine (PPC) synthetase (PPCS) catalyzes nucleoside triphosphate-dependent condensation reaction between 4'-phosphopantothenate (PPA) and l-cysteine to form PPC in CoA biosynthesis. The catalytic mechanism of PPCS has not been resolved yet. Coenzyme A biosynthesis protein 2 (Cab2) possesses activity of PPCS in Saccharomyces cerevisiae. Our enzymatic assays suggest that Cab2 could utilize both ATP and CTP to activate PPA in vitro. The results of isothermal titration calorimetry indicate that PPA, CTP, and ATP could bind to Cab2 individually, with PPA having the highest binding affinity. To provide further insight into the catalytic mechanism of Cab2, we determined the crystal structures of Cab2 and its complex with PPA, the reaction intermediate 4'-phosphopantothenoyl-CMP, the final reaction product PPC, and the product analogue phosphopantothenoylcystine. Except for PPA, all other ligands were generated in situ and present in the active-site pocket of Cab2. Structures of Cab2 in complex with ligands provide insight into substrates binding and its catalytic mechanism. Analysis of structures indicates that the carboxyl of PPA-moiety of ligands and the γ-amino group of Asn97 possess different conformations in these complex structures. The cysteine/cystine/serine selectivity assays for Cab2 indicate that the amino group rather than the thiol group of l-cysteine attacks the carbonyl of 4'-phosphopantothenoyl-CMP to form PPC. Based on structural and biochemical data, the catalytic mechanism of Cab2 was proposed for the first time.

Mutations in PPCS, Encoding Phosphopantothenoylcysteine Synthetase, Cause Autosomal-Recessive Dilated Cardiomyopathy

Coenzyme A (CoA) is an essential metabolic cofactor used by around 4% of cellular enzymes. Its role is to carry and transfer acetyl and acyl groups to other molecules. Cells can synthesize CoA de novo from vitamin B5 (pantothenate) through five consecutive enzymatic steps. Phosphopantothenoylcysteine synthetase (PPCS) catalyzes the second step of the pathway during which phosphopantothenate reacts with ATP and cysteine to form phosphopantothenoylcysteine. Inborn errors of CoA biosynthesis have been implicated in neurodegeneration with brain iron accumulation (NBIA), a group of rare neurological disorders characterized by accumulation of iron in the basal ganglia and progressive neurodegeneration. Exome sequencing in five individuals from two unrelated families presenting with dilated cardiomyopathy revealed biallelic mutations in PPCS, linking CoA synthesis with a cardiac phenotype. Studies in yeast and fruit flies confirmed the pathogenicity of identified mutations. Biochemical analysis revealed a decrease in CoA levels in fibroblasts of all affected individuals. CoA biosynthesis can occur with pantethine as a source independent from PPCS, suggesting pantethine as targeted treatment for the affected individuals still alive.

Biosynthesis of Pantothenic Acid and Coenzyme A

Pantothenate is vitamin B5 and is the key precursor for the biosynthesis of coenzyme A (CoA), a universal and essential cofactor involved in a myriad of metabolic reactions, including the synthesis of phospholipids, the synthesis and degradation of fatty acids, and the operation of the tricarboxylic acid cycle. CoA is also the only source of the phosphopantetheine prosthetic group for enzymes that shuttle intermediates between the active sites of enzymes involved in fatty acid, nonribosomal peptide, and polyketide synthesis. Pantothenate can be synthesized de novo and/or transported into the cell through a pantothenatepermease. Pantothenate uptake is essential for those organisms that lack the genes to synthesize this vitamin. The intracellular levels of CoA are controlled by the balance between synthesis and degradation. In particular, CoA is assembled in five enzymatic steps, starting from the phosphorylation of pantothenate to phosphopantothenatecatalyzed by pantothenate kinase, the product of the coaA gene. In some bacteria, the production of phosphopantothenate by pantothenate kinase is the rate limiting and most regulated step in the biosynthetic pathway. CoA synthesis additionally networks with other vitamin-associated pathways, such as thiamine and folic acid.

The antibiotic CJ-15,801 is an antimetabolite that hijacks and then inhibits CoA biosynthesis

The natural product CJ-15,801 is an inhibitor of Staphylococcus aureus, but not other bacteria. Its close structural resemblance to pantothenic acid, the vitamin precursor of coenzyme A (CoA), and its Michael acceptor moiety suggest that it irreversibly inhibits an enzyme involved in CoA biosynthesis or utilization. However, its mode of action and the basis for its specificity have not been elucidated to date. We demonstrate that CJ-15,801 is transformed by the uniquely selective S. aureus pantothenate kinase, the first CoA biosynthetic enzyme, into a substrate for the next enzyme, phosphopantothenoylcysteine synthetase, which is inhibited through formation of a tight-binding structural mimic of its native reaction intermediate. These findings reveal CJ-15,801 as a vitamin biosynthetic pathway antimetabolite with a mechanism similar to that of the sulfonamide antibiotics and highlight CoA biosynthesis as a viable antimicrobial drug target.

Crystal structure of a trapped catalytic intermediate suggests that forced atomic proximity drives the catalysis of mIPS

1-L-myo-inositol-phosphate synthase (mIPS) catalyzes the first step of the unique, de novo pathway of inositol biosynthesis. However, details about the complex mIPS catalytic mechanism, which requires oxidation, enolization, intramolecular aldol cyclization, and reduction, are not fully known. To gain further insight into this mechanism, we determined the crystal structure of the wild-type mIPS from Archaeoglobus fulgidus at 1.7 Å, as well as the crystal structures of three active-site mutants. Additionally, we obtained the structure of mIPS with a trapped 5-keto-glucose-6-phosphate intermediate at 2 Å resolution by a novel (to our knowledge) process of activating the crystal at high temperature. A comparison of all of the crystal structures of mIPS described in this work suggests a novel type of catalytic mechanism that relies on the forced atomic proximity of functional groups. The lysine cluster is contained in a small volume in the active site, where random motions of these side chains are responsible for the progress of the complex multistep reaction as well as for the low rate of catalysis. The mechanism requires that functional groups of Lys-274, Lys-278, Lys-306, and Lys-367 assume differential roles in the protonation/deprotonation steps that must occur during the mIPS reaction. This mechanism is supported by the complete loss of activity of the enzyme caused by the Leu-257 mutation to Ala that releases the lysine containment.

Selective inhibitors of bacterial phosphopantothenoylcysteine synthetase

Bacterial phosphopantothenolycysteine synthetase (PPCS) catalyzes the formation of phosphopantothenoylcysteine (PPC) from (R)-phosphopantothenate, l-cysteine, and cytidine-5'-triphosphate (CTP) and has been shown to be essential for growth and survival. The reaction proceeds through a phosphopantothenoyl cytidylate, mixed anhydride intermediate. Both structural and kinetic characterization studies on PPCS have shown differences in the nucleobase binding site between the bacterial and human enzyme. We report for the first time the design and synthesis of mimics of the phosphopantothenoyl cytidylate, which proved to be potent inhibitors of PPCS. These compounds were evaluated in vitro against PPCS from human and several species of bacteria and showed marked selectivity (up to 1000-fold) toward the bacterial enzymes. A phosphodiester intermediate mimic was the most potent of the compounds synthesized and displayed slow-onset, tight-binding kinetics toward E. faecalis PPCS.

Kinetic characterization of human phosphopantothenoylcysteine synthetase

Phosphopantothenoylcysteine synthetase (PPCS) catalyzes the formation of phosphopantothenoylcysteine from (R)-phosphopantothenate and L-cysteine with the concomitant consumption of a nucleotide triphosphate. Herein, the human coaB gene encoding PPCS is cloned into pET23a and overexpressed in E. coli BL21(DE3), to yield 10mg of purified enzyme per liter of culture. Detailed kinetic studies found that this PPCS follows a similar Bi Uni Uni Bi Ping Pong mechanism as previously described for the E. faecalis PPCS, except that the human enzyme can use both ATP and CTP with similar affinity. One significant difference for human PPCS catalysis with respect to ATP and CTP is that the enzyme shows cooperative binding of ATP, measured as a Hill constant of 1.7. PPCS catalysis under CTP conditions displayed Michaelis constants of 265 microM, 57 microM, and 16 microM for CTP, PPA, and cysteine, respectively, with a kcat of 0.53+/-0.01 s(-1) for the reaction. Taking into account the cooperativity under ATP condition, PPCS exhibited Michaelis constants of 269 microM, 13 microM, and 14 microM for ATP, PPA, and cysteine, respectively, with a kcat of 0.56 s(-1) for the reaction. Oxygen transfer studies found that 18O from [carboxyl-18O] phosphopantothenate is incorporated into the AMP or CMP produced during PPCS catalysis, consistent with the formation of a phosphopantothenoyl cytidylate or phosphopantothenoyl adenylate intermediate, supporting similar catalytic mechanisms under both CTP and ATP conditions. Inhibition studies with GTP and UTP as well as product inhibition studies with CMP and AMP suggest that human PPCS lacks strong nucleotide selectivity.

Characterization and kinetics of phosphopantothenoylcysteine synthetase from Enterococcus faecalis

The enzyme phosphopantothenoylcysteine synthetase (PPCS) catalyzes the nucleotide-dependent formation of phosphopantothenoylcysteine from (R)-phosphopantothenate and L-cysteine in the biosynthetic pathway leading to the formation of the essential biomolecule, coenzyme A. The Enterococcus faecalis gene coaB encodes a novel monofunctional PPCS which has been cloned into pET23a and expressed in Escherichia coli BL21 AI. The heterologous expression system yielded 30 mg of purified PPCS per liter of cell culture. The purified enzyme chromatographed as a homodimer of 28 kDa subunits on Superdex HR 200 gel filtration resin. The monofunctional protein displayed a nucleotide specificity for cytidine 5'-triphosphate (CTP) analogous to that seen for bifunctional PPCS expressed by most prokaryotes. Kinetic characterization, utilizing initial velocity and product inhibition studies, found the mechanism of PPCS to be Bi Uni Uni Bi Ping-Pong, with the nucleotide CTP binding first and CMP released last. Michaelis constants were 156, 17, and 86 microM for CTP, (R)-phosphopantothenate, and L-cysteine, respectively, and the k(cat) was 2.9 s(-1). [carboxyl-(18)O]Phosphopantothenate was prepared by hydrolysis of methyl pantothenate with Na(18)OH, followed by enzymatic phosphorylation with E. faecalis pantothenate kinase (PanK). The fate of the carboxylate oxygen of labeled phosphopantothenate, during the course of the PPCS-catalyzed reaction with CTP and L-cysteine, was monitored by (31)P NMR spectroscopy. The results show that the carboxylate oxygen of the phosphopantothenate is recovered with the CMP formed during the reaction, indicative of the formation of a phosphopantothenoyl cytidylate catalytic intermediate, which is consistent with the kinetic mechanism.

Coenzyme A biosynthesis: an antimicrobial drug target

Pantothenic acid, a precursor of coenzyme A (CoA), is essential for the growth of pathogenic microorganisms. Since the structure of pantothenic acid was determined, many analogues of this essential metabolite have been prepared. Several have been demonstrated to exert an antimicrobial effect against a range of microorganisms by inhibiting the utilization of pantothenic acid, validating pantothenic acid utilization as a potential novel antimicrobial drug target. This review commences with an overview of the mechanisms by which various microorganisms acquire the pantothenic acid they require for growth, and the universal CoA biosynthesis pathway by which pantothenic acid is converted into CoA. A detailed survey of studies that have investigated the inhibitory activity of analogues of pantothenic acid and other precursors of CoA follows. The potential of inhibitors of both pantothenic acid utilization and biosynthesis as novel antibacterial, antifungal and antimalarial agents is discussed, focusing on inhibitors and substrates of pantothenate kinase, the enzyme catalysing the rate-limiting step of CoA biosynthesis in many organisms. The best strategies are considered for identifying inhibitors of pantothenic acid utilization and biosynthesis that are potent and selective inhibitors of microbial growth and that may be suitable for use as chemotherapeutic agents in humans.

4'-phosphopantetheine biosynthesis in Archaea

Coenzyme A as the principal acyl carrier is required for many synthetic and degradative reactions in intermediary metabolism. It is synthesized in five steps from pantothenate, and recently the CoaA biosynthetic genes of eubacteria, plants, and human were all identified and cloned. In most bacteria, the so-called Dfp proteins catalyze the synthesis of the coenzyme A precursor 4'-phosphopantetheine. Dfp proteins are bifunctional enzymes catalyzing the synthesis of 4'-phosphopantothenoylcysteine (CoaB activity) and its decarboxylation to 4'-phosphopantetheine (CoaC activity). Here, we demonstrate the functional characterization of the CoaB and CoaC domains of an archaebacterial Dfp protein. Both domains of the Methanocaldococcus jannaschii Dfp protein were purified as His tag proteins, and their enzymatic activities were then identified and characterized by site-directed mutagenesis. Although the nucleotide binding motif II of the CoaB domain resembles that of eukaryotic enzymes, Methanocaldococcus CoaB is a CTP- and not an ATP-dependent enzyme, as shown by detection of the 4'-phosphopantothenoyl-CMP intermediate. The proposed 4'-phosphopantothenoylcysteine binding clamp of the Methanocaldococcus CoaC activity differs significantly from those of other characterized CoaC proteins. In particular, the active site cysteine residue, which otherwise is involved in the reduction of an aminoenethiol reaction intermediate, is not present. Moreover, the conserved Asn residue of the PXMNXXMW motif, which contacts the carboxyl group of 4'-phosphopantothenoylcysteine, is exchanged for His.