AccD6, a member of the Fas II locus, is a functional carboxyltransferase subunit of the acyl-coenzyme A carboxylase in Mycobacterium tuberculosis

Dissecting the Ca2+ dependence of DesA1 function in Mycobacterium tuberculosis

Mycobacterium tuberculosis (M. tb) has a complex cell wall, composed largely of mycolic acids, that are crucial to its structural maintenance. The M. tb desaturase A1 (DesA1) is an essential Ca2+-binding protein that catalyses a key step in mycolic acid biosynthesis. To investigate the structural and functional significance of Ca2+ binding, we introduced mutations at key residues in its Ca2+-binding βγ-crystallin motif to generate DesA1F303A, E304Q, and F303A-E304Q. Complementation of a conditional ΔdesA1 strain of Mycobacterium smegmatis, with the Ca2+ non-binders F303A or F303A-E304Q, failed to rescue its growth phenotype; these complements also exhibited enhanced cell wall permeability. Our findings highlight the criticality of Ca2+ in DesA1 function, and its implicit role in the maintenance of mycobacterial cellular integrity.

Host and bacterial lipid metabolism during tuberculosis infections: possibilities to synergise host- and bacteria-directed therapies

Mycobacterium tuberculosis (Mtb) is the causative pathogen of tuberculosis, the most lethal infectious disease resulting in 1.3 million deaths annually. Treatments against Mtb are increasingly impaired by the growing prevalence of antimicrobial drug resistance, which necessitates the development of new antibiotics or alternative therapeutic approaches. Upon infecting host cells, predominantly macrophages, Mtb becomes critically dependent on lipids as a source of nutrients. Additionally, Mtb produces numerous lipid-based virulence factors that contribute to the pathogen's ability to interfere with the host's immune responses and to create a lipid rich environment for itself. As lipids, lipid metabolism and manipulating host lipid metabolism play an important role for the virulence of Mtb, this review provides a state-of-the-art overview of mycobacterial lipid metabolism and concomitant role of host metabolism and host-pathogen interaction therein. While doing so, we will emphasize unexploited bacteria-directed and host-directed drug targets, and highlight potential synergistic drug combinations that hold promise for the development of new therapeutic interventions.

The PII protein interacts with the Amt ammonium transport and modulates nitrate/nitrite assimilation in mycobacteria

PII proteins are signal transduction proteins that belong to a widely distributed family of proteins involved in the modulation of different metabolisms in bacteria. These proteins are homotrimers carrying a flexible loop, named T-loop, which changes its conformation due to the recognition of diverse key metabolites, ADP, ATP, and 2-oxoglutarate. PII proteins interact with different partners to primarily regulate a set of nitrogen pathways. In some organisms, PII proteins can also control carbon metabolism by interacting with the biotin carboxyl carrier protein (BCCP), a key component of the acetyl-CoA carboxylase (ACC) enzyme complex, inhibiting its activity with the consequent reduction of fatty acid biosynthesis. Most bacteria contain at least two PII proteins, named GlnB and GlnK, with different regulatory roles. In mycobacteria, only one PII protein was identified, and the three-dimensional structure was solved, however, its physiological role is unknown. In this study we purified the Mycobacterium tuberculosis (M. tb) PII protein, named GlnB, and showed that it weakly interacts with the AccA3 protein, the α subunit shared by the three different, and essential, Acyl-CoA carboxylase complexes (ACCase 4, 5, and 6) present in M. tb. A M. smegmatis deletion mutant, ∆MsPII, exhibited a growth deficiency on nitrate and nitrite as unique nitrogen sources, and accumulated nitrite in the culture supernatant. In addition, M. tb PII protein was able to interact with the C-terminal domain of the ammonium transporter Amt establishing the ancestral role for this PII protein as a GlnK functioning protein.

Targeting polyketide synthase 13 for the treatment of tuberculosis

Tuberculosis (TB) is one of the most threatening diseases for humans, however, the drug treatment strategy for TB has been stagnant and inadequate, which could not meet current treatment needs. TB is caused by Mycobacterial tuberculosis, which has a unique cell wall that plays a crucial role in its growth, virulence, and drug resistance. Polyketide synthase 13 (Pks13) is an essential enzyme that catalyzes the biosynthesis of the cell wall and its critical role is only found in Mycobacteria. Therefore, Pks13 is a promising target for developing novel anti-TB drugs. In this review, we first introduced the mechanism of targeting Pks13 for TB treatment. Subsequently, we focused on summarizing the recent advance of Pks13 inhibitors, including the challenges encountered during their discovery and the rational design strategies employed to overcome these obstacles, which could be helpful for the development of novel Pks13 inhibitors in the future.

Genetic analysis of acyl-CoA carboxylases involved in lipid accumulation in Rhodococcus jostii RHA1

In actinomycetes, the acyl-CoA carboxylases, including the so-called acetyl-CoA carboxylases (ACCs), are biotin-dependent enzymes that exhibit broad substrate specificity and diverse domain and subunit arrangements. Bioinformatic analyses of the Rhodococcus jostii RHA1 genome found that this microorganism contains a vast arrange of putative acyl-CoA carboxylases domains and subunits. From the thirteen putative carboxyltransferase domains, only the carboxyltransferase subunit RO01202 and the carboxyltransferase domain present in the multidomain protein RO04222 are highly similar to well-known essential ACC subunits from other actinobacteria. Mutant strains in each of these genes showed that none of these enzymes is essential for R. jostii growth in rich or in minimal media with high nitrogen concentration, presumably because of their partial overlapping activities. A mutant strain in the ro04222 gene showed a decrease in triacylglycerol and mycolic acids accumulation in rich and minimal medium, highlighting the relevance of this multidomain ACC in the biosynthesis of these lipids. On the other hand, RO01202, a carboxyltransferase domain of a putative ACC complex, whose biotin carboxylase and biotin carboxyl carrier protein domain were not yet identified, was found to be essential for R. jostii growth only in minimal medium with low nitrogen concentration. The results of this study have identified a new component of the TAG-accumulating machinery in the oleaginous R. jostii RHA1. While non-essential for growth and TAG biosynthesis in RHA1, the activity of RO04222 significantly contributes to lipogenesis during single-cell oil production. Furthermore, this study highlights the high functional diversity of ACCs in actinobacteria, particularly regarding their essentiality under different environmental conditions. KEY POINTS: • R. jostii possess a remarkable heterogeneity in their acyl-carboxylase complexes. • RO04222 is a multidomain acetyl-CoA carboxylase involved in lipid accumulation. • RO01202 is an essential carboxyltransferase only at low nitrogen conditions.

Polar protein Wag31 both activates and inhibits cell wall metabolism at the poles and septum

Mycobacterial cell elongation occurs at the cell poles; however, it is not clear how cell wall insertion is restricted to the pole or how it is organized. Wag31 is a pole-localized cytoplasmic protein that is essential for polar growth, but its molecular function has not been described. In this study we used alanine scanning mutagenesis to identify Wag31 residues involved in cell morphogenesis. Our data show that Wag31 helps to control proper septation as well as new and old pole elongation. We have identified key amino acid residues involved in these essential functions. Enzyme assays revealed that Wag31 interacts with lipid metabolism by modulating acyl-CoA carboxylase (ACCase) activity. We show that Wag31 does not control polar growth by regulating the localization of cell wall precursor enzymes to the Intracellular Membrane Domain, and we also demonstrate that phosphorylation of Wag31 does not substantively regulate peptidoglycan metabolism. This work establishes new regulatory functions of Wag31 in the mycobacterial cell cycle and clarifies the need for new molecular models of Wag31 function.

Mining Fatty Acid Biosynthesis for New Antimicrobials

Antibiotic resistance is a serious public health concern, and new drugs are needed to ensure effective treatment of many bacterial infections. Bacterial type II fatty acid synthesis (FASII) is a vital aspect of bacterial physiology, not only for the formation of membranes but also to produce intermediates used in vitamin production. Nature has evolved a repertoire of antibiotics inhibiting different aspects of FASII, validating these enzymes as potential targets for new antibiotic discovery and development. However, significant obstacles have been encountered in the development of FASII antibiotics, and few FASII drugs have advanced beyond the discovery stage. Most bacteria are capable of assimilating exogenous fatty acids. In some cases they can dispense with FASII if fatty acids are present in the environment, making the prospects for identifying broad-spectrum drugs against FASII targets unlikely. Single-target, pathogen-specific FASII drugs appear the best option, but a major drawback to this approach is the rapid acquisition of resistance via target missense mutations. This complication can be mitigated during drug development by optimizing the compound design to reduce the potential impact of on-target missense mutations at an early stage in antibiotic discovery. The lessons learned from the difficulties in FASII drug discovery that have come to light over the last decade suggest that a refocused approach to designing FASII inhibitors has the potential to add to our arsenal of weapons to combat resistance to existing antibiotics.

Phylogenomic Reappraisal of Fatty Acid Biosynthesis, Mycolic Acid Biosynthesis and Clinical Relevance Among Members of the Genus Corynebacterium

The genus Corynebacterium encompasses many species of biotechnological, medical or veterinary significance. An important characteristic of this genus is the presence of mycolic acids in their cell envelopes, which form the basis of a protective outer membrane (mycomembrane). Mycolic acids in the cell envelope of Mycobacterium tuberculosis have been associated with virulence. In this study, we have analysed the genomes of 140 corynebacterial strains, including representatives of 126 different species. More than 50% of these strains were isolated from clinical material from humans or animals, highlighting the true scale of pathogenic potential within the genus. Phylogenomically, these species are very diverse and have been organised into 19 groups and 30 singleton strains. We find that a substantial number of corynebacteria lack FAS-I, i.e., have no capability for de novo fatty acid biosynthesis and must obtain fatty acids from their habitat; this appears to explain the well-known lipophilic phenotype of some species. In most species, key genes associated with the condensation and maturation of mycolic acids are present, consistent with the reports of mycolic acids in their species descriptions. Conversely, species reported to lack mycolic acids lacked these key genes. Interestingly, Corynebacterium ciconiae, which is reported to lack mycolic acids, appears to possess all genes required for mycolic acid biosynthesis. We suggest that although a mycolic acid-based mycomembrane is widely considered to be the target for interventions by the immune system and chemotherapeutics, the structure is not essential in corynebacteria and is not a prerequisite for pathogenicity or colonisation of animal hosts.

Pyrrolocin C and equisetin inhibit bacterial acetyl-CoA carboxylase

Antimicrobial resistance is a growing global health and economic concern. Current antimicrobial agents are becoming less effective against common bacterial infections. We previously identified pyrrolocins A and C, which showed activity against a variety of Gram-positive bacteria. Structurally similar compounds, known as pyrrolidinediones (e.g., TA-289, equisetin), also display antibacterial activity. However, the mechanism of action of these compounds against bacteria was undetermined. Here, we show that pyrrolocin C and equisetin inhibit bacterial acetyl-CoA carboxylase (ACC), the first step in fatty acid synthesis. We used transcriptomic data, metabolomic analysis, fatty acid rescue and acetate incorporation experiments to show that a major mechanism of action of the pyrrolidinediones is inhibition of fatty acid biosynthesis, identifying ACC as the probable molecular target. This hypothesis was further supported using purified proteins, demonstrating that biotin carboxylase is the inhibited component of ACC. There are few known antibiotics that target this pathway and, therefore, we believe that these compounds may provide the basis for alternatives to current antimicrobial therapy.

Transcriptional and post-translational regulation of AccD6 in Mycobacterium smegmatis

AccD6 is an important component of acetyl-CoA/propionyl-CoA carboxylase, which acts as a key role in mycolic acid synthesis and short chain fatty acyl-coenzyme A metabolism. In this study, we demonstrated that AccD6 of Mycobacterium smegmatis associates with AccA3 (α subunit of acetyl-CoA carboxylase, MSMEG_1807) and AccE (ε subunit, MSMEG_1812) to form the acetyl-CoA (propionyl-CoA) carboxylase. Results showed that the MSMEG_4331 subunit is a regulator that interacts with the promoter region of accD6 to inhibit its transcription. Transcription of accD6 was reduced by 50% in the mutant M. smegmatis strain overexpressing MSMEG_4331. Moreover, the activity of AccD6 was inhibited by acylation (such as acetylation and propionylation). These results demonstrate that AccD6 of M. smegmatis is regulated at both the transcriptional and post-translational levels. Our findings highlight the novel regulatory mechanism underlying mycolic acid biosynthesis in mycobacteria.

Kinase Targets for Mycolic Acid Biosynthesis in Mycobacterium tuberculosis

BACKGROUND

Mycolic acids (MAs) are the characteristic, integral building blocks for the mycomembrane belonging to the insidious bacterial pathogen Mycobacterium tuberculosis (M.tb). These C60-C90 long α-alkyl-β-hydroxylated fatty acids provide protection to the tubercle bacilli against the outside threats, thus allowing its survival, virulence and resistance to the current antibacterial agents. In the post-genomic era, progress has been made towards understanding the crucial enzymatic machineries involved in the biosynthesis of MAs in M.tb. However, gaps still remain in the exact role of the phosphorylation and dephosphorylation of regulatory mechanisms within these systems. To date, a total of 11 serine-threonine protein kinases (STPKs) are found in M.tb. Most enzymes implicated in the MAs synthesis were found to be phosphorylated in vitro and/or in vivo. For instance, phosphorylation of KasA, KasB, mtFabH, InhA, MabA, and FadD32 downregulated their enzymatic activity, while phosphorylation of VirS increased its enzymatic activity. These observations suggest that the kinases and phosphatases system could play a role in M.tb adaptive responses and survival mechanisms in the human host. As the mycobacterial STPKs do not share a high sequence homology to the human's, there have been some early drug discovery efforts towards developing potent and selective inhibitors.

OBJECTIVE

Recent updates to the kinases and phosphatases involved in the regulation of MAs biosynthesis will be presented in this mini-review, including their known small molecule inhibitors.

CONCLUSION

Mycobacterial kinases and phosphatases involved in the MAs regulation may serve as a useful avenue for antitubercular therapy.

Crystal structure of the essential biotin-dependent carboxylase AccA3 fromMycobacterium tuberculosis

Biotin‐dependent acetyl‐CoA carboxylases catalyze the committed step in type II fatty acid biosynthesis, the main route for production of membrane phospholipids in bacteria, and are considered a key target for antibacterial drug discovery. Here we describe the first structure of AccA3, an essential component of the acetyl‐CoA carboxylase system in Mycobacterium tuberculosis (MTb). The structure, sequence comparisons, and modeling of ligand‐bound states reveal that the ATP cosubstrate‐binding site shows distinct differences compared to other bacterial and eukaryotic biotin carboxylases, including all human homologs. This suggests the possibility to design MTb AccA3 subtype‐specific inhibitors.

Database

Coordinates and structure factors have been deposited in the Protein Data Bank with the accession number 5MLK.

Functional reconstitution of the Mycobacterium tuberculosis long-chain acyl-CoA carboxylase from multiple acyl-CoA subunits

Mycobacterium tuberculosis produces a large number of structurally diverse lipids that have been implicated in the pathogenicity, persistence and antibiotic resistance of this organism. Most building blocks involved in the biosynthesis of all these lipids are generated by acyl-CoA carboxylases whose subunit composition and physiological roles have not yet been clearly established. Inconclusive data in the literature refer to the exact protein composition and substrate specificity of the enzyme complex that produces the long-chain α-carboxy-acyl-CoAs, which are substrates involved in the last step of condensation mediated by the polyketide synthase 13 to synthesize mature mycolic acids. Here we have successfully reconstituted the long-chain acyl-CoA carboxylase (LCC) complex from its purified components, the α subunit (AccA3), the ε subunit (AccE5) and the two β subunits (AccD4 and AccD5), and demonstrated that the four subunits are essential for its activity. Furthermore, we also showed by substrate competition experiments and the use of a specific inhibitor that the AccD5 subunit's role in the carboxylation of the long acyl-CoAs, as part of the LCC complex, was structural rather than catalytic. Moreover, AccD5 was also able to carboxylate its natural substrates, acetyl-CoA and propionyl-CoA, in the context of the LCC enzyme complex. Thus, the supercomplex formed by these four subunits has the potential to generate the main substrates, malonyl-CoA, methylmalonyl-CoA and α-carboxy-C24-26 -CoA, used as condensing units for the biosynthesis of all the lipids present in this pathogen.

The influence of AccD5 on AccD6 carboxyltransferase essentiality in pathogenic and non-pathogenic Mycobacterium

Malonyl-coenzyme A (CoA) is a crucial extender unit for the synthesis of mycolic and other fatty acids in mycobacteria, generated in a reaction catalyzed by acetyl-CoA carboxylase. We previously reported on the essentiality of accD6_Mtb encoding the functional acetyl-CoA carboxylase subunit in Mycobacterium tuberculosis. Strikingly, the homologous gene in the fast-growing, non-pathogenic Mycobacterium smegmatis - (accD6_Msm) appeared to be dispensable, and its deletion did not influence the cell lipid content. Herein, we demonstrate that, despite the difference in essentiality, accD6_Msm and accD6_Mtb encode proteins of convergent catalytic activity in vivo. To identify an alternative, AccD6-independent, malonyl-CoA synthesis pathway in M. smegmatis, a complex genetic approach combined with lipid analysis was applied to screen all five remaining carboxyltransferase genes (accD1-accD5) with respect to their involvement in mycolic acid biosynthesis and ability to utilize acetyl-CoA as the substrate for carboxylation. This approach revealed that AccD1_Msm, AccD2_Msm and AccD3_Msm are not essential for mycolic acid biosynthesis. Furthermore, we confirmed in vivo the function of AccD4_Msm as an essential, long-chain acyl-CoA carboxyltransferase, unable to carboxylate short-chain substrate. Finally, our comparative studies unambiguously demonstrated between-species difference in in vivo ability of AccD5 carboxyltransferase to utilize acetyl-CoA that influences AccD6 essentiality in pathogenic and non-pathogenic mycobacteria.

Construction and application of a co-expression network in Mycobacterium tuberculosis

Because of its high pathogenicity and infectivity, tuberculosis is a serious threat to human health. Some information about the functions of the genes in Mycobacterium tuberculosis genome was currently available, but it was not enough to explore transcriptional regulatory mechanisms. Here, we applied the WGCNA (Weighted Gene Correlation Network Analysis) algorithm to mine pooled microarray datasets for the M. tuberculosis H37Rv strain. We constructed a co-expression network that was subdivided into 78 co-expression gene modules. The different response to two kinds of vitro models (a constant 0.2% oxygen hypoxia model and a Wayne model) were explained based on these modules. We identified potential transcription factors based on high Pearson’s correlation coefficients between the modules and genes. Three modules that may be associated with hypoxic stimulation were identified, and their potential transcription factors were predicted. In the validation experiment, we determined the expression levels of genes in the modules under hypoxic condition and under overexpression of potential transcription factors (Rv0081, furA (Rv1909c), Rv0324, Rv3334, and Rv3833). The experimental results showed that the three identified modules related to hypoxia and that the overexpression of transcription factors could significantly change the expression levels of genes in the corresponding modules.

Mycobacterial genes essential for the pathogen's survival in the host

Mycobacterium tuberculosis (Mtb) has evolved within the human immune system as both host and reservoir. The study of genes required for its growth and persistence in vivo thus offers linked insights into its pathogenicity and host immunity. Studies of Mtb mutants have implicated metabolic adaptation (consisting of carbon, nitrogen, vitamin, and cofactor metabolism), intrabacterial pH homeostasis, and defense against reactive oxygen and reactive nitrogen species, as key determinants of its pathogenicity. However, the mechanisms of host immunity are complex and often combinatorial. Growing evidence has thus begun to reveal that the determinants of Mtb's pathogenicity may serve a broader and more complex array of functions than the isolated experimental settings in which they were initially found. Here, we review select examples, which exemplify this complexity, highlighting the distinct phases of Mtb's life cycle and the diverse microenvironments encountered therein.

Mycolic acids: deciphering and targeting the Achilles' heel of the tubercle bacillus

Mycolic acids are unique long chain fatty acids found in the lipid-rich cell walls of mycobacteria including the tubercle bacillus Mycobacterium tuberculosis. Essential for viability and virulence, enzymes involved in the biosynthesis of mycolic acids represent novel targets for drug development. This is particularly relevant to the impact on global health given the rise of multidrug resistant and extensively drug resistant strains of M. tuberculosis. In this review, we discuss recent advances in our understanding of how mycolic acid are synthesised, especially the potential role of specialised fatty acid synthase complexes. Also, we examine the role of a recently reported mycolic acid transporter MmpL3 with reference to several reports of the targeting of this transporter by diverse compounds with anti-M. tuberculosis activity. Additionally, we consider recent findings that place mycolic acid biosynthesis in the context of the cell biology of the bacterium, viz its localisation and co-ordination with the bacterial cytoskeleton, and its role beyond maintaining cell envelope integrity.

Novel katG mutations causing isoniazid resistance in clinical M. tuberculosis isolates

We report the discovery and confirmation of 23 novel mutations with previously undocumented role in isoniazid (INH) drug resistance, in catalase-peroxidase (katG) gene of Mycobacterium tuberculosis (Mtb) isolates. With these mutations, a synonymous mutation in fabG1^g609a, and two canonical mutations, we were able to explain 98% of the phenotypic resistance observed in 366 clinical Mtb isolates collected from four high tuberculosis (TB)-burden countries: India, Moldova, Philippines, and South Africa. We conducted overlapping targeted and whole-genome sequencing for variant discovery in all clinical isolates with a variety of INH-resistant phenotypes. Our analysis showed that just two canonical mutations (katG 315AGC-ACC and inhA promoter-15C-T) identified 89.5% of resistance phenotypes in our collection. Inclusion of the 23 novel mutations reported here, and the previously documented point mutation in fabG1, increased the sensitivity of these mutations as markers of INH resistance to 98%. Only six (2%) of the 332 resistant isolates in our collection did not harbor one or more of these mutations. The third most prevalent substitution, at inhA promoter position -8, present in 39 resistant isolates, was of no diagnostic significance since it always co-occurred with katG 315. 79% of our isolates harboring novel mutations belong to genetic group 1 indicating a higher tendency for this group to go down an uncommon evolutionary path and evade molecular diagnostics. The results of this study contribute to our understanding of the mechanisms of INH resistance in Mtb isolates that lack the canonical mutations and could improve the sensitivity of next generation molecular diagnostics.

Characterization of the mycobacterial acyl-CoA carboxylase holo complexes reveals their functional expansion into amino acid catabolism

Biotin-mediated carboxylation of short-chain fatty acid coenzyme A esters is a key step in lipid biosynthesis that is carried out by multienzyme complexes to extend fatty acids by one methylene group. Pathogenic mycobacteria have an unusually high redundancy of carboxyltransferase genes and biotin carboxylase genes, creating multiple combinations of protein/protein complexes of unknown overall composition and functional readout. By combining pull-down assays with mass spectrometry, we identified nine binary protein/protein interactions and four validated holo acyl-coenzyme A carboxylase complexes. We investigated one of these - the AccD1-AccA1 complex from Mycobacterium tuberculosis with hitherto unknown physiological function. Using genetics, metabolomics and biochemistry we found that this complex is involved in branched amino-acid catabolism with methylcrotonyl coenzyme A as the substrate. We then determined its overall architecture by electron microscopy and found it to be a four-layered dodecameric arrangement that matches the overall dimensions of a distantly related methylcrotonyl coenzyme A holo complex. Our data argue in favor of distinct structural requirements for biotin-mediated γ-carboxylation of α−β unsaturated acid esters and will advance the categorization of acyl-coenzyme A carboxylase complexes. Knowledge about the underlying structural/functional relationships will be crucial to make the target category amenable for future biomedical applications.

Tuberculosis is deadly human disease caused by infection with the bacterium Mycobacterium tuberculosis. This pathogen has a complex metabolism with many genes required for the synthesis of components of its unique cell envelope. We have investigated a family of closely related genes coding for different acyl CoA carboxylase enzyme complexes with previously unexplained genetic redundancy that have been thought to have an involvement in the synthesis of these cell envelope components. We identified five functional multienzyme complexes. Of the two complexes with hitherto unknown function we chose to investigate, one specifically and to our surprise it is required for the degradation of the amino acid leucine. To our knowledge this is the first demonstration that mycobacteria have a specific pathway for leucine degradation and thus broaden the functional diversity associated with acyl CoA carboxylase coding genes.

The Molecular Genetics of Mycolic Acid Biosynthesis

Mycolic acids are major and specific long-chain fatty acids that represent essential components of the Mycobacterium tuberculosis cell envelope. They play a crucial role in the cell wall architecture and impermeability, hence the natural resistance of mycobacteria to most antibiotics, and represent key factors in mycobacterial virulence. Biosynthesis of mycolic acid precursors requires two types of fatty acid synthases (FASs), the eukaryotic-like multifunctional enzyme FAS I and the acyl carrier protein (ACP)–dependent FAS II systems, which consists of a series of discrete mono-functional proteins, each catalyzing one reaction in the pathway. Unlike FAS II synthases of other bacteria, the mycobacterial FAS II is incapable of de novo fatty acid synthesis from acetyl-coenzyme A, but instead elongates medium-chain-length fatty acids previously synthesized by FAS I, leading to meromycolic acids. In addition, mycolic acid subspecies with defined biological properties can be distinguished according to the chemical modifications decorating the meromycolate. Nearly all the genetic components involved in both elongation and functionalization of the meromycolic acid have been identified and are generally clustered in distinct transcriptional units. A large body of information has been generated on the enzymology of the mycolic acid biosynthetic pathway and on their genetic and biochemical/structural characterization as targets of several antitubercular drugs. This chapter is a comprehensive overview of mycolic acid structure, function, and biosynthesis. Special emphasis is given to recent work addressing the regulation of mycolic acid biosynthesis, adding new insights to our understanding of how pathogenic mycobacteria adapt their cell wall composition in response to environmental changes.

Structure, activity, and inhibition of the Carboxyltransferase β-subunit of acetyl coenzyme A carboxylase (AccD6) from Mycobacterium tuberculosis

In Mycobacterium tuberculosis, the carboxylation of acetyl coenzyme A (acetyl-CoA) to produce malonyl-CoA, a building block in long-chain fatty acid biosynthesis, is catalyzed by two enzymes working sequentially: a biotin carboxylase (AccA) and a carboxyltransferase (AccD). While the exact roles of the three different biotin carboxylases (AccA1 to -3) and the six carboxyltransferases (AccD1 to -6) in M. tuberculosis are still not clear, AccD6 in complex with AccA3 can synthesize malonyl-CoA from acetyl-CoA. A series of 10 herbicides that target plant acetyl-CoA carboxylases (ACC) were tested for inhibition of AccD6 and for whole-cell activity against M. tuberculosis. From the tested herbicides, haloxyfop, an arylophenoxypropionate, showed in vitro inhibition of M. tuberculosis AccD6, with a 50% inhibitory concentration (IC50) of 21.4 ± 1 μM. Here, we report the crystal structures of M. tuberculosis AccD6 in the apo form (3.0 Å) and in complex with haloxyfop-R (2.3 Å). The structure of M. tuberculosis AccD6 in complex with haloxyfop-R shows two molecules of the inhibitor bound on each AccD6 subunit. These results indicate the potential for developing novel therapeutics for tuberculosis based on herbicides with low human toxicity.

Pleiotropic effect of AccD5 and AccE5 depletion in acyl-coenzyme A carboxylase activity and in lipid biosynthesis in mycobacteria

Mycobacteria contain a large variety of fatty acids which are used for the biosynthesis of several complex cell wall lipids that have been implicated in the ability of the organism to resist host defenses. The building blocks for the biosynthesis of all these lipids are provided by a fairly complex set of acyl-CoA carboxylases (ACCases) whose subunit composition and roles within these organisms have not yet been clearly established. Previous biochemical and structural studies provided strong evidences that ACCase 5 from Mycobacterium tuberculosis is formed by the AccA3, AccD5 and AccE5 subunits and that this enzyme complex carboxylates acetyl-CoA and propionyl-CoA with a clear substrate preference for the latest. In this work we used a genetic approach to unambiguously demonstrate that the products of both accD5 and accE5 genes are essential for the viability of Mycobacterium smegmatis. By obtaining a conditional mutant on the accD5-accE5 operon, we also demonstrated that the main physiological role of this enzyme complex was to provide the substrates for fatty acid and mycolic acid biosynthesis. Furthermore, enzymatic and biochemical analysis of the conditional mutant provided strong evidences supporting the notion that AccD5 and/or AccE5 have an additional role in the carboxylation of long chain acyl-CoA prior to mycolic acid condensation. These studies represent a significant step towards a better understanding of the roles of ACCases in mycobacteria and confirm ACCase 5 as an interesting target for the development of new antimycobacterial drugs.

The Wag31 protein interacts with AccA3 and coordinates cell wall lipid permeability and lipophilic drug resistance in Mycobacterium smegmatis

Mycobacterium tuberculosis, especially drug resistant tuberculosis, is a serious threat to global human health. Compared with other bacterial pathogens, M. tuberculosis gains stronger natural drug resistance from its unusually lipid-rich cell wall. As a DivIVA homolog, Wag31 has been demonstrated to be closely involved in peptidoglycan synthesis, cell growth and cell division. Previous research rarely investigated the role of Wag31 in drug resistance. In this study, we found Wag31 knock-down in Mycobacterium smegmatis resulted in a co-decrease of the resistance to four lipophilic drugs (rifampicin, novobiocin, erythromycin and clofazimine) and an increase in the cell permeability to lipophilic molecules. Six proteins (AccA3, AccD4 and AccD5, Fas, InhA and MmpL3) that are involved in fatty acid and mycolic acid synthesis were identified in the Wag31 interactome through Co-Immunoprecipitation. The Wag31-AccA3 interaction was confirmed by the pull-down assay. AccA3 overexpression resulted in a decrease in lipid permeability and an increase in the resistance of rifampicin and novobiocin. It confirmed the close relationship of lipophilic drug resistance, lipid permeability and the Wag31-AccA3 interaction. These results demonstrated that Wag31 maintained the resistance to lipophilic drugs and that Wag31 could play a role in controlling the lipid permeability of the cell wall through the Wag31-AccA3 interaction.

bkaR is a TetR-type repressor that controls an operon associated with branched-chain keto-acid metabolism in Mycobacteria

This study describes how bkaR, a highly conserved mycobacterial TetR-like transcriptional repressor, regulates a number of nearby genes that have associations with branched-chain keto-acid metabolism. bkaR (MSMEG_4718) was deleted from the nonpathogenic species Mycobacterium smegmatis, and changes in global gene expression were assessed using microarray analysis and reporter gene studies. bkaR was found to directly control the expression of 10 genes in M. smegmatis, and its ortholog in Mycobacterium tuberculosis (Rv2506) is predicted to control at least 12 genes. A conserved operator motif was identified, and binding of purified recombinant M. tuberculosis BkaR to the motif was demonstrated. Analysis of the stoichiometry of binding showed that BkaR binds to the motif as a dimer.

Analysis of differentially expressed proteins in late-stationary growth phase of Mycobacterium tuberculosis H37Rv

Mycobacterium tuberculosis is a causative agent of tuberculosis (TB). The ability of M. tuberculosis to be quiescent in the cell has caused the emergence of latent infection. A comprehensive proteomic analysis of M. tuberculosis H37Rv over three growth phases, namely mid-log (14-day culture), early stationary (28-day culture), and late stationary (50-day culture), was performed in order to study the change in proteome from the mid-log phase to late-stationary phase. Combination methods of two-dimensional electrophoresis (2-DE) and tandem mass spectrometry were used to generate proteome maps of M. tuberculosis at different growth phases. Ten proteins were detected differentially expressed in the late-stationary phase compared with the other two phases. These proteins were SucD, TrpD, and Rv2161c, which belong to metabolic pathway proteins; FadE5, AccD5, DesA1, and Rv1139c are proteins involved in cell wall or lipid biosynthesis, whereas TB21.7 and Rv3224 are conserved hypothetical proteins with unknown function. A surface antigen protein, DesA1, was not detectable in the late-stationary phase, although present in both log and early-stationary phases. The changes in the expression levels of these proteins were in line with the growth environment changes of the bacteria from mid-log phase to late-stationary phase. The information gathered may be valuable in the intervention against latent TB infection.

Structure and function of biotin-dependent carboxylases

Biotin-dependent carboxylases include acetyl-CoA carboxylase (ACC), propionyl-CoA carboxylase (PCC), 3-methylcrotonyl-CoA carboxylase (MCC), geranyl-CoA carboxylase, pyruvate carboxylase (PC), and urea carboxylase (UC). They contain biotin carboxylase (BC), carboxyltransferase (CT), and biotin-carboxyl carrier protein components. These enzymes are widely distributed in nature and have important functions in fatty acid metabolism, amino acid metabolism, carbohydrate metabolism, polyketide biosynthesis, urea utilization, and other cellular processes. ACCs are also attractive targets for drug discovery against type 2 diabetes, obesity, cancer, microbial infections, and other diseases, and the plastid ACC of grasses is the target of action of three classes of commercial herbicides. Deficiencies in the activities of PCC, MCC, or PC are linked to serious diseases in humans. Our understanding of these enzymes has been greatly enhanced over the past few years by the crystal structures of the holoenzymes of PCC, MCC, PC, and UC. The structures reveal unanticipated features in the architectures of the holoenzymes, including the presence of previously unrecognized domains, and provide a molecular basis for understanding their catalytic mechanism as well as the large collection of disease-causing mutations in PCC, MCC, and PC. This review will summarize the recent advances in our knowledge on the structure and function of these important metabolic enzymes.

Expression, purification and preliminary crystallographic analysis of Rv2247, the β subunit of acyl-CoA carboxylase (ACCD6) from Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) acyl-CoA carboxylase is involved in the biosynthesis of mycolic acids, which are a key component of the bacillus cell wall. The Mtb genome encodes six acyl-CoA carboxylase β subunits (ACCD1-6), three of which (ACCD4-6) are essential for survival of the pathogen on minimal medium. Mtb ACCD6 has been expressed, purified and crystallized. The two forms of Mtb ACCD6 crystals belonged to space groups P4(1)2(1)2 and P2(1)2(1)2(1) and diffracted to 2.9 and 2.5 Å resolution, respectively, at a synchrotron-radiation source.

AccD6, a key carboxyltransferase essential for mycolic acid synthesis in Mycobacterium tuberculosis, is dispensable in a nonpathogenic strain

Acetyl coenzyme A carboxylase (ACC) is a key enzyme providing a substrate for mycolic acid biosynthesis. Although in vitro studies have demonstrated that the protein encoded by accD6 (Rv2247) may be a functional carboxyltransferase subunit of ACC in Mycobacterium tuberculosis, the in vivo function and regulation of accD6 in slow- and fast-growing mycobacteria remain elusive. Here, directed mutagenesis demonstrated that although accD6 is essential for M. tuberculosis, it can be deleted in Mycobacterium smegmatis without affecting its cell envelope integrity. Moreover, we showed that although it is part of the type II fatty acid synthase operon, the accD6 gene of M. tuberculosis, but not that of M. smegmatis, possesses its own additional promoter (P(acc)). The expression level of accD6(Mtb) placed only under the control of P(acc) is 10-fold lower than that in wild-type M. tuberculosis but is sufficient to sustain cell viability. Importantly, this limited expression level affects growth, mycolic acid content, and cell morphology. These results provide the first in vivo evidence for AccD6 as a key player in the mycolate biosynthesis of M. tuberculosis, implicating AccD6 as the essential ACC subunit in pathogenic mycobacteria and an excellent target for new antitubercular compounds. Our findings also highlight important differences in the mechanism of acetyl carboxylation between pathogenic and nonpathogenic mycobacterial species.

Evaluating the sensitivity of Mycobacterium tuberculosis to biotin deprivation using regulated gene expression

In the search for new drug targets, we evaluated the biotin synthetic pathway of Mycobacterium tuberculosis (Mtb) and constructed an Mtb mutant lacking the biotin biosynthetic enzyme 7,8-diaminopelargonic acid synthase, BioA. In biotin-free synthetic media, ΔbioA did not produce wild-type levels of biotinylated proteins, and therefore did not grow and lost viability. ΔbioA was also unable to establish infection in mice. Conditionally-regulated knockdown strains of Mtb similarly exhibited impaired bacterial growth and viability in vitro and in mice, irrespective of the timing of transcriptional silencing. Biochemical studies further showed that BioA activity has to be reduced by approximately 99% to prevent growth. These studies thus establish that de novo biotin synthesis is essential for Mtb to establish and maintain a chronic infection in a murine model of TB. Moreover, these studies provide an experimental strategy to systematically rank the in vivo value of potential drug targets in Mtb and other pathogens.

We evaluated the biotin synthetic pathway of Mycobacterium tuberculosis (Mtb) as a new drug target by first generating an Mtb deletion mutant, ΔbioA, in which the biotin biosynthetic enzyme 7,8-diaminopelargonic acid synthase (BioA) has been inactivated. This mutant grew in the presence of biotin or des-thiobiotin, but not with an intermediate of the biotin biosynthesis pathway that requires BioA to be converted into biotin. Without exogenous biotin or des-thiobiotin, ΔbioA, was unable to produce biotinylated proteins, which are required for the biosynthesis of fatty acids, and thus died in biotin-free media. Using a regulatable promoter and different ribosome binding sequences we next constructed tightly controlled TetON mutants, in which expression of BioA could be induced with tetracyclines, but was inhibited in their absence. Characterization of these mutants during infections demonstrated that de novo biotin synthesis is not only required to establish infections but also to maintain bacterial persistence. Inhibition of BioA or other enzymes of the biotin biosynthesis pathways could thus be used to kill Mtb during both acute and chronic infections. Biochemical and immunological analyses of different Mtb mutants indicate that drugs targeting BioA would have to inactive approximately 99% of its activity to be effective.

Binding of activated isoniazid with acetyl-CoA carboxylase from Mycobacterium tuberculosis

AccD6 (acetyl coenzyme A (CoA) carboxylase), plays an important role in mycolic acid synthesis of Mycobacterium tuberculosis (Mtb). Induced gene expression by isoniazid (isonicotinylhydrazine - INH), anti-tuberculosis drug) shows the expression of accD6. It is our interest to study the binding of activated INH with the AccD6 model using molecular docking procedures. The study predicts a primary binding site for activated INH (isonicotinyl acyl radical) in AccD6 as a potential target.

(1)H NMR-based metabolomic profiling in mice infected with Mycobacterium tuberculosis

Tuberculosis (TB) is one of three major infectious diseases, and the control of TB is becoming more difficult because of the emergence of multidrug-resistant and extensively drug-resistant strains. In this study, we explored the (1)H NMR-based metabolomics of TB using an aerobic TB infection model. Global profiling was applied to characterize the responses of C57Bl/6 mice to an aerobic infection with virulent Mycobacterium tuberculosis (MTB). The metabolic changes in organs (i.e., the lung, the target organ of TB, and the spleen and liver, remote systemic organs) and in serum from control and MTB-infected rats were investigated to clarify the host-pathogen interactions in MTB-infected host systems. Principal components analysis (PCA) and orthogonal partial least-squares discriminant analysis (OPLS-DA) score plots showed distinct separation between control and MTB-infected rats for all tissue and serum samples. Several tissue and serum metabolites were changed in MTB-infected rats, as compared to control rats. The precursors of membrane phospholipids, phosphocholine, and phosphoethanolamine, as well as glycolysis, amino acid metabolism, nucleotide metabolism, and the antioxidative stress response were altered based on the presence of MTB infection. This study suggests that NMR-based global metabolite profiling of organ tissues and serum could provide insight into the metabolic changes in host infected aerobically with virulent Mycobacterium tuberculosis.

Fatty acid biosynthesis in actinomycetes

All organisms that produce fatty acids do so via a repeated cycle of reactions. In mammals and other animals, these reactions are catalyzed by a type I fatty acid synthase (FAS), a large multifunctional protein to which the growing chain is covalently attached. In contrast, most bacteria (and plants) contain a type II system in which each reaction is catalyzed by a discrete protein. The pathway of fatty acid biosynthesis in Escherichia coli is well established and has provided a foundation for elucidating the type II FAS pathways in other bacteria (White et al., 2005). However, fatty acid biosynthesis is more diverse in the phylum Actinobacteria: Mycobacterium, possess both FAS systems while Streptomyces species have only the multienzyme FAS II system and Corynebacterium species exclusively FAS I. In this review, we present an overview of the genome organization, biochemical properties and physiological relevance of the two FAS systems in the three genera of actinomycetes mentioned above. We also address in detail the biochemical and structural properties of the acyl-CoA carboxylases (ACCases) that catalyzes the first committed step of fatty acid synthesis in actinomycetes, and discuss the molecular bases of their substrate specificity and the structure-based identification of new ACCase inhibitors with antimycobacterial properties.

Structural ordering of disordered ligand-binding loops of biotin protein ligase into active conformations as a consequence of dehydration

Mycobacterium tuberculosis (Mtb), a dreaded pathogen, has a unique cell envelope composed of high fatty acid content that plays a crucial role in its pathogenesis. Acetyl Coenzyme A Carboxylase (ACC), an important enzyme that catalyzes the first reaction of fatty acid biosynthesis, is biotinylated by biotin acetyl-CoA carboxylase ligase (BirA). The ligand-binding loops in all known apo BirAs to date are disordered and attain an ordered structure only after undergoing a conformational change upon ligand-binding. Here, we report that dehydration of Mtb-BirA crystals traps both the apo and active conformations in its asymmetric unit, and for the first time provides structural evidence of such transformation. Recombinant Mtb-BirA was crystallized at room temperature, and diffraction data was collected at 295 K as well as at 120 K. Transfer of crystals to paraffin and paratone-N oil (cryoprotectants) prior to flash-freezing induced lattice shrinkage and enhancement in the resolution of the X-ray diffraction data. Intriguingly, the crystal lattice rearrangement due to shrinkage in the dehydrated Mtb-BirA crystals ensued structural order of otherwise flexible ligand-binding loops L4 and L8 in apo BirA. In addition, crystal dehydration resulted in a shift of approximately 3.5 A in the flexible loop L6, a proline-rich loop unique to Mtb complex as well as around the L11 region. The shift in loop L11 in the C-terminal domain on dehydration emulates the action responsible for the complex formation with its protein ligand biotin carboxyl carrier protein (BCCP) domain of ACCA3. This is contrary to the involvement of loop L14 observed in Pyrococcus horikoshii BirA-BCCP complex. Another interesting feature that emerges from this dehydrated structure is that the two subunits A and B, though related by a noncrystallographic twofold symmetry, assemble into an asymmetric dimer representing the ligand-bound and ligand-free states of the protein, respectively. In-depth analyses of the sequence and the structure also provide answers to the reported lower affinities of Mtb-BirA toward ATP and biotin substrates. This dehydrated crystal structure not only provides key leads to the understanding of the structure/function relationships in the protein in the absence of any ligand-bound structure, but also demonstrates the merit of dehydration of crystals as an inimitable technique to have a glance at proteins in action.

ACCase 6 is the essential acetyl-CoA carboxylase involved in fatty acid and mycolic acid biosynthesis in mycobacteria

Mycolic acids are essential for the survival, virulence and antibiotic resistance of the human pathogen Mycobacterium tuberculosis. Inhibitors of mycolic acid biosynthesis, such as isoniazid and ethionamide, have been used as efficient drugs for the treatment of tuberculosis. However, the increase in cases of multidrug-resistant tuberculosis has prompted a search for new targets and agents that could also affect synthesis of mycolic acids. In mycobacteria, the acyl-CoA carboxylases (ACCases) provide the building blocks for de novo fatty acid biosynthesis by fatty acid synthase (FAS) I and for the elongation of FAS I products by the FAS II complex to produce meromycolic acids. By generating a conditional mutant in the accD6 gene of Mycobacterium smegmatis, we demonstrated that AccD6 is the essential carboxyltransferase component of the ACCase 6 enzyme complex implicated in the biosynthesis of malonyl-CoA, the substrate of the two FAS enzymes of Mycobacterium species. Based on the conserved structure of the AccD5 and AccD6 active sites we screened several inhibitors of AccD5 as potential inhibitors of AccD6 and found that the ligand NCI-172033 was capable of inhibiting AccD6 with an IC(50) of 8 microM. The compound showed bactericidal activity against several pathogenic Mycobacterium species by producing a strong inhibition of both fatty acid and mycolic acid biosynthesis at minimal inhibitory concentrations. Overexpression of accD6 in M. smegmatis conferred resistance to NCI-172033, confirming AccD6 as the main target of the inhibitor. These results define the biological role of a key ACCase in the biosynthesis of membrane and cell envelope fatty acids, and provide a new target, AccD6, for rational development of novel anti-mycobacterial drugs.

Defining mycobacteria: Shared and specific genome features for different lifestyles

During the last decade, the combination of rapid whole genome sequencing capabilities, application of genetic and computational tools, and establishment of model systems for the study of a range of species for a spectrum of biological questions has enhanced our cumulative knowledge of mycobacteria in terms of their growth properties and requirements. The adaption of the corynebacterial surrogate system has simplified the study of cell wall biosynthetic machinery common to actinobacteria. Comparative genomics supported by experimentation reveals that superimposed on a common core of 'mycobacterial' gene set, pathogenic mycobacteria are endowed with multiple copies of several protein families that encode novel secretion and transport systems such as mce and esx; immunomodulators named PE/PPE proteins, and polyketide synthases for synthesis of complex lipids. The precise timing of expression, engagement and interactions involving one or more of these redundant proteins in their host environments likely play a role in the definition and differentiation of species and their disease phenotypes. Besides these, only a few species specific 'virulence' factors i.e., macromolecules have been discovered. Other subtleties may also arise from modifications of shared macromolecules. In contrast, to cope with the broad and changing growth conditions, their saprophytic relatives have larger genomes, in which the excess coding capacity is dedicated to transcriptional regulators, transporters for nutrients and toxic metabolites, biosynthesis of secondary metabolites and catabolic pathways. In this review, we present a sampling of the tools and techniques that are being implemented to tease apart aspects of physiology, phylogeny, ecology and pathology and illustrate the dominant genomic characteristics of representative species. The investigation of clinical isolates, natural disease states and discovery of new diagnostics, vaccines and drugs for existing and emerging mycobacterial diseases, particularly for multidrug resistant strains are the challenges in the coming decades.

Identifying residues in antigenic determinants by chemical modification

Chemical modification of the side chains of amino acid residues was one of the first methods developed to investigate epitopes in protein antigens. The principle of the method is that alteration of the structure of a key residue of an epitope by a chemical modification will alter reactivity with antibody by affecting either specificity or avidity or both. Chemical modification has the advantage that it can be applied to discontinuous as well as continuous epitopes and may be of value in identifying cryptic epitopes. We consider here the several recent studies that have applied site-specific chemical modification to the identification of epitopes on antigens, including the use of formaldehyde, glutaraldehyde, and acid anhydrides, to produce allergoids where determinants important to reaction with IgE are modified but the ability to elicit an IgG response is retained. It is noteworthy that modification of amino groups with charge reversal appears to be the most useful approach. The approach to the use of site-specific chemical modification as a tool for the study of protein function is discussed, and emphasis is placed on the necessity to (1) validate the specificity of modification and (2) assess potential conformational change that may occur secondary to modification. Finally, a list of chemical reagents used for protein modification is presented, together with properties and references to use.

The genetics of cell wall biosynthesis in Mycobacterium tuberculosis

Despite an available vaccine and effective antibiotics, Mycobacterium tuberculosis is still the causative agent of almost 2 million deaths every year. The cell wall of M. tuberculosis is composed of sugars and lipids of exotic structure, many of which contribute to its pathogenicity. The majority of the enzymes responsible for building this structure are essential. However, they share very little homology with well-characterized enzymes, which makes their identification in the genome difficult. Despite this, our knowledge of the structure of the cell wall of M. tuberculosis is fairly complete and an increasing number of genes have been identified that are involved in its biosynthesis. By contrast, data concerning regulation of the expression of these genes and control of the cell wall composition are restricted. This review summarizes current information on the genetics of cell wall biosynthesis in M. tuberculosis, incorporating available data on gene organization and regulation.

Comparative genomics provides evidence for the 3-hydroxypropionate autotrophic pathway in filamentous anoxygenic phototrophic bacteria and in hot spring microbial mats

Stable carbon isotope signatures of diagnostic lipid biomarkers have suggested that Roseiflexus spp., the dominant filamentous anoxygenic phototrophic bacteria inhabiting microbial mats of alkaline siliceous hot springs, may be capable of fixing bicarbonate via the 3-hydroxypropionate pathway, which has been characterized in their distant relative, Chloroflexus aurantiacus. The genomes of three filamentous anoxygenic phototrophic Chloroflexi isolates (Roseiflexus sp. RS-1, Roseiflexus castenholzii and Chloroflexus aggregans), but not that of a non-photosynthetic Chloroflexi isolate (Herpetosiphon aurantiacus), were found to contain open reading frames that show a high degree of sequence similarity to genes encoding enzymes in the C. aurantiacus pathway. Metagenomic DNA sequences from the microbial mats of alkaline siliceous hot springs also contain homologues of these genes that are highly similar to genes in both Roseiflexus spp. and Chloroflexus spp. Thus, Roseiflexus spp. appear to have the genetic capacity for carbon dioxide reduction via the 3-hydroxypropionate pathway. This may contribute to heavier carbon isotopic signatures of the cell components of native Roseiflexus populations in mats compared with the signatures of cyanobacterial cell components, as a similar isotopic signature would be expected if Roseiflexus spp. were participating in photoheterotrophic uptake of cyanobacterial photosynthate produced by the reductive pentose phosphate cycle.

The two carboxylases of Corynebacterium glutamicum essential for fatty acid and mycolic acid synthesis

The suborder Corynebacterianeae comprises bacteria like Mycobacterium tuberculosis and Corynebacterium glutamicum, and these bacteria contain in addition to the linear fatty acids, unique alpha-branched beta-hydroxy fatty acids, called mycolic acids. Whereas acetyl-coenzyme A (CoA) carboxylase activity is required to provide malonyl-CoA for fatty acid synthesis, a new type of carboxylase is apparently additionally present in these bacteria. It activates the alpha-carbon of a linear fatty acid by carboxylation, thus enabling its decarboxylative condensation with a second fatty acid to afford mycolic acid synthesis. We now show that the acetyl-CoA carboxylase of C. glutamicum consists of the biotinylated alpha-subunit AccBC, the beta-subunit AccD1, and the small peptide AccE of 8.9 kDa, forming an active complex of approximately 812,000 Da. The carboxylase involved in mycolic acid synthesis is made up of the two highly similar beta-subunits AccD2 and AccD3 and of AccBC and AccE, the latter two identical to the subunits of the acetyl-CoA carboxylase complex. Since AccD2 and AccD3 orthologues are present in all Corynebacterianeae, these polypeptides are vital for mycolic acid synthesis forming the unique hydrophobic outer layer of these bacteria, and we speculate that the two beta-subunits present serve to lend specificity to this unique large multienzyme complex.