Ethylenic mycolic acid biosynthesis: extension of the biosynthetic model using cell-free extracts of Mycobacterium aurum and Mycobacterium smegmatis

Mycobacterium alvei (ω-1)-methoxy mycolic acids: Absolute stereochemistry and synthesis

Cells of Mycobacterium alvei are known to contain a unique set of mycolic acids with a (ω-1)-methoxy group; although the various enzymes in the biosynthesis of other types of mycolic acid have been widely studied, the biosynthetic route to this substituent is unclear. We now define the stereochemistry of the (ω-1)-methoxy fragment as R, and describe the synthesis of a major R-(ω-1)-methoxy-mycolic acid and its sugar esters, and of two natural M. alvei diene mycolic acids.

The Crucial Role of Trehalose and Structurally Related Oligosaccharides in the Biosynthesis and Transfer of Mycolic Acids in Corynebacterineae

Trehalose (alpha-D-glucopyranosyl-alpha'-D-glucopyranoside) is essential for the growth of the human pathogen Mycobacterium tuberculosis but not for the viability of the phylogenetically related corynebacteria. To determine the role of trehalose in the physiology of these bacteria, the so-called Corynebacterineae, mutant strains of Corynebacterium glutamicum unable to synthesize trehalose due to the knock-out of the genes of the three pathways of trehalose biosynthesis, were biochemically analyzed. We demonstrated that the synthesis of trehalose under standard conditions is a prerequisite for the production of mycolates, major and structurally important constituents of the cell envelope of Corynebacterineae. Consistently, the trehalose-less cells also lack the cell wall fracture plane that typifies mycolate-containing bacteria. Importantly, however, the mutants were able to synthesize mycolates when grown on glucose, maltose, and maltotriose but not on other carbon sources known to be used for the production of internal glucose phosphate such as fructose, acetate, and pyruvate. The mycoloyl residues synthesized by the mutants grown on alpha-D-glucopyranosyl-containing oligosaccharides were transferred both onto the cell wall and free sugar acceptors. A combination of chemical analytical approaches showed that the newly synthesized glycolipids consisted of 1 mol of mycolate located on carbon 6 of the non reducing glucopyranosyl unit. Additionally, experiments with radioactively labeled trehalose showed that the transfer of mycoloyl residues onto sugars occurs outside the plasma membrane. Finally, and in contradiction to published data, we demonstrated that trehalose 6-phosphate has no impact on mycolate synthesis in vivo.

Lipid biosynthesis as a target for antibacterial agents

Fatty acid biosynthesis, the first stage in membrane lipid biogenesis, is catalyzed in most bacteria by a series of small, soluble proteins that are each encoded by a discrete gene (Fig. 1; Table 1). This arrangement is termed the type II fatty acid synthase (FAS) system and contrasts sharply with the type I FAS of eukaryotes which is a dimer of a single large, multifunctional polypeptide. Thus, the bacterial pathway offers several unique sites for selective inhibition by chemotherapeutic agents. The site of action of isoniazid, used in the treatment of tuberculosis for 50 years, and the consumer antimicrobial agent triclosan were revealed recently to be the enoyl-ACP reductase of the type II FAS. The fungal metabolites, cerulenin and thiolactomycin, target the condensing enzymes of the bacterial pathway while the dehydratase/isomerase is inhibited by a synthetic acetylenic substrate analogue. Transfer of fatty acids to the membrane has also been inhibited via interference with the first acyltransferase step, while a new class of drugs targets lipid A synthesis. This review will summarize the data generated on these inhibitors to date, and examine where additional efforts will be required to develop new chemotherapeutics to help combat microbial infections.

Crystal structure of the Mycobacterium tuberculosis enoyl-ACP reductase, InhA, in complex with NAD+ and a C16 fatty acyl substrate

Enoyl-ACP reductases participate in fatty acid biosynthesis by utilizing NADH to reduce the trans double bond between positions C2 and C3 of a fatty acyl chain linked to the acyl carrier protein. The enoyl-ACP reductase from Mycobacterium tuberculosis, known as InhA, is a member of an unusual FAS-II system that prefers longer chain fatty acyl substrates for the purpose of synthesizing mycolic acids, a major component of mycobacterial cell walls. The crystal structure of InhA in complex with NAD+ and a C16 fatty acyl substrate, trans-2-hexadecenoyl-(N-acetylcysteamine)-thioester, reveals that the substrate binds in a general "U-shaped" conformation, with the trans double bond positioned directly adjacent to the nicotinamide ring of NAD+. The side chain of Tyr158 directly interacts with the thioester carbonyl oxygen of the C16 fatty acyl substrate and therefore could help stabilize the enolate intermediate, proposed to form during substrate catalysis. Hydrophobic residues, primarily from the substrate binding loop (residues 196-219), engulf the fatty acyl chain portion of the substrate. The substrate binding loop of InhA is longer than that of other enoyl-ACP reductases and creates a deeper substrate binding crevice, consistent with the ability of InhA to recognize longer chain fatty acyl substrates.

A mutant of Mycobacterium smegmatis defective in the biosynthesis of mycolic acids accumulates meromycolates

Mycolic acids are a major constituent of the mycobacterial cell wall, and they form an effective permeability barrier to protect mycobacteria from antimicrobial agents. Although the chemical structures of mycolic acids are well established, little is known on their biosynthesis. We have isolated a mycolate-deficient mutant strain of Mycobacterium smegmatis mc2-155 by chemical mutagenesis followed by screening for increased sensitivity to novobiocin. This mutant also was hypersensitive to other hydrophobic compounds such as crystal violet, rifampicin, and erythromycin. Entry of hydrophobic probes into mutant cells occurred much more rapidly than that into the wild-type cells. HPLC and TLC analysis of fatty acid composition after saponification showed that the mutant failed to synthesize full-length mycolic acids. Instead, it accumulated a series of long-chain fatty acids, which were not detected in the wild-type strain. Analysis by 1H NMR, electrospray and electron impact mass spectroscopy, and permanganate cleavage of double bonds showed that these compounds corresponded to the incomplete meromycolate chain of mycolic acids, except for the presence of a beta-hydroxyl group. This direct identification of meromycolates as precursors of mycolic acids provides a strong support for the previously proposed pathway for mycolic acid biosynthesis involving the separate synthesis of meromycolate chain and the alpha-branch of mycolic acids, followed by the joining of these two branches.

Synthesis of mycolic acids of mycobacteria: an assessment of the cell-free system in light of the whole genome

Mycolic acids are 70-90 carbon, alpha-alkyl, beta-hydroxy fatty acids constituting a major component of the cell envelope of Mycobacterium tuberculosis. The fact that the mycolic acid biosynthetic pathway is both essential in mycobacteria and the target for many first-line anti-TB drugs necessitates a detailed understanding of its biochemistry. A whole cell-free, but cell particulate- and membrane-containing enzyme preparation for mycolic acid biosynthesis was developed a few years ago and studied extensively. This system was shown to catalyze the synthesis of mature mycolic acids from [14C]acetate, but allows only minimal deposition into the cell wall proper. In the meantime the sequence of the entire genome of M. tuberculosis has been elucidated and its analysis using numerous protein sequence-based algorithms predicted cytoplasmic localization and a soluble, not a particulate, nature for the enzymes involved in the mycolic acid synthetic pathway. Accordingly, we re-assessed the 'cell-free' system for mycolic acid synthesis and concluded that it is probably due to the presence of unbroken cells, since viable cells were recovered from the cell wall preparation. The amount of whole cells depended upon the efficiency of the cell disruption method and conditions, and the amount of mycolic acid synthesized by the putative cell-free system correlated with the content of whole cells. Thus, accumulated results from the use of this 'cell-free' cell wall-based system should be re-evaluated in the light of these new data.

The biosynthesis of mycolic acids in Mycobacterium tuberculosis. Enzymatic methyl(ene) transfer to acyl carrier protein bound meromycolic acid in vitro

A closely related family of enzymes from Mycobacterium tuberculosis has been shown by heterologous expression to catalyze the modification of mycolic acids through the addition of a methyl (or methylene) group derived from S-adenosyl-L-methionine (SAM). Overproduction of all six of these enzymes in Escherichia coli and subsequent in vitro reactions with heat-inactivated acceptor fractions derived from Mycobacterium smegmatis in the presence of [methyl-3H]SAM demonstrated that the immediate substrate to which methyl group addition occurs was a family of very long-chain fatty acids. Inhibitors of methyl transfer, such as S-adenosyl-L-homocysteine and sinefungin, were shown to inhibit this reaction but had no effect on whole cells of either M. smegmatis or M. tuberculosis. Purified mycolic acids from M. tuberculosis were pyrolyzed, and the resulting meroaldehyde was oxidized and methylated to produce full-length methyl meromycolates. These esters were shown to comigrate with a fraction of the acceptor from the in vitro reactions, suggesting that methyl group addition occurs up to the level of the meromycolate. Protease and other treatments destroyed the activity of the acceptor fraction, which was also found to be extremely sensitive to basic pH. Antibody to the acyl carrier protein AcpM, which has recently been shown to be the carrier of full-length meromycolate produced by a unique type II fatty acid synthase system, inhibited the cell-free methyl(en)ation of these acids. These results suggest that mycolate modification reactions occur parallel with the synthesis of the AcpM-bound meromycolate chain.

Structure of a hydroxymycolic acid potentially involved in the synthesis of oxygenated mycolic acids of the Mycobacterium tuberculosis complex

Mycolic acids are believed to play a crucial role in the architecture of the mycobacterial envelope. However, very few steps of their biosynthetic pathway have yet been elucidated. We previously isolated [Dubnau, E., Lanéelle, M. A., Soares, S., Bénichou, A., Vaz, T., Promé, D., Promé, J. C., Daffé, M. & Quémard, A. (1997) Mycobacterium bovis BCG genes involved in the biosynthesis of cyclopropyl keto- and hydroxy-mycolic acids, Mol. Microbiol. 23, 313-322] a gene cluster from Mycobacterium bovis BCG, cmaA-D, which confers upon M. smegmatis the ability to synthesize cyclopropanated ketomycolic acid, and a new type of mycolic acid which is hydroxylated. A meticulous analysis of all the mycolic-like fatty acids of M. bovis BCG and M. tuberculosis showed that these organisms produce small amounts of the hydroxymycolic acid. The structure of this molecule, determined by NMR spectroscopy, mass spectrometry and stereochemical studies, strongly suggests that there is a direct biosynthetic relationship between the keto- and the hydroxy-mycolic acids.

The envelope layers of mycobacteria with reference to their pathogenicity

The review discusses current knowledge of the biosynthesis, composition and arrangement of the mycobacterial envelope, describes the biological activities of the constituents and considers how these activities may be relevant to the pathology of mycobacterial disease. The envelope possesses three structural components: plasma membrane, wall and capsule. Although the major biomolecules occurring in each of these parts are known, the distribution of numerous minor substances is poorly understood; an attempt has been made to assign them to particular positions on rational grounds. The plasma membrane appears to be a typical bacterial membrane but, though vital to the mycobacterium, probably plays little part in pathological processes. The wall partly resembles a Gram-positive wall, but is unusual in having a layer of lipid (mycolate esters) which is probably arranged to form a permeability barrier to polar molecules. The capsule, whose chemical composition has only recently been recognized, consists of polysaccharide and protein with traces of lipid; the arrangement of these components is imperfectly understood. Constituents of all parts of the envelope have biological activities which may be relevant. The likely importance of these activities in the overall effect of the envelope is considered.

inhA, a gene encoding a target for isoniazid and ethionamide in Mycobacterium tuberculosis

Isoniazid (isonicotinic acid hydrazide, INH) is one of the most widely used antituberculosis drugs, yet its precise target of action on Mycobacterium tuberculosis is unknown. A missense mutation within the mycobacterial inhA gene was shown to confer resistance to both INH and ethionamide (ETH) in M. smegmatis and in M. bovis. The wild-type inhA gene also conferred INH and ETH resistance when transferred on a multicopy plasmid vector to M. smegmatis and M. bovis BCG. The InhA protein shows significant sequence conservation with the Escherichia coli enzyme EnvM, and cell-free assays indicate that it may be involved in mycolic acid biosynthesis. These results suggest that InhA is likely a primary target of action for INH and ETH.

Mycolic acid synthesis: a target for ethionamide in mycobacteria?

Striking structural analogies exist between the two specific antimycobacterial drugs ethionamide (ETH) and isoniazid (INH), and they share several inhibitory properties in susceptible species of mycobacteria. The effect of ETH on mycolic acid synthesis was studied in whole cells and in cell extracts of various species, since this synthesis is one direct target for INH, as we recently demonstrated in cell extracts of Mycobacterium aurum. It was shown in the present study that there is not a direct relationship between ETH susceptibility and mycolic acid inhibition. This observation could explain the lack of cross-resistance between the two drugs. The presence of ETH disturbed mycolic acid synthesis in both resistant and susceptible mycobacteria. Synthesis of oxygenated species of mycolic acid was inhibited, while that of diunsaturated acids was either slightly altered or even increased. In contrast, INH inhibited the synthesis of all kinds of mycolic acids in the same way in all susceptible strains and had no effect on mycolic acid synthesis in resistant strains. In the presence of ETH, the unsaturated mycolic acid molecules presented a methyl end different from the usual one. These data strongly suggest that the normal unsaturated mycolic acid species are not the precursors of the oxygenated types. Moreover, they show that ETH probably acts early in the pathway leading to oxygenated mycolic acid.