Impact of the epoxide hydrolase EphD on the metabolism of mycolic acids in mycobacteria.

Mycolic acids are the hallmark of the cell envelope in mycobacteria, which include the important human pathogens Mycobacterium tuberculosis and Mycobacterium leprae Mycolic acids are very long C60-C90 α-alkyl ß-hydroxy fatty acids having a variety of functional groups on their hydrocarbon chain that define several mycolate types. Mycobacteria also produce an unusually large number of putative epoxide hydrolases, but the physiological functions of these enzymes are still unclear. Here, we report that the mycobacterial epoxide hydrolase EphD is involved in mycolic acid metabolism. We found that orthologs of EphD from M. tuberculosis and M. smegmatis are functional epoxide hydrolases, cleaving a lipophilic substrate, 9,10-cis-epoxystearic acid, in vitro and forming a vicinal diol. The results of EphD overproduction in M. smegmatis and M. bovis BCG Δhma strains producing epoxymycolic acids indicated that EphD is involved in the metabolism of these forms of mycolates in both fast- and slow-growing mycobacteria. Moreover, using MALDI-TOF-MS and 1H NMR spectroscopy of mycolic acids and lipids isolated from EphD-overproducing M. smegmatis, we identified new oxygenated mycolic acid species that accumulated during epoxymycolate depletion. Disruption of the ephD gene in M. tuberculosis specifically impaired the synthesis of ketomycolates and caused accumulation of their precursor, hydroxymycolate, indicating either direct or indirect involvement of EphD in ketomycolate biosynthesis. Our results clearly indicate that EphD plays a role in metabolism of oxygenated mycolic acids in mycobacteria.

Mycolic acids: structures, biosynthesis, and beyond.

Mycolic acids are major and specific lipid components of the mycobacterial cell envelope and are essential for the survival of members of the genus Mycobacterium that contains the causative agents of both tuberculosis and leprosy. In the alarming context of the emergence of multidrug-resistant, extremely drug-resistant, and totally drug-resistant tuberculosis, understanding the biosynthesis of these critical determinants of the mycobacterial physiology is an important goal to achieve, because it may open an avenue for the development of novel antimycobacterial agents. This review focuses on the chemistry, structures, and known inhibitors of mycolic acids and describes progress in deciphering the mycolic acid biosynthetic pathway. The functional and key biological roles of these molecules are also discussed, providing a historical perspective in this dynamic area.

Breaking down the wall: fractionation of mycobacteria.

Mycobacterium spp. possess a complex cell envelope that consists of a plasma membrane, a peptidoglycan-arabinogalactan complex which in turn is esterified by mycolic acids that form with other non-bound lipids an asymmetric permeability barrier and an outer layer, also called a capsule in the case of pathogenic species. In order to investigate the functional roles of the cell envelope components, especially those of the major pathogens Mycobacterium tuberculosis and Mycobacterium leprae, it is necessary to fractionate the envelope by breaking the unusual wall that covers these bacteria. To this aim we first compared the efficiency of high pressure (cell disrupter/French press) with those of pathogen-compatible breakage methods such as sonication, bead beater and lysozyme treatment using the non-pathogenic Mycobacterium smegmatis. When the distribution of various specific markers of the cell envelope compartments, which include mycolic acids, arabinose, NADH oxidase activity, cell wall and cytosolic proteins, were determined sonication combined with lysozyme treatment was found to be the best option. The protocol of subcellular fractionation was then validated for pathogenic species by applying the method to Mycobacterium bovis BCG cells, an attenuated strain of the M. tuberculosis complex.

The acyl-AMP ligase FadD32 and AccD4-containing acyl-CoA carboxylase are required for the synthesis of mycolic acids and essential for mycobacterial growth: identification of the carboxylation product and determination of the acyl-CoA carboxylase components.

Mycolic acids are major and specific long-chain fatty acids of the cell envelope of several important human pathogens such as Mycobacterium tuberculosis, M. leprae, and Corynebacterium diphtheriae. Their biosynthesis is essential for mycobacterial growth and represents an attractive target for developing new antituberculous drugs. We have previously shown that the pks13 gene encodes condensase, the enzyme that performs the final condensation step of mycolic acid biosynthesis and is flanked by two genes, fadD32 and accD4. To determine the functions of the gene products we generated two mutants of C. glutamicum with an insertion/deletion within either fadD32 or accD4. The two mutant strains were deficient in mycolic acid production and exhibited the colony morphology that typifies the mycolate-less mutants of corynebacteria. Application of multiple analytical approaches to the analysis of the mutants demonstrated the accumulation of a tetradecylmalonic acid in the DeltafadD32::km mutant and its absence from the DeltaaccD4::km strain. The parental corynebacterial phenotype was restored upon the transfer of the wild-type fadD32 and accD4 genes in the mutants. These data demonstrated that both FadD32 and AccD4-containing acyl-CoA carboxylase are required for the production of mycolic acids. They also prove that the proteins catalyze, respectively, the activation of one fatty acid substrate and the carboxylation of the other substrate, solving the long-debated question of the mechanism involved in the condensation reaction. We used comparative genomics and applied a combination of molecular biology and proteomic technologies to the analysis of proteins that co-immunoprecipitated with AccD4. This resulted in the identification of AccA3 and AccD5 as subunits of the acyl-CoA carboxylase. Finally, we used conditionally replicative plasmids to show that both the fadD32 and accD4 genes are essential for the survival of M. smegmatis. Thus, in addition to Pks13, FadD32 and AccD4 are promising targets for the development of new antimicrobial drugs against pathogenic species of mycobacteria and related microorganisms.

Role of the pks15/1 gene in the biosynthesis of phenolglycolipids in the Mycobacterium tuberculosis complex. Evidence that all strains synthesize glycosylated p-hydroxybenzoic methyl esters and that strains devoid of phenolglycolipids harbor a frameshift mutation in the pks15/1 gene.

Diesters of phthiocerol and phenolphthiocerol are important virulence factors of Mycobacterium tuberculosis and Mycobacterium leprae, the two main mycobacterial pathogens in humans. They are both long-chain beta-diols, and their biosynthetic pathway is beginning to be elucidated. Although the two classes of molecules share a common lipid core, phthiocerol diesters have been found in all the strains of the M. tuberculosis complex examined although phenolphthiocerol diesters are produced by only a few groups of strains. To address the question of the origin of this diversity 8 reference strains and 10 clinical isolates of M. tuberculosis were analyzed. We report the presence of glycosylated p-hydroxybenzoic acid methyl esters, structurally related to the type-specific phenolphthiocerol glycolipids, in the culture media of all reference strains of M. tuberculosis, suggesting that the strains devoid of phenolphthiocerol derivatives are unable to elongate the putative p-hydroxybenzoic acid precursor. We also show that all the strains of M. tuberculosis examined and deficient in the production of phenolphthiocerol derivatives are natural mutants with a frameshift mutation in pks15/1 whereas a single open reading frame for pks15/1 is found in Mycobacterium bovis BCG, M. leprae, and strains of M. tuberculosis that produce phenolphthiocerol derivatives. Complementation of the H37Rv strain of M. tuberculosis, which is devoid of phenolphthiocerol derivatives, with the fused pks15/1 gene from M. bovis BCG restored phenolphthiocerol glycolipids production. Conversely, disruption of the pks15/1 gene in M. bovis BCG led to the abolition of the synthesis of type-specific phenolphthiocerol glycolipid. These data indicate that Pks15/1 is involved in the elongation of p-hydroxybenzoic acid to give p-hydroxyphenylalkanoates, which in turn are converted, presumably by the PpsA-E synthase, to phenolphthiocerol derivatives.