Drugs that Inhibit Mycolic Acid Biosynthesis in Mycobacterium tuberculosis

Abstract

Tuberculosis resurged in the late 1980s and now kills more than 2 million people a year. The reemergence of tuberculosis as a potential public health threat, the high susceptibility of human immunodeficiency virus-infected persons to the disease, and the proliferation of multi-drug-resistant (MDR) strains have created much scientific interest in developing new antimycobacterial agents to both treat Mycobacterium tuberculosis strains resistant to existing drugs, and shorten the duration of short-course treatment to improve patient compliance. Bacterial cell-wall biosynthesis is a proven target for new antibacterial drugs. Mycolic acids, which are key components of the mycobacterial cell wall, are alpha-alkyl, beta-hydroxy fatty acids, with a species-dependent saturated "short" arm of 20-26 carbon atoms and a "long" meromycolic acid arm of 50-60 carbon atoms. The latter arm is functionalized at regular intervals by cyclopropyl, alpha-methyl ketone, or alpha-methyl methylethers groups. The mycolic acid biosynthetic pathway has been proposed to involve five distinct stages: (i) synthesis of C20 to C26 straight-chain saturated fatty acids to provide the alpha-alkyl branch; (ii) synthesis of the meromycolic acid chain to provide the main carbon backbone, (iii) modification of this backbone to introduce other functional groups; (iv) the final Claisen-type condensation step followed by reduction; and (v) various mycolyltransferase processes to cellular lipids. The drugs shown to inhibit mycolic acid biosynthesis are isoniazid, ethionamide, isoxyl, thiolactomycin, and triclosan. In addition, pyrazinamide was shown to inhibit fatty acid synthase type I which, in turn, provides precursors for fatty acid elongation to long-chain mycolic acids by fatty acid synthase II. Here we review the biosynthesis of mycolic acids and the mechanism of action of antimicrobial agents that act upon this pathway. In addition, we describe molecular modeling studies on InhA, the bona-fide target for isoniazid, which should improve our understanding of the amino acid residues involved in the enzyme's mechanism of action and, accordingly, provide a rational approach to the design of new drugs.

An inorganic complex that inhibits Mycobacterium tuberculosis enoyl reductase as a prototype of a new class of chemotherapeutic agents to treat tuberculosis

“…However, it should be pointed out that the mechanism of action of INH is complex, as mutations in at least five different genes (katG, inhA, ahpC, kasA, and ndh) have been found to correlate with INH resistance. 10 At any rate, the IQG607 M. tuberculosis growth inhibition correlates well with in vitro inhibitory activity against WT InhA enzyme 21 and InhA mutant enzymes identified in INH-resistant clinical isolates of M. tuberculosis. 22 Although we have not been able to obtain crystals to provide an experimental three-dimensional structure of InhA-IQG607 binary complex, results of docking simulations of IQG607 in the cofactor binding site of WT InhA crystal structure suggest that the inhibitor molecule may undergo small conformational changes after initial docking in the active site of WT InhA and that IQG607 preferentially occupies the pyrophosphate and nicotinamide sites in the NAD(H) binding site.…”

“…13 InhA is a member of the mycobacterial FAS-II system, which elongates acyl fatty acid precursors yielding the long carbon chain of the meromycolate branch of mycolic acids, the hallmark of mycobacteria. 10 Consistent with InhA being a target for INH, mutations in the inhA structural gene were identified in INH-resistant clinical isolates of M. tuberculosis. 14 More recently, it was shown by specialized linkage transduction that the introduction of inhA (S94A) point mutation in M. tuberculosis confers clinically relevant levels of resistance to INH killing and inhibition of mycolic acid biosynthesis.…”

“…11 Mycobacteria, unlike most organisms, have both FAS-I and FAS-II systems. 10 The product of the M. tuberculosis inhA gene (InhA) has been shown to be a major target for isoniazid (INH, isonicotinic acid hydrazide), the most prescribed drug for active TB infection and prophylaxis. 12 InhA was identified as an NADH-dependent enoyl-ACP (acyl carrier protein) reductase enzyme, which exhibits specificity for long-chain (C 18 > C 16 ) enoyl thioester substrates.…”

“…9,10 Mycolic acids are high-molecular-weight a-alkyl, b-hydroxy fatty acids, which appear mostly as bound esters in tetramycolylpentaarabinosyl clusters in the mycobacterial cell wall. The condensation, b-keto reduction, dehydration, and enoyl reduction chemical reactions of fatty acid elongation are catalyzed by two types of fatty acid synthase (FAS) systems.…”

The resumption of consumption: a review on tuberculosis

“…Isoniazid is a synthetic pro-drug that requires the product of the katG structural gene for its activation (Telenti & Iseman 2000); this drug becomes an active compound once it is metabolized by the M. tuberculosis catalase-peroxidase enzyme, and inhibits the activity of the enoyl-ACP (CoA) reductase enzyme (encoded by the inhA gene) in the presence of NADH or NAD + (reviewed in , Schroeder et al 2002. Resistance to isoniazid is more complex, as it involves at least 4 genes: katG, which mediates both susceptibility and resistance to isoniazid, and encodes the catalase-peroxidase enzyme; inhA, which is involved in the elongation of fatty acids (Zhang et al 1992); ahpC, which encodes the hydroperoxide alquil reductase C; and oxyR, which is an important regulator of oxidative stress (Telenti & Iseman 2000).…”