# Mycobacterial Cell Wall: A Source of Successful Targets for Old and New Drugs
## 2 The Mycobacterial Cell Wall
mycolic acids are found attached to the arabin part of the arabin galactan complex but can also be found as a free form or bound to other saccharides to form trehalose mono/dimycolates (TMM/TDM) and glucose monomycolate. Mycolic acids are long-chain fatty acids composed of a meromycolate chain containing up to 62 carbons with various modifications (_cis/trans_ cyclopropane, keto, or methoxy groups) to which a saturated C\({}_{26}\) alkyl chain is attached at the \(\alpha\) position (Figure 2). The biosynthesis of mycolic acids starts with two fatty acid synthases (FAS): the eukaryotic-like type I (FAS-I) and the prokaryotic-like type II (FAS-II) (Figure 2). FAS-I is a multidomain polypeptide [30] that synthesizes the \(\alpha\)-C\({}_{26}\) alkyl chain and also provides a C\({}_{14/16}\) fatty acyl-CoA to be elongated into the meromycolate chain by the FAS-II system. This elongation reaction starts with the condensation of C\({}_{14/16}\) fatty acyl-CoA with malonyl-Acyl Carrier Protein (ACP) by the \(\beta\)-ketoacyl-ACP synthase III FabH. The resulting \(\beta\)-ketoacyl-ACP intermediate is delivered to FAS-II, which performs cycles of elongation using independent enzymes: the reductase MabA, the heterodimer dehydatases HadAB and HadBC, the enoyl-reductase InhA and the condensases KasA and KasB (Figure 2). The modifications (_cis/trans_ cyclopropane, keto, or methoxy groups) of the meromycolate chains are introduced either during the elongation by FAS-II or when the meromycolate chain is fully formed [31]. The meromycolate chain is activated by the fatty acid AMP synthase FadD32 to a meromycolyl-AMP, which is coupled to the carboxylated \(\alpha\)-C\({}_{26}\) fatty acyl-CoA (from FAS-I) by the polyketide synthase Pks13. Reduction of the resulting mycolic \(\beta\)-ketoester by the mycolyl reductase CmrA yields a mature mycolic acid. The biosynthesis of mycolic acids takes place in the cytoplasm. Transfer of the mycolic acids to the cell envelope occurs via the formation of a trehalose monomycolate (TMM) which is translocated by the efflux pump MmpL3 (Mycobacterial membrane protein Large). The mycolyltransferase Antigen 85 complex (_fbpA,fbpB,fbpC_) then condenses mycolic acids to the arabin galactan releasing a molecule of trehalose. Figure 2: Schematic representation of the biosynthesis of mycolic acids. The three families of mycolic acids in _Mycobacterium tuberculosis_ are the \(\alpha\)-, methoxy- and keto-mycolic acids. Only the cis configuration of the cyclopropane group of the methoxy- and keto-mycolic acids is shown, but _M. tuberculosis_ also produces the trans-methoxy- and trans-keto-mycolic acids. The known targets of the mycolic acid inhibitors in clinical use or in clinical trial are InhA for isoniazid (INH) and ethonamide (ETH); HadAB for thioacetazone (TAC); and MmpL3 for SQ109. Besides the mAGP complex, the mycobacterial cell wall also includes lipomannan (LM), lipoarabinomannan (LAM) and phosphatidyl-_myo_-inositol mannosides (PIMs). These lipids are anchored in the inner membrane and play an important role in _M. tuberculosis_ growth, survival and virulence [32; 33; 34]. In the outer membrane, trehalose-containing glycolipids and phthiocerol dimycoserates are found interlaced with the mycolic acids. These lipids are crucial for their interactions with the host and its immune system [35]. Specific components of the cell wall are essential for _M. tuberculosis_ survival in the host. It is, therefore, not surprising that two of the four first-line TB drugs target the cell wall. In the past 20 years, potent new TB drugs have been discovered that target the peptidoglycan, arabinogalactan, LAM, and mycolic acids or their transfer to the cell wall. This article focuses on the cell wall inhibitors included in the TB pharmacopeia since the 1950s/1960s (old TB drugs) and the new ones in clinical development.