Overview of the Mycobacterial Mycolic Acid Biosynthesis Pathway

What is Mycolic Acid?

Mycolic acids, a homologous series of C60-C90 long-chain alpha-alkyl-&beta-hydroxy fatty acids, represent essential components of the mycobacterial cell wall. They are important for mycobacterial growth, survival, and pathogenicity. They are found as esters of an arabinogalactan as well as free lipids in the form of trehalose di mycolate (TDM). Arabinogalactan-mycolate is covalently linked to the cell wall peptidoglycan via a phosphodiester bond located on the inner leaflet of the outer membrane. Both arabinogalactan-mycolate and TDM provide a protective thick cell wall and protect the tubercle bacillus from antibiotics and host's immune system. TDM also inhibits phago-lysosome fusion and is often considered to be an indicator of virulent strains.

Three distinct structural classes of mycolic acids namely alpha- (more than 70 percent), methoxy- and keto-mycolic acids (10-15 percent) are found in this bacillus. The alpha-mycolic acid is a cis,cis-dicyclopropyl fatty acid. Both methoxy- and keto-mycolic acids have either cis- or trans-cyclopropane rings. Cyclopropane rings in mycolic acids protect the bacillus from oxidative stress [ 15653820, 17555433, 15353567 ].

Mycolic acid pathway as TB drug targets

Several front-line drugs used for treating tuberculosis inhibit mycolic acid synthesis. Understanding the pathway of mycolate biosynthesis therefore helps understand the underlying molecular mechanisms of the disease tuberculosis as well as the identification of new anti-tuberculosis drug targets. InhA (EC 1.3.1.9, enoyl-[acyl-carrier-protein] reductase), involved in mycolic acid synthesis, is a target for front-line anti-tubercular drugs, such as isoniazid and ethionamide [16261191]. Enzymes needed for biosynthesis of mycolic acids, such as methyl transferase (PcaA), beta-ketoacyl-acyl carrier protein synthase (KasAB and FabH), acyl-AMP ligase (Fad32) and polyketide synthase (Psk13) are promising drug targets for new anti-TB agents [20477209].

Critical steps of the pathway

Mycolates are synthesized by at least two discrete elongation systems in Mycobacteria - the type I and type II fatty acid synthases (FAS-I and FAS-II respectively). The eukaryotic-like FAS-I is a single polypeptide with multiple catalytic activities (encoded by the fas gene, Rv2524c) that catalyses the de novo synthesis of fatty acids from acetyl-CoA. The domains of M. tuberculosis FAS are organized in the following order: acyltransferase, enoyl reductase, dehydratase, malonyl/palmitoyl transferase, acyl carrier protein, beta-ketoacyl reductase, and -ketoacyl synthase. FAS-I exhibits a bimodal product distribution: C20 and C26 acyl CoAs. These form the substrates for the FAS-II reaction cycle and the polyketide synthase enzyme respectively. Beta-ketoacyl-ACP synthase III forms a pivotal link between FAS-I and FAS-II. FAS II, similar to that in other bacteria, consists of dissociable enzyme components, which act on a substrate bound to an acyl-carrier protein (ACP). Four condensases participate in initiation (FabH), elongation (KasA and KasB) and termination (Pks13) steps, leading to full-length mycolates. Synthesis of mycolic acids in the cell is followed by extensive postsynthetic modifications and unsaturation reactions. Condensation of the fully functionalized and preformed meromycolate chain with a 26-carbon alpha-branch generates full-length mycolic acids that must be transported to their final location for attachment to the cell-wall arabinogalactan. Three well-known immunogenic proteins of the antigen 85 complex mediate the transfer and subsequent transesterification. The basic reactions in FAS-I and FAS-II are a repetition of a cycle of four reactions, each cycle culminating in the extension of the alkyl chain by a two-carbon unit [15653820, 15353567]. A mycobacterial mycolic acid biosynthesis model has been developed in pathway logic. A guided tour of this model can be found here . You can browse and analyse the Pathway Logic mycolic acid biosynthesis model using the Pathway Logic Assistant client . Just click on the link and follow instructions. The guide includes suggestions for subnets to explore and instructions for creating them using the Pathway Logic Assistant viewer.