Pathway to Synthesis and Processing of Mycolic Acids in Mycobacterium tuberculosis

Abstract

SUMMARY Mycobacterium tuberculosis is known to synthesize α-, methoxy-, and keto-mycolic acids. We propose a detailed pathway to the biosynthesis of all mycolic acids in M. tuberculosis . Fatty acid synthetase I provides C 20 -S-coenzyme A to the fatty acid synthetase II system (FAS-IIA). Modules of FAS-IIA and FAS-IIB introduce cis unsaturation at two locations on a growing meroacid chain to yield three different forms of cis , cis -diunsaturated fatty acids (intermediates to α-, methoxy-, and keto-meroacids). These are methylated, and the mature meroacids and carboxylated C 26 -S-acyl carrier protein enter into the final Claisen-type condensation with polyketide synthase-13 (Pks13) to yield mycolyl-S-Pks13. We list candidate genes in the genome encoding the proposed dehydrase and isomerase in the FAS-IIA and FAS-IIB modules. We propose that the processing of mycolic acids begins by transfer of mycolic acids from mycolyl-S-Pks13 to d -mannopyranosyl-1-phosphoheptaprenol to yield 6- O -mycolyl-β- d -mannopyranosyl-1-phosphoheptaprenol and then to trehalose 6-phosphate to yield phosphorylated trehalose monomycolate (TMM-P). Phosphatase releases the phosphate group to yield TMM, which is immediately transported outside the cell by the ABC transporter. Antigen 85 then catalyzes the transfer of a mycolyl group from TMM to the cell wall arabinogalactan and to other TMMs to produce arabinogalactan-mycolate and trehalose dimycolate, respectively. We list candidate genes in the genome that encode the proposed mycolyltransferases I and II, phosphatase, and ABC transporter. The enzymes within this total pathway are targets for new drug discovery.