Identification and substrate specificity of beta-ketoacyl (Acyl carrier protein) synthase III (mtFabH) from Mycobacterium tuberculosis

The long-chain alpha-alkyl-beta-hydroxy fatty acids, termed mycolic acids, which are characteristic components of the mycobacterial cell wall are prod uced by successive rounds of elongation catalyzed by a multifunctional (typ e I) fatty acid synthase complex followed by a dissociated (type II) fatty acid synthase, In bacterial type II systems, the first initiation step in e longation is the condensation of acetyl-CoA with malonyl-acyl carrier prote in (ACP) catalyzed by beta-ketoacyl-ACP III (FabH), An open reading frame i n the Mycobacterium tuberculosis genome (Rv0533c), now termed mtfabH, was 3 7.3% identical to Escherichia coli ecFabH and contained the Cys-His-Asn cat alytic triad signature. However, the purified recombinant mtFabH clearly pr eferred long-chain acyl-CoA substrates rather than acyl-ACP primers and did not utilize acetyl-CoA as a primer in comparison to ecFabH, In addition, p urified mtFabH was sensitive to thiolactomycin and resistant to cerulenin i n an in vitro assay, However, mtFabH overexpression in Mycobacterium bovis BCG did not confer thiolactomycin resistance, suggesting that mtFabH may no t be the primary target of thiolactomycin inhibition in vivo and led to sev eral changes in the lipid composition of the bacilli. The data presented is consistent with a role for mtFabH as the pivotal link between the type I a nd type II fatty acid elongation systems in M. tuberculosis. This study ope ns up new avenues for the development of selective and novel anti-mycobacte rial agents targeted against mtFabH.