Extensively drug resistant M. tuberculosis contributed further to deteriorate the control of tuberculosis in developing countries. In this context, the effective control of this major public health problem requires the identification of novel drug targets suitable for the development of new anti-mycobacterial drugs. Extensive enzymology and genetic evidences concerning the biosynthesis of mycolic acids and the other mycobacterial complex lipids are available; although several steps remain obscure. In particular, the information regarding the metabolic pathways involved in the biosynthesis of the elongation units used by Fatty Acid Synthases type I and II and PKSs in vivo is still very limited. These enzymes, whose molecular composition appear to be unique within the phylum Actinobacteria, are attractive targets for the development of new and specific LEE011 CDK inhibitor antimycobacterial agents. The reaction catalyzed by the ACCases occurs in two catalytic steps ; in the first step, the biotin carboxylase component couples carbonate to a biotin residue attached to a biotin carboxyl carrier protein to form carboxybiotin. A third ACCase complex, the so-called “long chain acylCoA carboxylase”, has been less characterized at the biochemical and structural levels; however there are some genetic and biochemical evidences suggesting that this enzyme complex could have a fairly complex subunit composition in actinomycetes. The first genetic studies that generated information regarding the long chain ACCase subunit composition were carried out in Corynebacterium glutamicum. The generation of an accD4 mutant in this organism resulted in a lack of mycolic acid production and in the absence of tetradecylmalonic acid, suggesting that AccD4 is the b component of the long-chain ACCase that generates the C16 a-carboxy acyl-CoA that, after its condensation with the meromycolyl-AMP forms the corynomycolic acid a-branch. An additional support to this conclusion came from the genomic organization of accD4 which is found clustered with and transcribed in the same orientation as pks13 and fadD32, the genes encoding for the enzyme system involved in the final step of biosynthesis of mycolic acids. Interestingly, the same genetic organization occurs for the orthologues of accD4, pks13 and fadD32 in mycobacteria suggesting that AccD4 of this organism plays the same role that its counterpart in C. glutamicum. Phylogenetic analyses showed that AccD4 is present exclusively in mycolic acid containing bacteria, confirming a specific role for this CT subunit in the biosynthesis of these complex lipids. Furthermore, co-immunoprecipitation and copurification studies, carried out in cell-free extracts of M. smegmatis, demonstrated that AccD4 interacts with both AccA3 and AccD5 subunits, suggesting that the ACCase4 complex is formed by the a subunit AccA3 and two b subunits, AccD4 and AccD5. However, so far, there are no concluding biochemical evidences regarding the exact subunit composition of an active long chain acyl-CoA carboxylase complex. Furthermore, the ACCase5 complex has only been studied in vitro and the question of its physiological role in mycobacteria still needs to be addressed.