The biosynthesis of mycolic acids by Mycobacteria: current and alternative hypotheses.

Experimental observations, accumulated during several decades, have allowed an overall scheme for the biosynthesis of the mycolic acids, which are very long chain fatty acids of Mycobacteria to be proposed. But, in almost every step, several hypotheses are compatible with the experimental results, leading to variations of the overall scheme. The aim of this review is to point to some additional possibilities. It is generally assumed that the classical elongation process of fatty acid synthesis produces two long chains, the condensation of which leads to the direct precursors of mycolic acids. But three condensations of four fatty acids, usually synthesized by Mycobacteria, is another hypothesis that could be considered. In the first hypothesis, some methyl or methylene substituents or oxygenated functions are added to the double bonds of an unsaturated precursor, whereas in the second hypothesis, the methylations could help in the building of very long aliphatic chains, and determine the location of double bonds or ramifications. The hypothetical coexistence of two pathways for mycolate biosynthesis is discussed.