Biosynthesis of iso-fatty acids in myxobacteria

The fatty acid (FA) profiles of the myxobacteria Stigmatella aurantiaca and Myxococcus xanthus were investigated by acidic methanolysis of total cell extracts and GC or GC-MS analysis. The main components were 13-methyltetradecanoic acid (iso-15:0) and (Z)-hexadec-11-enoic acid (16:1, ω-5 cis). The biosynthesis of iso-FAs was investigated in several feeding experiments. Feeding of isovaleric acid (IVA) to a mutant impaired in the degradation of leucine to isovaleryl-CoA (IV–CoA) (bkd mutant) of M. xanthus only increased the amount of iso-odd FAs, whereas feeding of isobutyric acid (IBA) gave increased amounts only of iso-even FAs. In contrast, a bkd mutant of S. aurantiaca gave increased amounts of iso-odd and iso-even fatty acids in both experiments. We assumed that in S. aurantiaca α-oxidation takes place. [D₇]-15-Methylhexadecanoic acid (8) was synthesised and fed to S. aurantiaca as well as [D₁₀]leucine and [D₈]valine to elucidate this pathway in more detail. The iso-fatty acid 8 was degraded by α- and β-oxidation steps. [D₁₀]Leucine was strongly incorporated into iso-odd and iso-even fatty acids, whereas the incorporation rates for [D₈]valine into both types of fatty acids were low. Thus α-oxidation plays an important role in the biosynthesis of iso-fatty acids in S. aurantiaca. The incorporation rates observed after feeding of [D₁₀]leucine and [D₈]valine are the highest for iso-17:0 compared to the other acids. This indicates the central role of iso-17:0 in the biosynthesis of iso-FAs. The shorter homologues seem to be formed mainly by α-oxidation and β-oxidation of this acid. After feeding of 8 traces of unsaturated counterparts of this labelled FA occurred in the extracts indicating that desaturases are active in the biosynthesis of unsaturated fatty acids in S. aurantiaca.