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A Biosynthetic Pathway to Isovaleryl‐CoA in Myxobacteria: The Involvement of the Mevalonate Pathway

A biosynthetic shunt pathway branching from the mevalonate pathway and providing starter units for branched‐chain fatty acid and secondary metabolite biosynthesis has been identified in strains of the myxobacterium Stigmatella aurantiaca. This pathway is upregulated when the branched‐chain α‐keto acid dehydrogenase gene (bkd) is inactivated, thus impairing the normal branched‐chain amino acid degradation process. We previously proposed that, in this pathway, isovaleryl‐CoA is derived from 3,3‐dimethylacrylyl‐CoA (DMA‐CoA). Here we show that DMA‐CoA is an isomerization product of 3‐methylbut‐3‐enoyl‐CoA (3MB‐CoA). This compound is directly derived from 3‐hydroxy‐3‐methylglutaryl‐CoA (HMG‐CoA) by a decarboxylation/ dehydration reaction resembling the conversion of mevalonate 5‐diphosphate to isopentenyl diphosphate. Incubation of cell‐free extracts of a bkd mutant with HMG‐CoA gave product(s) with the molecular mass of 3MB‐CoA or DMA‐CoA. The shunt pathway most likely also operates reversibly and provides an alternative source for the monomers of isoprenoid biosynthesis in myxobacteria that utilize L‐leucine as precursor.