The Role of Leucine in Terpenoid Metabolism: Incorporation of Leucine into Sesquiterpenoids and Phytosterols by Andrographis Tissue Cultures

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 alpha-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.

3-Methylcrotonyl-coenzyme A carboxylase is a component of the mitochondrial leucine catabolic pathway in plants

3-Methylcrotonyl-coenzyme A carboxylase (MCCase) is a mitochondrial biotin-containing enzyme whose metabolic function is not well understood in plants. In soybean (Glycine max) seedlings the organ-specific and developmentally induced changes in MCCase expression are regulated by mechanisms that control the accumulation of MCCase mRNA and the activity of the enzyme. During soybean cotyledon development, when seed-storage proteins are degraded, leucine (Leu) accumulation peaks transiently at 8 d after planting. The coincidence between peak MCCase expression and the decline in Leu content provides correlative evidence that MCCase is involved in the mitochondrial catabolism of Leu. Direct evidence for this conclusion was obtained from radiotracer metabolic studies using extracts from isolated mitochondria. These experiments traced the metabolic fate of [U-14C]Leu and NaH14CO3, the latter of which was incorporated into methylglutaconyl-coenzyme A (CoA) via MCCase. These studies directly demonstrate that plant mitochondria can catabolize Leu via the following scheme: Leu --> alpha-ketoisocaproate --> isovaleryl-CoA --> 3-methylcrotonyl-CoA --> 3-methylglutaconyl-CoA --> 3-hydroxy-3-methylglutaryl-CoA --> acetoacetate + acetyl-CoA. These findings demonstrate for the first time, to our knowledge, that the enzymes responsible for Leu catabolism are present in plant mitochondria. We conclude that a primary metabolic role of MCCase in plants is the catabolism of Leu.