An Alternative Isovaleryl CoA Biosynthetic Pathway Involving a Previously Unknown 3-Methylglutaconyl CoA Decarboxylase

摘要

Myxobacteria are swarming bacteria characterized by their social behavior and ability to undergo a complex developmental life cycle. They prey on other bacteria or fungi for food. Under starvation conditions, the vegetative cells aggregate to form multicellular fruiting bodies. To support such a remarkable lifestyle, myxobacteria exhibit an enormous metabolic potential, which is mirrored by discoveries of new compounds and unusual biochemical pathways in recent years. One example is the biosynthesis of isovaleryl coenzyme A (IV-CoA) for which an additional alternative pathway has been recently suggested. IV-CoA is generally derived from leucine degradation by transamination and subsequent oxidative decarboxylation by the branched-chain-keto acid dehydrogenase complex (Bkd). The Bkd complex is also involved in degradation pathways of valine and isoleucine to produce isobutyryl CoA and 2-methylbutyryl CoA. These precursors are the starter units of branched-chain (or iso-) fatty acids (FAs), which are of particular importance in myxobacteria. IV-CoA-derived iso-odd FAs are the majority of FAs, as shown in the model myxobacterium, Myxococcus xanthus. These FAs maintain the membrane fluidity for thermal adaptation, and play key roles in signaling during myxobacterial developmental differentiation. Besides its role in iso-odd FA biogenesis, IV-CoA is the precursor of a number of myxobacterial secondary metabolites, such as myxothiazol and aurafuron. During our study of myxothiazol biosynthesis, we analyzed the bkd— mutants of M. xanthus and Stigmatella aurantiaca and showed that iso-FAs and myxothiazol were still produced in these mutants, albeit at lower amounts, which provided the first evidence for the existence of an alternative pathway to IV-CoA. The bkd— mutants could incorporate labeled acetate, but not leucine, into IV-CoA-derived compounds and exhibited a labeling pattern similar to that found in mevalonate-dependent isoprenoids, thus establishing a likely link between the hypothetical IV-CoA and the mevalonate pathway. 3-Hydroxy-3-methylglutaryl CoA (HMG-CoA) was proposed to be the branching point, from which reverse reactions of leucine degradation would happen and lead to TV-Co A (Figure 1). Feeding studies and analysis of a double-mutant bkd-/mvaS— (mvaS encodes HMG-CoA synthase) demonstrated the involvement of HMG-CoA and the intermediacy of 3,3-dimethylacrylyl CoA (DMA-CoA; as shown by incorporation of labeled 3,3-dimethylacrylic acid, which needs to undergo activation to the CoA ester) in the alternative pathway. Importantly, this pathway was found to be highly active in the bkd— mutant and during fruiting body formation when leucine-derived IV-CoA is limited, reflecting essential roles of IV-CoA-derived compounds in the life cycle of myxobacteria.