Synthesis of C 5 -dicarboxylic acids from C 2 -units involving crotonyl-CoA carboxylase/reductase: The ethylmalonyl-CoA pathway

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Abstract
Fifty years ago, Kornberg and Krebs established the glyoxylate cycle as the pathway for the synthesis of cell constituents from C 2 -units. However, since then, many bacteria have been described that do not contain isocitrate lyase, the key enzyme of this pathway. Here, a pathway termed the ethylmalonyl-CoA pathway operating in such organisms is described. Isotopically labeled acetate and bicarbonate were transformed to ethylmalonyl-CoA by cell extracts of acetate-grown, isocitrate lyase-negative Rhodobacter sphaeroides as determined by NMR spectroscopy. Crotonyl-CoA carboxylase/reductase, catalyzing crotonyl-CoA + CO 2 + NADPH → ethylmalonyl-CoA − + NADP + was identified as the key enzyme of the ethylmalonyl-CoA pathway. The reductive carboxylation of an enoyl-thioester is a unique biochemical reaction, unprecedented in biology. The enzyme from R. sphaeroides was heterologously produced in Escherichia coli and characterized. Crotonyl-CoA carboxylase/reductase (or its gene) can be used as a marker for the presence of the ethylmalonyl-CoA pathway, which functions not only in acetyl-CoA assimilation. In Streptomyces sp., it may also supply precursors (ethylmalonyl-CoA) for antibiotic biosynthesis. For methylotrophic bacteria such as Methylobacterium extorquens , extension of the serine cycle with reactions of the ethylmalonyl-CoA pathway leads to a simplified scheme for isocitrate lyase-independent C 1 assimilation.