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Synthesis of C₅-dicarboxylic acids from C₂-units involving crotonyl-CoA carboxylase/reductase: The ethylmalonyl-CoA pathway

Erb, Tobias J., Berg, Ivan A., Brecht, Volker, Müller, Michael, Fuchs, Georg, Alber, Birgit E.
Proceedings of the National Academy of Sciences of the United States of America 2007 v.104 no.25 pp. 10631-10636
Escherichia coli, Methylobacterium extorquens, NADP (coenzyme), Rhodobacter sphaeroides, Streptomyces, acetates, acetyl coenzyme A, acids, antibiotics, bacteria, bicarbonates, biosynthesis, carbon dioxide, carboxylation, genes, glyoxylate cycle, isocitrate lyase, isotope labeling, nuclear magnetic resonance spectroscopy, serine
Fifty years ago, Kornberg and Krebs established the glyoxylate cycle as the pathway for the synthesis of cell constituents from C₂-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 + CO₂ + NADPH [rightward arrow] 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 C₁ assimilation.