A bicyclic autotrophic CO2 fixation pathway in Chloroflexus aurantiacus

Abstract

Phototrophic CO2 assimilation by the primitive, green eubacterium Chloroflexus aurantiacus has been shown earlier to proceed in a cyclic mode via 3-hydroxypropionate, propionyl-CoA, succinyl-CoA, and malyl-CoA. The metabolic cycle could be closed by cleavage of malyl-CoA affording glyoxylate (the primary CO2 fixation product) with regeneration of acetyl-CoA serving as the starter unit of the cycle. The pathway of glyoxylate assimilation to form gluconeogenic precursors has not been elucidated to date. We could now show that the incubation of cell extract with a mixture of glyoxylate and [1,2,3-13C3]propionyl-CoA afforded erythro-β-[1,2,2′-13C3]methylmalate and [1,2,2′-13C3]citramalate. Similar experiments using a partially purified protein fraction afforded erythro-β-[1,2,2′13C3]methylmalyl-CoA and [1,2,2′-13C3]mesaconyl-CoA. Cell extracts of C. aurantiacus were also shown to catalyze the conversion of citramalate into pyruvate and acetyl-CoA in a succinyl-CoA-dependent reaction. The data suggest that glyoxylate obtained by the cleavage of malyl-CoA can be utilized by condensation with propionyl-CoA affording erythro-β-methylmalyl-CoA, which is converted to acetyl-CoA and pyruvate. This reaction sequence regenerates acetyl-CoA, which serves as the precursor of propionyl-CoA in the 3-hydroxypropionate cycle. Autotrophic CO2 fixation proceeds by combination of the 3-hydroxypropionate cycle with the methylmalyl-CoA cycle. The net product of that bicyclic autotrophic CO2 fixation pathway is pyruvate serving as an universal building block for anabolic reactions.