Abstract
Phototrophic CO(2) 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 CO(2) 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-(13)C(3)]propionyl-CoA afforded erythro-beta-[1,2,2'-(13)C(3)]methylmalate and [1,2,2'-(13)C(3)]citramalate. Similar experiments using a partially purified protein fraction afforded erythro-beta-[1,2,2'-(13)C(3)]methylmalyl-CoA and [1,2,2'-(13)C(3)]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-beta-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 CO(2) fixation proceeds by combination of the 3-hydroxypropionate cycle with the methylmalyl-CoA cycle. The net product of that bicyclic autotrophic CO(2) fixation pathway is pyruvate serving as an universal building block for anabolic reactions.
Highlights
Autotrophic CO2 fixation in the phototrophic bacterium Chloroflexus aurantiacus has been proposed to proceed via a novel pathway, the 3-hydroxypropionate cycle (Fig. 1) (1–7)
We found that cell extracts of C. aurantiacus could cleave L-citramalate in the presence of succinyl-CoA with formation of acetyl-CoA and pyruvate
The present study aimed at elucidating the fate of glyoxylate in autotrophic CO2 fixation in C. aurantiacus
Summary
Autotrophic CO2 fixation in the phototrophic bacterium Chloroflexus aurantiacus has been proposed to proceed via a novel pathway, the 3-hydroxypropionate cycle (Fig. 1) (1–7). Acetyl-CoA (1) serves as starting unit, and biotin-dependent carboxylation of acetyl-CoA and propionyl-CoA (4) are the main CO2 fixation reactions. An acetyl-CoA-dependent conversion of glyoxylate to malyl-CoA and malate was ascribed to the reverse reaction of malyl-CoA lyase forming malyl-CoA, combined with a side reaction of citrate synthase or acyl-CoA thioesterase, which hydrolyzes malyl-CoA to malate and CoA (7, 13–15). To serve as a central intermediate for anaplerotic reactions, it should be formed from one of the intermediates of the 3-hydroxypropionate cycle and/or from glyoxylate. We show that a reaction sequence starting with glyoxylate and propionyl-CoA affords acetyl-CoA and pyruvate
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