Enzymes of a novel autotrophic CO2 fixation pathway in the phototrophic bacterium Chloroflexus aurantiacus, the 3‐hydroxypropionate cycle

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Abstract
The phototrophic bacterium Chloroflexus aurantiacus can grow autotrophically but seems not to assimilate CO2 via any of the known autotrophic pathways. Holo [Holo, H. (1989) Arch. Microbiol. 151, 252–256] proposed a new pathway in which 3‐hydroxypropionate is formed from acetyl‐CoA. Previous studies excluded the operation of known CO2 fixation pathways and provided indirect evidence for the suggested pathway based on 13C‐labelling experiments. Here all enzyme activities of the postulated cyclic CO2 fixation mechanism are demonstrated in vitro. In essence, acetyl‐CoA is carboxylated and reductively converted via 3‐hydroxypropionate to propionyl‐CoA. Propionyl‐CoA is carboxylated and converted via succinyl‐CoA and CoA transfer to malyl‐CoA. Malyl‐CoA is cleaved to acetyl‐CoA and glyoxylate. Thereby, the first CO2 acceptor molecule acetyl‐CoA is regenerated, completing the cycle and the net CO2 fixation product glyoxylate is released. This cycle represents the fourth autotrophic pathway in nature and is designated the 3‐hydroxypropionate cycle.