The reverse Krebs cycle, also known as reverse TCA cycle (rTCA) or reductive citric acid cycle, is an alternative to the standard Calvin-Benson cycle for carbon fixation. It has been found in strict anaerobic or microaerobic bacteria (as Aquificales) and anaerobic archea. It was discovered by Evans, Buchanan and Arnon in 1966 working with the photosynthetic green sulfur bacterium Chlorobium limicola. In particular, it is one of the most used pathways in hydrothermal vents by the Campylobacterota. This feature is very important in oceans. Without it, there would be no primary production in aphotic environments, which would lead to habitats without life. So this kind of primary production is called "dark primary production".
The cycle involves the biosynthesis of acetyl-CoA from two molecules of CO. The key steps of the reverse Krebs cycle are:
* Oxaloacetate to malate, using NADH + H
* Fumarate to succinate, catalyzed by an oxidoreductase, Fumarate reductase
* Succinate to succinyl-CoA, an ATP dependent step
* Succinyl-CoA to alpha-ketoglutarate, using one molecule of CO
* Alpha-ketoglutarate to isocitrate, using NADPH + H and another molecule of CO
* Citrate converted into oxaloacetate and acetyl-CoA, this is an ATP dependent step and the key enzyme is the ATP citrate lyase
This pathway is cyclic due to the regeneration of the oxaloacetate.
The bacteria Gammaproteobacteria and Riftia pachyptila switch from the Calvin-Benson cycle to the rTCA cycle in response to concentrations of HS.