High CO2 levels drive the TCA cycle backwards towards autotrophy

In the deltaproteobacterium Hippea maritima, the tricarboxylic acid (TCA) cycle can be reversed by high partial pressures of CO2 for the autotrophic fixation of carbon. It has recently been shown that in anaerobic microorganisms the tricarboxylic acid (TCA) cycle, including the seemingly irreversible citrate synthase reaction, can be reversed and used for autotrophic fixation of carbon(1,2). This reversed oxidative TCA cycle requires ferredoxin-dependent 2-oxoglutarate synthase instead of the NAD-dependent dehydrogenase as well as extremely high levels of citrate synthase (more than 7% of the proteins in the cell). In this pathway, citrate synthase replaces ATP-citrate lyase of the reductive TCA cycle, which leads to the spending of one ATP-equivalent less per one turn of the cycle. Here we show, using the thermophilic sulfur-reducing deltaproteobacterium Hippea maritima, that this route is driven by high partial pressures of CO2. These high partial pressures are especially important for the removal of the product acetyl coenzyme A (acetyl-CoA) through reductive carboxylation to pyruvate, which is catalysed by pyruvate synthase. The reversed oxidative TCA cycle may have been functioning in autotrophic CO2 fixation in a primordial atmosphere that is assumed to have been rich in CO2.

Automated summary

In Deltoproteobacterium Hippea maritima, the TCA cycle can be reversed by high CO2 partial pressures for autotrophic carbon fixation. This reversed oxidative TCA cycle could have functioned in autotrophic CO2 fixation in a primordial atmosphere rich in CO2.