Scientists provide new insights into the citric acid cycle

Lydia Steffens and Eugenio Pettinato (University of Münster, left) and Thomas M. Steiner (TUM, right) in the laboratory; the three doctoral students share first authorship of the Nature publication. In the middle, a fermenter system for growing bacteria can be seen. AG Berg/AG Eisenreich High carbon dioxide concentrations are required to allow the central metabolic pathway to run "backwards" / publication in "Nature" The citric acid cycle is an important metabolic pathway that enables living organisms to generate energy by degrading organic compounds into carbon dioxide (CO₂). The first step in the cycle is usually performed by the enzyme citrate synthase, which builds citrate. But, in the absence of oxygen (under anaerobic conditions), some bacteria can perform the reverse cycle: They can build up biomass from CO₂. In this so-called reversed citric acid cycle, citrate synthase is replaced by ATP-citrate lyase, which consumes cells' universal energy carrier ATP (adenosine triphosphate) to cleave citrate instead of forming it. However, a few years ago, a research team led by Ivan Berg (University of Münster) and Wolfgang Eisenreich (Technical University of Munich) discovered that instead of requiring ATP-citrate lyase for the reversed cycle, some anaerobic bacteria can use citrate synthase itself to catalyze citrate cleavage without consuming ATP.