Role of cytochromes and quinones in acetogenic bacteria

• Applicant: Professor Dr. Volker Müller
• Subject Area: Metabolism, Biochemistry and Genetics of Microorganisms
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 503149329

Acetogenic bacteria are a polyphyletic group of organisms that fix carbon dioxide under anoxic, non-phototrophic conditions by reduction of two mol of CO2 to acetyl-CoA and further to acetate via the Wood-Ljungdahl pathway. CO2 can be reduced with H2 as electron donor allowing for chemolithoautotrophic growth and acetogenesis from H2 + CO2 is considered to be one of the oldest metabolic pathways on Earth, since it couples CO2 reduction to the net synthesis of ATP. However, the ATP gain is only a fraction of an ATP per mol of acetate formed, making acetogens prime candidates to study the basis of microbial life at thermodynamic equilibrium. Moreover, acetogens are on an impressive rise as production plattforms for value-added compounds from the greenhouse gas CO2, but their industrial application is still limited due to the low ATP gain which yields only low titers and a limited number of products that can be produced. How ATP is synthesized during acetogenesis from H2 + CO2 has been an enigma since the discovery of this lithothrophic life style. In the last decade two ferredoxin-dependent respiratory chains were discovered. Those respiratory chains comprise of a ferredoxin-dependent respiratory enzyme complex, which is either a ferredoxin:NAD oxidoreductase (Rnf) or a ferredoxin:H+ oxidoreductase (Ech) that generate an electrochemical ion gradient across the cytoplasmic membrane that then drives ATP synthesis via a membrane-bound ATP synthase. These multisubunit, membrane-bound respiratory enzymes have iron-sulfur centers and flavins as electron carriers, but no cytochromes. This is astonishing since cytochromes were already discovered 46 and quinones 32 years ago in the acetogenic model organism Moorella thermoacetica. Later on, they were also found in other acetogens but their role had only a shadowy existence. Nevertheless, it is often speculated that there is a third type of energy conservation in acetogens, besides Rnf and Ech, that is based on cytochrome- and/or quinone-dependent electron transport phosphorylation. We will address the role of cytochromes/quinones and hypothesize that cytochromes-/quinones are part of to be identified electron transport chains (ETCs). These ETCs are speculated to lead from electron donors such as H2, CO, reduced ferredoxin or NAD(P)H to acceptors such as methylene-tetrahydrofolate or alternative electron acceptors such as, for example, nitrate and sulfur compounds. These ETCs will be identified, their constituents will be described and the mechanism of energy conservation will be unravelled. The final goal is to find the last piece of the puzzle of energy conservation in acetogenic bacteria and unravel whether there is a third way of energy conservation in these bacteria. The outcome of the project will also reach as far as to understand the ecological fitness of acetogens and to improve their use in a sustainable, carbon dioxide-based biotechnology.

• DFG Programme: Reinhart Koselleck Projects