title: Prebiotic metabolism: production by mineral photoelectrochemistry of alpha-ketocarboxylic acids in the reductive tricarboxylic acid cycle.
abstract: A reductive tricarboxylic acid (rTCA) cycle could have fixed carbon dioxide as biochemically useful energy-storage molecules on early Earth. Nonenzymatic chemical pathways for some steps of the rTCA cycle, however, such as the production of the alpha-ketocarboxylic acids pyruvate and alpha-ketoglutarate, remain a challenging problem for the viability of the proposed prebiotic cycle. As a class of compounds, alpha-ketocarboxylic acids have high free energies of formation that disfavor their production. We report herein the production of pyruvate from lactate and of alpha-ketoglutarate from pyruvate in the millimolar concentration range as promoted by ZnS mineral photoelectrochemistry. Pyruvate is produced from the photooxidation of lactate with 70% yield and a quantum efficiency of 0.009 at 15 degrees C across the wavelength range of 200-400 nm. The produced pyruvate undergoes photoreductive back reaction to lactate at a 30% yield and with a quantum efficiency of 0.0024. Pyruvate alternatively continues in photooxidative forward reaction to alpha-ketoglutarate with a 50% yield and a quantum efficiency of 0.0036. The remaining 20% of the carbon follows side reactions that produce isocitrate, glutarate, and succinate. Small amounts of acetate are also produced. The results of this study suggest that alpha-ketocarboxylic acids produced by mineral photoelectrochemistry could have participated in a viable enzyme-free cycle for carbon fixation in an environment where light, sulfide minerals, carbon dioxide, and other organic compounds interacted on prebiotic Earth.
authors.affiliation: School of Engineering and Applied Sciences, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: Marcelo I
authors.initials: MI
authors.lastname: Guzman
authors.forename: Scot T
authors.initials: ST
authors.lastname: Martin
mesh_terms: D002264:Carboxylic Acids / Q000378:metabolism*; D002952:Citric Acid Cycle; D018534:Earth, Planet; D004563:Electrochemistry; D019418:Evolution, Chemical; D007656:Ketoglutaric Acids / Q000378:metabolism; D019344:Lactic Acid / Q000378:metabolism; D008027:Light; D008903:Minerals / Q000378:metabolism*; D000069497:Origin of Life; D010084:Oxidation-Reduction / Q000528:radiation effects; D010777:Photochemistry; D019289:Pyruvic Acid / Q000378:metabolism
chemical_list: D002264:Carboxylic Acids; D007656:Ketoglutaric Acids; D008903:Minerals; D019344:Lactic Acid; D019289:Pyruvic Acid