School of Engineering and Applied Sciences, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
Astrobiology. 2009 Nov;9(9):833-42. doi: 10.1089/ast.2009.0356.
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.
还原性三羧酸 (rTCA) 循环可以将二氧化碳固定为生物化学上有用的储能分子，这在早期地球上是可能的。然而，rTCA 循环的一些步骤的非酶化学途径，例如丙酮酸和α-酮戊二酸的产生，仍然是提议的前生物循环可行性的一个具有挑战性的问题。作为一类化合物，α-酮羧酸具有较高的生成自由能，不利于其产生。我们在此报告了 ZnS 矿物光电化学在毫摩尔浓度范围内促进的丙酮酸从乳酸和α-酮戊二酸从丙酮酸的产生。丙酮酸是通过乳酸的光氧化产生的，在 15 摄氏度下，在 200-400nm 的波长范围内，产率为 70%，量子效率为 0.009。产生的丙酮酸在 30%的产率和 0.0024 的量子效率下经历光还原的反向反应回到乳酸。丙酮酸替代地以 50%的产率和 0.0036 的量子效率继续进行光氧化正向反应生成α-酮戊二酸。剩余的 20%的碳遵循产生异柠檬酸、戊二酸和琥珀酸的副反应。也产生少量的乙酸盐。本研究的结果表明，矿物光电化学产生的α-酮羧酸可能参与了在光、硫化物矿物、二氧化碳和其他有机化合物在原始地球上相互作用的环境中，无酶的可行碳固定循环。
