Elements of Metabolic Evolution

Research into the origin of evolution is polarized between a genetics-first approach, with its focus on polymer replication, and a metabolism-first approach that takes aim at chemical reaction cycles. Taking the latter approach, we explored reductive carbon fixation in a volcanic hydrothermal setting, driven by the chemical potential of quenched volcanic fluids for converting volcanic C1 compounds into organic products by transition-metal catalysts. These catalysts are assumed to evolve by accepting ever-new organic products as ligands for enhancing their catalytic power, which in turn enhances the rates of synthetic pathways that give rise to ever-new organic products, with the overall effect of a self-expanding metabolism. We established HCN, CO, and CH3SH as carbon nutrients, CO and H2 as reductants, and iron-group transition metals as catalysts. In one case, we employed the cyano-system [Ni(OH)(CN)] with [Ni(CN)4]2- as the dominant nickelcyano species. This reaction mainly produced a-amino acids and a-hydroxy acids as well as various intermediates and derivatives. An organo-metal-catalyzed mechanism is suggested that mainly builds carbon skeletons by repeated cyano insertions, with minor CO insertions in the presence of CO. The formation of elemental nickel (Ni0) points to an active reduced-nickel species. In another case, we employed the mercaptocarbonyl system [Co2(CO)8]/Ca(OH)2/CO for the double-carbonylation of mercaptans. In a hybrid system, we combined benzyl mercaptan with the cyano system, in which [Ni(OH)(CN)] was the most productive for the double-carbon-fixation reaction. Finally, we demonstrated that the addition of products of the cyano system (Gly, Ala) to the hybrid system increased productivity. These results demonstrate the chemical possibility of metabolic evolution through rate-promotion of one synthetic reaction by the products of another.