Serpentinizing hydrothermal vents are likely sites for the origin of metabolism because they produce H2 as a source of electrons for CO2 reduction while depositing zero-valent iron, cobalt, and nickel Show more
Serpentinizing hydrothermal vents are likely sites for the origin of metabolism because they produce H2 as a source of electrons for CO2 reduction while depositing zero-valent iron, cobalt, and nickel as catalysts for organic reactions. Recent work has shown that solid-state nickel can catalyze the H2-dependent reduction of CO2 to various organic acids and their reductive amination with H2 and NH3 to biological amino acids under the conditions of H2-producing hydrothermal vents and that amino acid synthesis from NH3, H2, and 2-oxoacids is facile in the presence of Ni0. Such reactions suggest a metallic origin of metabolism during early biochemical evolution because single metals replace the function of over 130 enzymatic reactions at the core of metabolism in microbes that use the acetyl-CoA pathway of CO2 fixation. Yet solid-state catalysts tether primordial amino synthesis to a mineral surface. Many studies have shown that pyridoxal catalyzes transamination reactions without enzymes. Here we show that pyridoxamine, the NH2-transferring intermediate in pyridoxal-dependent transamination reactions, is generated from pyridoxal by reaction with NH3 (as little as 5 mm) and H2 (5 bar) on Ni0 as catalyst at pH 11 and 80 °C within hours. These conditions correspond to those in hydrothermal vents undergoing active serpentinization. The results indicate that at the origin of metabolism, pyridoxamine provided a soluble, organic amino donor for aqueous amino acid synthesis, mediating an evolutionary transition from NH3-dependent amino acid synthesis on inorganic surfaces to pyridoxamine-dependent organic reactions in the aqueous phase. Show less
Metabolic theories for the origin of life posit that inorganic catalysts enabled self-organized chemical precursors to the pathways of metabolism, including those that make genetic molecules. Recently Show more
Metabolic theories for the origin of life posit that inorganic catalysts enabled self-organized chemical precursors to the pathways of metabolism, including those that make genetic molecules. Recently, experiments showing nonenzymatic versions of a number of core metabolic pathways have started to support this idea. However, experimental demonstrations of nonenzymatic reaction sequences along the de novo ribonucleotide biosynthesis pathways are limited. Here we show that all three reactions of pyrimidine nucleobase biosynthesis that convert aspartate to orotate proceed at 60 °C without photochemistry under aqueous conditions in the presence of metals such as Cu2+ and Mn4+ . Combining reactions into one-pot variants is also possible. Life may not have invented pyrimidine nucleobase biosynthesis from scratch, but simply refined existing nonenzymatic reaction channels. This work is a first step towards uniting metabolic theories of life's origin with those centered around genetic molecules. Show less