Prebiotic aqueous reactions catalyzed by native nickel without hydrogen

Compared to iron, nickel is comparatively rare as a transition metal in enzymes. However, it is essential in several enzymes of carbon and energy metabolism in acetogens (bacteria) and methanogens (archaea), which use the acetyl-CoA pathway of H₂-dependent CO₂ fixation. Nickel containing enzymes of acetogens and methanogens include FeNi hydrogenase, carbon monoxide dehydrogenase, acetyl-CoA synthase and, in methanogens, methyl-CoM reductase in the last step of methane synthesis. Several lines of evidence implicate the acetyl-CoA pathway as the most ancient pathway of CO₂ fixation, most notably recent findings that the overall reaction of the enzymatic pathway from H₂ (E₀' = -414 mV) and CO₂ to pyruvate can be replaced by Ni⁰ alone in water as the lone catalyst. Here, we studied Ni⁰ as a catalyst and reductant for nonenzymatic redox reactions that require only a mild reductant, as the midpoint potential of Ni⁰ oxidation to Ni²⁺ is E₀' = -260 mV. We showed that Ni⁰ in water can convert 2-oxo acids to 2-hydroxy acids and, in the presence of NH₃, to amino acids at 25-100 °C without the addition of H₂, and that it functions as a catalyst and reductant for the fumarate reductase reaction. The findings expand the repertoire of ancient metabolic reactions that Ni⁰ can catalyze without proteins, cofactors, or sulfur, shedding light on the broad catalytic activity and substrate specificity of Ni⁰ at metabolic origin.