Prebiotic aqueous reactions catalyzed by native nickel without hydrogen
Compared to iron, nickel is comparatively rare as a transition metal in enzymes. But 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 the ability of Ni0 to serve as 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 show 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{degrees}C without addition of H2, and that it will function as 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.