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 H 2 -dependent CO 2 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 CO 2 fixation, most notably recent findings that the overall reaction of the enzymatic pathway from H 2 ( E 0 ′ = –414 mV) and CO 2 to pyruvate can be replaced by Ni 0 alone in water as the lone catalyst. Here we studied the ability of Ni 0 to serve as catalyst and reductant for nonenzymatic redox reactions that require only a mild reductant, as the midpoint potential of Ni 0 oxidation to Ni 2+ is E 0 ′ = –260 mV. We show that Ni 0 in water can convert 2-oxo acids to 2-hydroxy acids and, in the presence of NH 3 , to amino acids at 25-100°C without addition of H 2 , and that it will function as catalyst and reductant for the fumarate reductase reaction. The findings expand the repertoire of ancient metabolic reactions that Ni 0 can catalyze without proteins, cofactors, or sulfur, shedding light on the broad catalytic activity and substrate specificity of Ni 0 at metabolic origin.