The dinucleotide structure of NAD enables specific reduction on mineral surfaces
Nucleotide-derived cofactors could function as a missing link between the informational and the metabolic part at lifes emergence. One well-known example is nicotinamide dinucleotide (NAD), one of the evolutionarily most conserved redox cofactors found in metabolism. Here, we propose that the role of these cofactors could even extend to missing links between geo- and biochemistry. We show NAD+ can be reduced under close-to nature conditions with nickel-iron-alloys found in water-rock-interaction settings rich in hydrogen (serpentinizing systems) and that nicotinamide mononucleotide (NMN), a precursor molecule to NAD, has different properties regarding reduction specificity and sensitivity than NAD. The additional adenosine monophosphate (AMP) "tail" of the dinucleotide, a shared trait between many organic cofactors, seems to play a crucial mechanistic role in preventing overreduction of the nicotinamide-bearing nucleotide. This specificity is also connected to the used transition metals. While the combination of nickel and iron promotes the reduction of NAD+ to 1,4-NADH most efficiently, in the case of NMN, the presence of nickel leads to the accumulation of overreduction products. Testing the reducing abilities of both NADH and NMNH under abiotic conditions showed that both molecules act as equally effective, soluble hydride donors in non-enzymatic, proto-metabolic stages of lifes emergence.