Mechanisms for Flavin-Mediated Oxidation: Hydride or Hydrogen-Atom Transfer?

Flavins are versatile biological cofactors which catalyze proton-coupled electron transfers (PCET) with varying number and coupling of electrons. Flavin-mediated oxidations of nicotinamide adenine dinucleotide (NADH) and of succinate, initial redox reactions in cellular respiration, were examined here with multiconfigurational quantum chemical calculations and a simple analysis of the wave function proposed to quantify electron transfer along the proton reaction coordinate. The mechanism of NADH oxidation is a prototypical hydride transfer, with two electrons moving concerted with the proton to the same acceptor group. However, succinate oxidation depends on the elimination step and can proceed through the transfer of a hydride or hydrogen atom, with proton and electrons moving to different groups in both cases. These results help to determine the mechanism of fundamental but still debated biochemical reactions and illustrate a new diagnostic tool for electron transfer that can be useful to characterize a broad class of PCET processes.