Co2+ acireductone dioxygenase: Fe2+ mechanism, Ni2+ mechanism, or something else?

Acireductone dioxygenase (ARD) oxidizes 1,2-dihydroxy-3-keto-5-(methylthio)pentene to either formate and an α-keto acid, or formate, methylthiopropionate and CO, depending on the nature of the catalytic metal, Fe2+ or Ni2+. We recently showed that, contrary to established hypotheses, the mechanistic preference is driven solely by the RedOx behavior of the metal. Here, we address the functionality of Co2+-ARD. Using mixed quantum–classical dynamics simulations and density functional theory calculations, we show that both Fe2+-like and Ni2+-like routes are accessible to Co2+-ARD, but the mechanism involves a bifurcating transition state, and so the exact product distribution would be determined by the reaction dynamics.