Hydrogenases are promising candidates for large-scale photobiological hydrogen production and biofuel cells. However, they are generally unstable to O2, making their direct application to currently used electrode systems difficult. To overcome this difficulty, it is interest in the reactivity of the hydrogenases with O2. Horch et al. proposed a mechanism for the O2-tolerance of [NiFe]-hydrogenase and suggested that the reversible sulfoxygenation played an important role in the O2 tolerance.1 However, the detailed reaction mechanism has not been yet clarified. Herein, density functional theory (DFT) calculations were utilized to characterize the geometrical and electronic structures of active site for the formation of the sulfenates species by O2 in the [NiFe]-hydrogenase and to determine their reaction pathways.
We explored the formation pathways of a hydroperoxo species (Ni(OOH)Fe) (step 1), cysteine sulfenate intermediate (Ni(SO)(OH)Fe) (step 2), and a hydroxo species (Ni(OH)Fe) (step 3) based on the previous proposed mechanism1 for the formation of the oxidized forms in [NiFe]-hydrogenase. Calculated energy diagrams showed that the whole reaction in step 1 was exothermic by 38.3 kcal/mol with the highest activation energy of 17.7 kcal/mol, and that in step 2 was exothermic by 32.9 kcal/mol with the highest activation energy of 14.0 kcal/mol, suggesting to be feasible. In step 3, two-electron reduction occurs to generate Ni(OH)Fe. Our DFT calculations showed that, after two-electron reduction of the Ni(SO)Fe(OH2) species, the whole reaction in subsequent reactions was exothermic by 66.8 kcal/mol with the highest activation energy of 18.3 kcal/mol, suggesting to be feasible thermodynamically.
In the whole reactions, the most oxidized state was Ni(SO)(OH)Fe. The results indicate that oxygen atoms are fully reduced to the O−2 state in Ni(SO)(OH)Fe, while the sulfenato sulfur assumes the increased oxidation state S0 by the redox reaction.
(1) Horch, M.; Lauterbach, L.; Mroginski, M. A.; Hildebrandt, P.; Lenz, O.; Zebger, I. J. Am. Chem. Soc., 2015, 137, 2555.