Mechanistic and Physiological Implications of the Interplay among Iron-Sulfur Clusters in [FeFe]-Hydrogenases. A QM/MM Perspective

Key stereoelectronic properties of Desulfovibrio desulfuricans [FeFe]-hydrogenase (DdH) were investigated by quantum mechanical description of its complete inorganic core, which includes a Fe6S6 active site (the H-cluster), as well as two ancillary Fe4S4 assemblies (the F and F' clusters). The partially oxidized, active-ready form of DdH is able to efficiently bind dihydrogen, thus starting H-2 oxidation catalysis. The calculations allow us to unambiguously assign a mixed Fe(H)Fe(I) state to the catalytic core of the active-ready enzyme and show that H-2 uptake exerts subtle, yet crucial influences on the redox properties of DdH. In fact, H-2 binding can promote electron transfer from the H-cluster to the solvent-exposed F'-cluster, thanks to a 50% decrease of the energy gap between the HOMO (that is localized on the H-cluster) and the LUMO (which is centered on the F'-cluster). Our results also indicate that the binding of the redox partners of DdH in proximity of its F'-cluster can trigger one-electron oxidation of the H-2-bound enzyme, a process that is expected to have an important role in H-2 activation. Our findings are analyzed not only from a mechanistic perspective, but also in consideration of the physiological role of DdH. In fact, this enzyme is known to be able to catalyze both the oxidation and the evolution of H-2, depending on the cellular metabolic requirements. Hints for the design of targeted mutations that could lead to the enhancement of the oxidizing properties of DdH are proposed and discussed.