Functionally Relevant Interplay between the Fe4S4 Cluster and CN- Ligands in the Active Site of [FeFe]-Hydrogenases

[FeFe]-hydrogenases are highly efficient H-2-evolving metalloenzymes that include cyanides and carbonyls in the active site. The latter is an Fe6S6 cluster (the so-called H-cluster) that can be subdivided into a binuclear portion carrying the CO and CN- groups and a tetranuclear subcluster. The fundamental role of cyanide ligands in increasing the basicity of the H-cluster has been highlighted previously. Here a more subtle but crucial role played by the two CN- ligands in the active site of [FeFe]-hydrogenases is disclosed. In fact, QM/MM calculations on all-atom models of the enzyme from Desulfovibrio desulfuricans show that the cyanide groups fine-tune the electronic and redox properties of the active site, affecting both the protonation regiochemistry and electron transfer between the two subclusters of the H-cluster. Despite the crucial role of cyanides in the protein active site, the currently available bioinspired electrocatalysts generally lack CN- groups in order to avoid competition between the latter and the catalytic metal centers for proton binding. In this respect, we show that a targeted inclusion of phosphine ligands in hexanuclear biomimetic clusters may restore the electronic and redox features of the wild-type H-cluster.