Resonance Raman characterization of archaeal and bacterial Rieske protein variants with modified hydrogen bond network around the [2Fe‐2S] center

The rate of quinol oxidation by cytochrome bc1/b6f complex is in part associated with the redox potential (Em) of its Rieske [2Fe‐2S] center, for which an approximate correlation with the number of hydrogen bonds to the cluster has been proposed. Here we report comparative resonance Raman (RR) characterization of bacterial and archaeal high‐potential Rieske proteins and their site‐directed variants with a modified hydrogen bond network around the cluster. Major differences among their RR spectra appear to be associated in part with the presence or absence of Tyr‐156 (in the Rhodobacter sphaeroides numbering) near one of the Cys ligands to the cluster. Elimination of the hydrogen bond between the terminal cysteinyl sulfur ligand (St) and Tyr‐Oη (as with the Y156W variant, which has a modified histidine Nε pKa,ox) induces a small structural bias of the geometry of the cluster and the surrounding protein in the normal coordinate system, and significantly affects some Fe‐Sb/t stretching vibrations. This is not observed in the case of the hydrogen bond between the bridging sulfide ligand (Sb) and Ser‐Oγ, which is weak and/or unfavorably oriented for extensive coupling with the Fe‐Sb/t stretching vibrations.