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 (E_m) 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 (S^t) and Tyr-Oη (as with the Y156W variant, which has a modified histidine N_ε pK_a,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-S^b/t stretching vibrations. This is not observed in the case of the hydrogen bond between the bridging sulfide ligand (S^b) and Ser-Oγ, which is weak and/or unfavorably oriented for extensive coupling with the Fe-S^b/t stretching vibrations. Published by Cold Spring Harbor Laboratory Press.