Atomic Resolution Structures of Rieske Iron-Sulfur Protein: Role of Hydrogen Bonds in Tuning the Redox Potential of Iron-Sulfur Clusters

SummaryThe Rieske [2Fe-2S] iron-sulfur protein of cytochrome bc1 functions as the initial electron acceptor in the rate-limiting step of the catalytic reaction. Prior studies have established roles for a number of conserved residues that hydrogen bond to ligands of the [2Fe-2S] cluster. We have constructed site-specific variants at two of these residues, measured their thermodynamic and functional properties, and determined atomic resolution X-ray crystal structures for the native protein at 1.2 Å resolution and for five variants (Ser-154→Ala, Ser-154→Thr, Ser-154→Cys, Tyr-156→Phe, and Tyr-156→Trp) to resolutions between 1.5 Å and 1.1 Å. These structures and complementary biophysical data provide a molecular framework for understanding the role hydrogen bonds to the cluster play in tuning thermodynamic properties, and hence the rate of this bioenergetic reaction. These studies provide a detailed structure-function dissection of the role of hydrogen bonds in tuning the redox potentials of [2Fe-2S] clusters.