STRUCTURE OF A WATER-SOLUBLE FRAGMENT OF THE RIESKE IRON-SULFUR PROTEIN OF THE BOVINE HEART MITOCHONDRIAL CYTOCHROME-BC(1) COMPLEX DETERMINED BY MAD PHASING AT 1.5-ANGSTROM RESOLUTION

Background: The 'Rieske' iron-sulfur protein is the primary electron a cceptor during hydroquinone oxidation in cytochrome be complexes, The spectroscopic and electrochemical properties of the 'Rieske' [2Fe-2S] cluster differ significantly from those of other iron-sulfur clusters, A 129-residue water soluble fragment containing the intact [2Fe-2S] c luster was isolated following proteolytic digestion of the be, complex and used for structural studies. Results: The structure of the Rieske iron-sulfur fragment containing the reduced [2Fe-2S] cluster has been determined using the multiwavelength anomalous diffraction (MAD) tech nique and refined at 1.5 Angstrom resolution, The fragment has a novel overall fold that includes three sheets of beta strands, The iron ato ms of the [2Fe-2S] cluster are coordinated by two cysteine (Fe-1) and two histidine (Fe-2) residues, respectively, with the histidine ligand s completely exposed to the solvent, This is in contrast to the four c ysteine coordination pattern observed in previously characterised [2Fe -2S] ferredoxins. The cluster-binding fold is formed by two loops conn ected by a disulfide bridge; these loops superpose with the metal-bind ing loops of rubredoxins. The environment of the cluster is stabilised by an extensive hydrogen-bond network. Conclusions: The high-resoluti on structure supports the proposed coordination pattern involving hist idine ligands and provides a basis for a detailed analysis of the spec troscopic and electrochemical properties. As the cluster is located at the tip of the protein, it might come into close contact with cytochr ome b. The exposed NE atoms of the histidine ligands of the cluster ar e readily accessible to quinones and inhibitors within the hydroquinon e oxidation (Q(P)) pocket of the bc(1) complex and may undergo redox-d ependent protonation/deprotonation.