The high-resolution structure supports the proposed coordination pattern involving histidine ligands and provides a basis for a detailed analysis of the spectroscopic and electrochemical properties. As the cluster is located at the tip of the protein, it might come into close contact with cytochrome b. The exposed N epsilon atoms of the histidine ligands of the cluster are readily accessible to quinones and inhibitors within the hydroquinone oxidation (QP) pocket of the bc1 complex and may undergo redox-dependent protonation/deprotonation.

A water-soluble fragment of the bc, complex from bovine heart mitochondria was isolated containing the intact Rieske [2Fe-2S] cluster. The fragment consists of the last 129 amino acid residues of the Rieske iron-sulfur protein and has a molecular mass of 14592 Da including two iron atoms. The absorption, visible CD, and EPR spectra of the fragment are indistinguishable from those of the membrane-bound iron-sulfur protein. The redox potential as determined by EPR-monitored redox titration was +306 mV. The far-ultraviolet CD spectrum is indicative of a protein with little regular secondary structure, while significant a-helix content was detected in the membrane anchor of the complete iron-sulfur protein. The fragment could be crystallized using poly(ethy1ene glycol) 6000 as precipitant. Needle-shaped single crystals have been grown by the hanging-drop vapor diffusion technique. These crystals belong to the space group P2, and diffract well beyond 0.2 nm resolution. Phase determination using the multiplewavelength anomalous-scattering technique is underway.

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