Hybrid-cluster protein (HCP) from Desulfovibrio vulgaris (Hildenborough) at 1.6 angstrom resolution

The three-dimensional structure of the hybrid cluster protein from Desulfov ibrio vulgaris (Hildenborough) has been determined at 1.6 Angstrom resoluti on using synchrotron X-ray radiation. The protein can be divided into three domains: an N-terminal mainly alpha -helical domain and two similar domain s comprising a central beta -sheet flanked by alpha -helices. The protein c ontains two 4Fe clusters with an edge-to-edge distance of 10.9 Angstrom. Fo ur cysteine residues at the N-terminus of the protein are ligands to the ir on atoms of a conventional [4Fe-4S] cubane cluster. The second cluster has an unusual asymmetric structure and has been named the hybrid cluster to re flect the variety of protein ligands, namely two mu -sulfido bridges, two m u (2)-oxo bridges, and a further disordered bridging ligand. Anomalous diff erences in data collected at 1.488 Angstrom, and close to the iron edge at 1.743 Angstrom have been used to confirm the identity of the metal and sulf ur atoms. The hybrid cluster is buried in the center of the protein, but is accessible through a large hydrophobic cavity that runs the length of doma in 3. Hydrophobic channels have previously been identified as access routes to the active centers in redox enzymes with gaseous substrates. The hybrid cluster is also accessible by a hydrophilic channel. The [4Fe-4S] cubane c luster is close to an indentation on the surface of the protein and can als o be approached on the opposite side by a long solvent channel. At the pres ent time, neither the significance of these channels nor, indeed, the funct ion of the hybrid cluster protein is known.