REFINED CRYSTAL-STRUCTURE OF THE 2[4FE-4S] FERREDOXIN FROM CLOSTRIDIUM-ACIDURICI AT 1.84-ANGSTROM RESOLUTION

The crystal structure of the 2[4Fe-4S] ferredoxin from Clostridium aci durici has been determined at a resolution of 1.84 Angstrom and refine d to an R-factor of 0.169. Crystals belong to space group P4(3)2(1)2 w ith unit cell dimensions a = b = 34.44 Angstrom and c = 74.78 Angstrom . The structure was determined by molecular replacement using the prev iously published model of an homologous ferredoxin and re tined by mol ecular dynamics techniques. The model contains the protein and 46 wate r molecules. Only two amino acid residues, Asp27 and Asp28, are poorly defined in the electron density maps. The molecule has an overall cha in fold similar to that of other [4Fe-4S] bacterial ferredoxins of kno wn structure. The two [4Fe-4S] clusters display similar bond distances and angles. In both of them the co-ordination of one iron atom (bound to Cys11 and Cys40) is slightly distorted as compared with that of th e other iron atoms. A core of hydrophobic residues and a few water mol ecules contribute to the stability of the structure. The [4Fe-4S] clus ters interact with the polypeptide chain through eight hydrogen bonds each, in addition to the covalent Fe-Scys bonds. The ferredoxin from C lostridium acidurici is the most typical clostridial ferredoxin crysta llized so far and the biological implications of the newly determined structure are discussed.