STRUCTURE OF THE FERREDOXIN FROM CLOSTRIDIUM-ACIDURICI - MODEL AT 1.8-ANGSTROM RESOLUTION

Ferredoxins (Fd) are electron-carrier proteins, the active sites of wh ich are organized around dusters made of iron and inorganic sulfur. Th e Fd from Clostridium acidurici is 55 amino acids long and contains tw o [4Fe-4S] clusters. Crystals have been obtained in the space group P4 (3)2(1)2, a = b = 34.441 (5), c = 74.778(9) Angstrom. The structure wa s solved by molecular replacement using the Fd from Peptostreptcoccus asaccharolyticus as a search model, these two ferredoxins having 37 re sidues in common. Refinement using molecular-dynamics techniques was t hen initiated. Successive rounds of model building and refinement gave a structure that includes 45 water molecules with R = 15%. At this st age, the electron-density map clearly revealed discrepancies in the po sition of two amino acids in the published primary sequence. Refinemen t based on these modifications led to R = 14.3% for 3921 reflections u p to 1.8 Angstrom resolution. The geometry of the two clusters has bee n found to be in good agreement with that previously obtained at a low er resolution. Interactions of polypeptide chain with the [4Fe-4S] dus ters, the cluster geometry as well as the hydrogen bonds involving S, S gamma, N and water molecules are reported.