STRUCTURE OF [2FE-2S] FERREDOXIN-I FROM EQUISETUM-ARVENSE AT 1.8 ANGSTROM RESOLUTION

Ferredoxin I (Fd I) from Equisetum arvense is an iron-sulfur protein c omposed of 95 amino-acid residues and one [2Fe-2S] cluster. It crystal lized in the space group P2(1), a = 30.4, b = 57.4, c = 47.5 angstrom and beta = 78.7-degrees with two molecules per asymmetric unit. X-ray diffraction data up to 1.8 angstrom resolution were collected by using a Rigaku four-circle diffractometer. The initial model of Fd I, which was derived by the molecular replacement method using a structure of the Fd I from the blue-green alga Aphanothece sacrum, was refined by m olecular dynamics simulation and a least-squares minimization with ste reochemical restraints. Positional parameters and isotropic temperatur e factors for 1420 non-H protein atoms and 183 water molecules were re fined on 13 838 observed structure factors (F0 > sigma(F0)) between 10 .0 and 1.8 angstrom resolution. The final R factor was 17.0%, and the standard deviation of atomic position estimated by Luzzati plot [Luzza ti (1952). Acta Cryst. 5, 802-810] was 0.2 angstrom. The electron-dens ity map was well defined for the two independent molecules except for the N-terminal residue and the three C-terminal residues. Equivalent C alpha atoms of two independent molecules in the asymmetric unit were s uperposed by the least-squares method with root-mean-square deviations of 0.26 angstrom. Reasonable structural differences were observed at a polypeptide segment having few intramolecular interactions. Highly f lexible regions of the molecule were assigned from the structural diff erences between the two independent molecules in the crystal and the d istribution of temperature factors along the polypeptide chain.