STRUCTURE OF THE [2FE-2S] FERREDOXIN-I FROM THE BLUE-GREEN-ALGA APHANOTHECE-SACRUM AT 2.2A RESOLUTION

Crystals of a [2Fe-2S]ferredoxin (Fd) I with a relative molecular mass of 10,480 were obtained from the blue-green alga Aphanothece sacrum. Each asymmetric unit of the crystal contains four molecules. An electron density map calculated by the single isomorphous replacement method with the anomalous dispersion at 2·5 Å resolution was refined by averaging the four molecules in the asymmetric unit. Positional and isotropic thermal parameters for the non-hydrogen atoms of the four molecules and 158 water molecules were refined to an R-factor (R = Σ|Fo - Fc|/ΣFo) of 0·23 by the restrained least-squares method. The estimated root-mean-square (r.m.s.) error for the atomic positions is 0·3 Å. The r.m.s. deviations of equivalent Cα atoms of the asymmetric-unit molecules superposed by the least-squares method average 0·35 Å. The Fd molecule has a structure like the β-barrel in the molecule of the [2Fe-2S] Fd from Spirulina platensis. A [2Fe-2S] cluster is bonded covalently to the protein molecule by four Fe-S, in which three of the Fe-S bonds are in a loop segment from position 38 to 47. The hydrophobic core inside the β-barrel is formed by seven conservative residues: Val15, Val18, Ile24, Leu51, Ile74, Ala79 and Ile87. The molecular surface around Tyr23, Tyr80 and the active center may interact with ferredoxin-NADP+ reductase. One of the two iron atoms of the [2Fe-2S] cluster should be more easily reduced than the other because of differences in the hydrogen-bonding scheme and the hydrophobicity around the atoms. © 1990 Academic Press Limited.