STRUCTURE OF AZOTOBACTER-VINELANDII 7FE FERREDOXIN AT 1.35 ANGSTROM RESOLUTION AND DETERMINATION OF THE [FE-S] BONDS WITH 0.01 ANGSTROM ACCURACY

The crystal structure of Azotobacter vinelandii ferredoxin I (FdI) at 100 K has been refined at 1.35 Angstrom resolution by full matrix bloc k diagonal least-squares methods with anisotropic temperature factors for all non-hydrogen atoms and with hydrogen atoms included in the mod el. Fe-S bonds within the [3Fe-4S](+) and [4Fe-4S](2+) clusters of the protein are determined with an accuracy of at least 0.01 Angstrom. An alysis of metric parameters reveals greater variation in bonds and ang les within the [3Fe-4S](+) cluster than in the [4Fe-4S](2+) cluster, w hereas the opposite is true regarding the cysteine S gamma atoms ligat ing to the two [Fe-S] cores. The [3Fe-4S](+) core is asymmetrically di storted by the protein matrix but relatively uniformly ligated by its three Cys ligands; in contrast the tetrahedral [4Fe-4S](2+) core is re latively :symmetric but non-uniformily ligated by its four Cys ligands , three of which occur in a conserved CysxxCysxxCys residue motif. Com parison of the [3Fe- 4S](+) clusters in FdI and Desulfovibrio gigas fe rredoxin II, refined at 1.7 Angstrom resolution, indicates that within the limit of accuracy of the two refinements the cuboidal core is dif ferently distorted in the two proteins. Comparison of the [3Fe-4S]+ co re in FdI with the structure of a reduced [Fe3S4]degrees synthetic ana log dagger indicates that the protein-bound cluster displays distortio ns not intrinsic to the core itself. Nevertheless, both [3Fe-4S](+) an d [Fe3S4]degrees cores have metric features consistent with expected t rends due to net charge on Fe and valency of S, and both exhibit a spl ayed configuration with respect to their three mu(2)S atoms in the abs ence of a fourth Fe. Comparison of the [4Fe-4S](2+) cluster in FdI wit h the structures of [Fe4S4](2+) synthetic analogs shows that the prote in bound and synthetic cubanes are very similar in geometric parameter s, including the presence of tetragonal distortion in the FdI cluster common to this oxidation state. (C) 1998 Academic Press Limited.