CRYSTAL-STRUCTURES OF FERRICYANIDE-OXIDIZED [FE-S] CLUSTERS IN AZOTOBACTER-VINELANDII FERREDOXIN-I

Crystals of Azotobacter vinelandii ferredoxin I (FdI) have been soaked in solutions containing K3Fe(CN)(6) in order to study the oxidation o f the [3Fe-4S] and [4Fe-4S] clusters in the protein. Ferricyanide trea tment results in partial loss of Fe and S from each cluster accompanie d by alteration of Fe-S bonds. The effects of oxidation can be quantit ated by crystallographic refinement when each [Fe-S] cluster is modele d as having a single, average structure with non-standard geometry. Th e oxidized clusters refined at 2.1-Angstrom resolution display statist ically significant deviations from geometric ideality. If interpreted in terms of atomic shifts these deviations indicate that each cluster first loses an inorganic S atom. In each case an Fe atom bonded to thi s S separates from the remaining atoms of the cluster such that the [3 Fe-4S] and [4Fe-4S] clusters partially decompose into a single Fe plus 2Fe and 3Fe fragments. The extent of structural changes observed are essentially the same in crystals soaked at 3:1, 9:1 and 30:1 mole rati o of K3Fe(CN)(6):FdI, suggesting that the crystal lattice permits limi ted oxidation reactions to occur at a low mole ratio but restricts con formational changes from occurring that may be required for more exten sive oxidative reactions at higher mole ratio. The results are relevan t to understanding the transformations which may take place when [Fe-S ] proteins are deliberately oxidized with ferricyanide.