V. Sridhar et al., CRYSTAL-STRUCTURES OF FERRICYANIDE-OXIDIZED [FE-S] CLUSTERS IN AZOTOBACTER-VINELANDII FERREDOXIN-I, JBIC. Journal of biological inorganic chemistry, 3(2), 1998, pp. 140-149
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.