On the nature of the iron-sulfur centers in a ferredoxin from Azotobacter vinelandii. Mössbauer studies and cluster displacement experiments.

Abstract

We have studied the high and low potential ironsulfur centers of an Azotobacter vinelandii ferredoxin with Mossbauer spectroscopy.The data suggest that the high potential cluster is of the Fe4Sc type and that the low potential center (Eo = -420 mV) represents a novel type of cluster.The oxidized low potential center which exhibits an isotropic EPR signal at g = 2.01 has at least 2 distinct iron sites with identical quadrupole splittings, AEQ = 0.63 mm/s, and isomeric shifts, B = 0.27 mm/s, suggesting high spin ferric ions with a tetrahedral sulfur ligation.The two sites display dras-.tically different magnetic hyperfine interactions.Studies in strong applied magnetic fields show that the magnetic hyperfine coupling constants have opposite signs, a strong indication of spin-coupling.Upon reduction, the g = 2.01 center is found to have an electronic ground state of integer, nonzero spin, Le. the center is paramagnetic.The Mossbauer data force us to consider the possibility that the low potential center might contain 3 iron atoms, a suggestion compatible with the data obtained for the oxidized center.Since 2 high spin ( S = 5/2) ferric ions cannot be coupled to yield the halfintegral spin observed for the oxidized center, a third, half-integral spin is required.If this spin is not furnished by a 3rd iron atom, as suggested by the data obtained for the reduced center, a ligand radical should be considered.Data reported in the accompanying paper (Stout, C. D., Ghosh, D., Pattabhi, V., and Robbins, A. H. (1980) J BioL Chem 266,1797-1800) support the concept of a 3-Fe center, as well as the presence of a well defined Fe4S4 cluster in this protein.Cluster displacement experiments reveal that FeaSr and Fe4S4 cores can be transferred from this ferredoxin into appropriate protein acceptors.These experiments do not rule out the presence of a trinuclear iron cluster.