Tetrahedral Iron in the Active Center of Plant Ferredoxins and Beef Adrenodoxin

Abstract

The coordination structure of the ironsulfur complex in spinach ferredoxin and adrenodoxin is investigated by optical spectroscopy. The circular-dichroism and absorption spectra of these two-iron iron-sulfur proteins reveal weak electronic transitions in the nearinfrared wavelength range, 0.8-2.5 ,Am (12,500-4000 cm-l).On the basis of the low absorption intensities and large anisotropy factors, d d transitions of the iron can be identified in the reduced proteins at about 4000 cm-' and 6000 cm-. The low energy of these one-center ligand-field transitions, together with the similarity to the ligandfield spectrum of the one-iron protein rubredoxin, leads to the conclusion that the reduced two-iron iron-sulfur proteins also contain a high-spin ferrous ion in a distorted tetrahedral site.A currently attractive proposal for the structure of the active center of the two-iron iron-sulfur proteins such as the plant ferredoxins, adrenodoxin, and putidaredoxin, was put forth several years ago, in part to explain the absence of electron paramagnetic resonance (EPR) of the oxidized protein and the unusually low average g value observed upon one-electron reduction (4, 5). The proposed structure consists of a binuclear complex in which each iron atom is bonded to four sulfur atoms tetrahedrally disposed, as depicted schematically in Fig 1. Two sulfur atoms-the "acid-labile sulfides"-are common bridging ligands to the metal ions, while the four terminal mercaptide sulfur atoms are provided by the cysteinyl residues from the protein. It was further proposed (5) that the oxidized protein contains two high-spin ferric (d$, S = 5/2) ions antiferromagnetically coupled to give a net spin of zero in the ground state, while in the reduced protein a high-spin ferrous (d6, S = 2) ion is antiferromagnetically coupled to a high-spin ferric ion, resulting in a ground state with a net spin of 2. Recent reviews by Tsibris and Woody (6) and Palmer and Brintzinger (7) have discussed the chemical, magnetic resonance, magnetic susceptibility, and M6ss-bauer studies, which have all been consistent with this model, though no demonstration of the coordination geometry of the iron-sulfur complex was possible. The basis for investigating this and other aspects of the proposed structure by optical spectroscopy was recently developed by Eaton and Lovenberg, from their absorption and circular-dichroism studies in the near-infrared on the one-iron protein rubredoxin (8).Oxidized and reduced rubredoxin are known from x-ray diffraction (9), magnetic susceptibility (10), M6ssbauer (10), and optical studies (8) to contain a high-spin ferric and high-spin ferrous ion, repsectively, coordinated to four sulfurs of cysteinyl residues in an approximately tetrahedral complex (Fig. 1). The d -d electronic transitions of the tetrahedral ferrous ion, predicted from ligand-field theory to be characteristically very optically active, were found by Eaton and Lovenberg at about 6250 cm-' (1.6 jm) (8). In the absence of a very strong interaction between the irons,...

The kinetics and mechanism of oxidation of reduced spinach ferredoxin by molecular oxygen and its reduced products

“…X-ray analysis of the structure of the [Fe-S]2 protein from Spirulina platensis [20] shows that the most likely mode of reaction proceeds via the orbitals of cysteinyl sulfur from residues 39 and 44 which are located on the surface of the molecule. The ferrous iron constituent of the binuclear cluster has tetrahedral symmetry [21,22], but data on the coordination number and symmetry of the ferric component are not available for either oxidized or reduced states• However, there is a report that the ferric sites in oxidized spinach ferredoxin are inequivalent, and the nonreducible site in reduced ferredoxin is proposed to be a highly distorted (squashed toward D2d) tetrahedral structure with the possibility of an additional weak coordination to an available nitrogen or oxygen donor atom [23]. Conceivably, oxygen can distort or partially displace one of the sulfur ligands and form a complex with the iron atom bonded by Cys-39 and Cys-44.…”

Cluster characterization in iron-sulfur proteins by magnetic circular dichroism.

“…Natural CD spectra (3) Madison, WI. 1 1 In addition to the well-known "390" band, all C2-state proteins exhibit a variably resolved peak at -v9600 cm-' (-u1050 nm); this band has been previously reported (6) (11). In the reduced clusters, the CD (11) shows that electronic transitions exist to well below 5000 cm-1; the lack of observation of MCD here is due to the small magnitude of the MCD anisotropy ratio.…”

The plant ferredoxins and their relationship to the evolution of ferredoxins from primitive life

“…These changes in protein conformation may be similar to those which cause changes in the optical absorption and CD when ferredoxin forms a complex with ferredoxin NADP red uctase26 27 Eaton et a!. 28 have investigated the nature of the iron sulphur complex in spinach ferredoxin and adrenodoxin by observing the CD and optical spectra in the near IR wavelength region. They identified d -+ d transitions of the iron atoms in the spectra of the reduced proteins at about 4000 cm and 6000 cm' and from the low energy of these transitions have concluded that the reduced iron-sulphur proteins contain a high spin ferrous ion in a distorted tetrahedral site.…”