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Tetrahedral Iron in the Active Center of Plant Ferredoxins And Proc. Nat. Acad. Sci. USA Vol. 68, No. 12, pp. 3015-3020, December 1971 Tetrahedral Iron in the Active Center of Plant Ferredoxins and Beef Adrenodoxin* (iron-sulfur proteins/rubredoxin/near-infrared circular dichroism/ligand-field spectrum/EPR) WILLIAM A. EATONt, GRAHAM PALMERI, JAMES A. FEEt§, TOKUJI KIMURA'[, AND WALTER LOVENBERG11 t Laboratory of Physical Biology, National Institute of Arthritis and Metabolic Diseases, National In- stitutes of Health, Bethesda, Md. 20014; t Biophysics Research Division, Institute of Science and Tech- nology, University of Michigan, Ann Arbor, Mich. 48105; 'I Department of Chemistry, Wayne State University, Detroit, Mich. 48202; and 11 Experimental Therapeutics Branch, National Heart and Lung Institute, National Institutes of Health, Bethesda, Md. 20014 Communicated by Harry B. Gray, September 30, 1971 ABSTRACT The coordination structure of the iron- with a net spin of 2. Recent reviews by Tsibris and Woody sulfur complex in spinach ferredoxin and adrenodoxin is Palmer and have discussed the chem- investigated by optical spectroscopy. The circular-dichro- (6) and Brintzinger (7) ism and absorption spectra of these two-iron iron-sulfur ical, magnetic resonance, magnetic susceptibility, and M6ss- proteins reveal weak electronic transitions in the near- bauer studies, which have all been consistent with this model, infrared wavelength range, 0.8-2.5 ,Am (12,500-4000 cm-l). though no demonstration of the coordination geometry of On the basis of the low absorption intensities and large the iron-sulfur complex was possible. The basis for investigat- anisotropy factors, d d transitions of the iron can be identified in the reduced proteins at about 4000 cm-' and ing this and other aspects of the proposed structure by optical 6000 cm-. The low energy of these one-center ligand-field spectroscopy was recently developed by Eaton and Loven- transitions, together with the similarity to the ligand- berg, from their absorption and circular-dichroism studies in field spectrum of the one-iron protein rubredoxin, leads the near-infrared on the one-iron protein rubredoxin (8). to the conclusion that the reduced two-iron iron-sulfur and reduced are known from proteins also contain a high-spin ferrous ion in a distorted Oxidized rubredoxin x-ray tetrahedral site. diffraction (9), magnetic susceptibility (10), M6ssbauer (10), and optical studies (8) to contain a high-spin ferric and A currently attractive proposal for the structure of the active high-spin ferrous ion, repsectively, coordinated to four sulfurs center of the two-iron iron-sulfur proteins such as the plant of cysteinyl residues in an approximately tetrahedral complex ferredoxins, adrenodoxin, and putidaredoxin, was put forth (Fig. 1). The d - d electronic transitions of the tetrahedral several years ago, in part to explain the absence of electron ferrous ion, predicted from ligand-field theory to be char- paramagnetic resonance (EPR) of the oxidized protein and acteristically very optically active, were found by Eaton the unusually low average g value observed upon one-electron and Lovenberg at about 6250 cm-' (1.6 jm) (8). In the ab- reduction (4, 5). The proposed structure consists of a bi- sence of a very strong interaction between the irons, the above nuclear complex in which each iron atom is bonded to four model would predict that a similar one-center ligand-field sulfur atoms tetrahedrally disposed, as depicted schematically spectrum exists in the two-iron iron-sulfur proteins. In this in Fig 1. Two sulfur atoms-the "acid-labile sulfides"-are communication we demonstrate that this is indeed the case common bridging ligands to the metal ions, while the four for plant ferredoxins and adrenodoxin, thereby providing terminal mercaptide sulfur atoms are provided by the cys- strong evidence that the reduced proteins contain a high-spin teinyl residues from the protein. It was further proposed (5) ferrous ion in an approximately tetrahedral site. that the oxidized protein contains two high-spin ferric (d$, S = 5/2) ions antiferromagnetically coupled to give a net MATERIALS AND METHODS spin of zero in the ground state, while in the reduced protein a Rubredoxin from the bacterium Cloetridium pasteurianum, high-spin ferrous (d6, S = 2) ion is antiferromagnetically ferredoxin from spinach chloroplasts, and adrenodoxin from coupled to a high-spin ferric ion, resulting in a ground state beef adrenal glands were prepared by previously described * This is one in a series of papers describing the electronic proper- ties of spinach ferredoxin and other iron sulfur proteins; related publications (refs. 1-3) contain the results of Mossbauer, electron- nuclear double-resonance spectroscopy, and variable-temperature magnetic-susceptibility measurements. A summary of all our physical data, their bearing on a discrete model for two-iron iron- sulfur proteins, and an analysis of the consequences of this model for nuclear magnetic resonance contact-shifted proton resonances will be published in Biochim. Biophys. Acta, vol. 253. § Present address: Department of Chemistry, Rensselaer Poly- FIG. 1. Schematic structure of the active centers of one- and technic Institute, Troy, N.Y. two-iron iron-sulfur proteins. 3015 Downloaded by guest on September 23, 2021 3016 Chemistry: Eaton et al. Proc. Nat. Acad. Sci. USA 68 (1971) Wavelength (micrometers) 0.8 1.0 1.5 2.0 2.5 light in the two cell compartments. At wavelengths shorter I than about 1.8 pm, the measured circular-dichroism mag- 61-E, (M'1cmr|) RED nitudes at the extrema are probably within 10-20% of the +8 true values. However, only qualitative estimates -+6 Circular Dichroism of the circular -+4 dichroism could be made in the 2.1-2.5 um region because -+2 of high D20 and protein absorption from vibrational overtone transitions and because of considerable deviation from perfect -2 ox circular polarization. The low degree of polarization would E'(M'Icm|) result in a measured value that is lower than the true value Absorption (14). The absorption and circular-dichroism spectra of reduced e (M-1cm-,) rubredoxin and reduced adrenodoxin showed no change during II % the 2- to 5-hr period of data collection. Some samples of fer- redoxin exhibited a slowly increasing absorbance over the I ,T 300 entire near-infrared spectral range (0.8-2.5 Mum); this ab- sorbance did not, however, significantly alter the circular- f dichroism spectrum. We have not yet determined the nature of the optically inactive material responsible for this, but it Rubredoxin dI could possibly be a polymeric complex of iron and sulfide, -200 ', which might be expected to show intense near-infrared ab- sorption. In any event, the spectra for ferredoxin reported 1l RED below have no contribution from this undefined material. RESULTS -100 '., II Figs. 2-4 show the near-infrared absorption and circular- dichroism spectra of the oxidized and reduced proteins: rubredoxin, ferredoxin, and adrenoxdoin. In the region from 4000 cm-' to about 9000 cm-', the oxidized proteins exhibit 14,000 12,000 10,000 8,000 only optically-inactive vibrational overtone bands of the Frequency (cm-I) protein and some residual HOD. The broad electronic ab- sorption of the reduced proteins in this spectral region is FIG. 2. Circular-dichroism and absorption spectra of oxidized more clearly seen in the reduced-minus-oxidized difference (OX) and reduced (RED) Clostridium pasteurianum rubredoxin in spectra shown in the middle of the Figures, where the con- D20-0.1 M Tris buffer (pH 7) at room temperature. The middle tribution from the relatively sharp vibrational transitions curve (RED - OX) is the difference absorption spectrum, (reduced rubredoxin) minus (oxidized rubredoxin). Extinction has been subtracted out by use of the oxidized proteins as a coefficients are based on the value 8800 Al-' cm-' for the 4900- reference. band of oxidized rubredoxin (11). For all three proteins, several of the electronic transitions in the near-infrared have large anisotropy factors (greater than 0.01, see Table 1) compared to the spectra in the visible methods (11-13). After two cycles of freeze-drying and soaking and ultraviolet regions (8, 15), where the (point) anisotropy in D20, the optical ratios for rubredoxin, ferredoxin, and factor does not exceed 0.005. Furthermore, there are very important similarities among the spectra of the reduced adrenodoxin were A4go/A2so = 0.35-0.4, A42o/A276 = 0.43- forms of and 0.46 and A414/A274 = 0.65, respectively. These ratios for the rubredoxin, ferredoxin, adrenodoxin, par- oxidized proteins are somewhat less than the best possible ticularly in the region 4000-8000 cm-', where all three proteins values, but should not influence the spectral results in the show weak absorption bands with large anisotropy factors. near-infrared, where the apoproteins exhibit no electronic Quantitative aspects of the spectra are summarized in absorption. Reduction was performed with excess solid sodium Table 1, together with the spectral assignments discussed dithionite and, in the case of adrenodoxin, methyl viologen below. The center of each transition (v) is obtained from either the maximum or the (about 10jum) was added as a redox mediator. absorption circular-dichroism In cases are All spectral measurements were made with D20 as the extremum. where both reasonably well resolved, solvent, since it absorbs much less strongly in the near- the agreement is within 300 cm-'. Most of the oscillator infrared than H20. Spectra could be measured to 1.8 jum strengths (f) and, consequently, the anisotropy factors (g), (5600 cm-') with 25-mm pathlength cells; to penetrate the are only reported to one significant figure, reflecting the con- in of of 2.1-2.5 ,tm (4800-4000 cm-') D20 "window" 2- or 5-mm siderable ambiguity the resolution certain parts the into its cells were used. Absorption spectra were obtained with either a absorption spectra component bands. Similar data Cary 14 or Zeiss DMR 21 recording spectrophotometer.
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