Two-iron ferredoxins

Fritz, J., Anderson, R., Fee, J., Palmer, G, Sands, R.H., Tsibris, J.C.M., Gunsalus, I.C., Orme-Johnson, W.H., and Beinert, H. 1971. The iron electron-nuclear double resonance (ENDOR) of two-iron ferredoxins from spinach, parsley, pig adrenal cortex and Pseudomonas putida. Biochimica et Biophysica Acta 253 110-133. 

Fig. 46. Schematic structure of the iron-sulfur complex in two-iron ferredoxin and adrenodoxin. (Adapted from Ref. 267)...

Table 17. 57Fe hf principal values of two-iron ferredoxins (data from Fritz et al.268) and Anderson et al.269 in MHz)... 

This enzyme complex [EC 1.10.2.2], also known as cytochrome bci and complex 111, catalyzes the reaction of ubiquinol (QH2) with two ferricytochrome c to produce ubiquinone (Q) and two ferrocytochrome c. The complex also contains cytochrome h-562, cytochrome h-566, cytochrome Ci, and a two-iron ferredoxin. 

A wide variety of synthetic binuclear iron complexes (39) bridged by sulfide, disulfide and/or thiolate groups are known. They have proven to be good models for the two-iron ferredoxin proteins, as demonstrated by comparisons of structure and properties. Early attempts to isolate two-iron complexes by direct reaction of a monothiol with FeCl3, NaSH and NaOMe afforded only four-iron products suggesting that a particularly high stability is associated with the tetrameric... 

Besides a poorly resolved triplet structure in the EPR spectrum of Fe enriched two-iron ferredoxins at gj, no hf interactions are observed in all these iron-sulfur clusters. The distribution of principal axes of the g tensors gives rise to an inhomogeneous, field-dependent EPR linewidth, and obscures the resolution of ligand hfs This distribution, whose origin is not completely established, does not, however, prevent application of the ENDOR technique. 

Iron-Sulphur Proteins.— A review article on the properties of iron-sulphur protein found in chloroplast membranes has been published. The oxidations of dithionite-reduced parsley and spinach two-iron ferredoxins, Fd(iii)(ii), with cobalt(in) complexes are independent of pH over the range 7.0—9.0. A change in the order of reaction for the tripositive reagents [Co(NH3)6] and [ 0(60)3] + from one to zero with increasing concentration is ascribed to association with the negatively charged protein followed by rate-determining electron transfer ... 

Clostridial ferredoxin, which is characterized by two clusters containing four sulphur-linked iron atoms each, has been studied by Mossbauer and e.s,r. spectroscopy. The iron of oxidized ferredoxin is high-spin Fe and reduction leads to an uptake of one electron per cluster. The two Fe centres in oxidized spinach ferredoxin are not equivalent and it seems that the metal co-ordination geometry at the non-reducible site is basically tetrahedral. The first well-defined synthetic analogue of the active sites of two-iron ferredoxins has been prepared and charac-... 

Dunham WR, Sands RH. 2003. g-strain, ENDOR, and structure of active centers of two-iron ferredoxins. Biochem Biophys Res Comm 312 255-261. 

Hill and Walton have examined the interaction of horse heart cytochrome c with azurin and cyt C551 using an electrochemical technique and a detailed theoretical study of cytochrome c has been published. The interaction with cytochrome and redox potentials for this latter protein have also been studied. The redox-inactive [Cr(en)3] " inhibits oxidation of cyt hs with [Co(NH3)6] and other positively charged reagents by blocking the heme edge site used by cyt Electron transfer reactions of iron-sulfur proteins have been reviewed and NMR studies " have been used to pinpoint reaction sites with inorganic complexes on the two iron ferredoxin from Anabena variabilis at residues 25 and 83. 

Dunham et have recently summarized their spectroscopic and magnetic susceptibility results on spinach and other two-iron ferredoxins they show how the results allow a precise definition of the active centre and provide a stringent set of criteria against which any further structural information may be tested. The c.d. and absorption spectra of spinach ferredoxin and another two-iron iron-sulphur protein, adrenodoxin, suggest that the reduced forms contain a high-spin ferrous ion in a distorted tetrahedral environment. The reaction between adrenodoxin and excess 1,10-phenanthroline apparently follows zero-order kinetics under aerobic conditions but first-order kinetics under anaerobic conditions the activation energies are 3.3 and 12.6 kcal mol, respectively. A reaction sequence is proposed. ... 

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