Iron-sulphur complex

As with some other enzymes involved in the generation of ATP in the mitochondria (e.g. oxoglutarate dehydrogenase), aconitase possesses an iron-sulphur complex in its... 

Fig. 9. A proposed structural configuration for the polynuclear iron-sulphur complex in clostridial ferredoxin.

An EPR signal centred at about g=1.94 is observed from the reduced iron-sulphur complex (27) and poseed a considerable problem in its interpretation until the suggestion came that one was dealing with a coupled pair having Si = f, S2 = 2 and a large positive J (22, 24). From Eqs. (8) and (15) it follows that the g values and hyperfine constants for the coupled system are given by... 

Figure 5.2 Energy surface associated with two variable torsional angles in one of the side chains of an iron sulphur complex (structure taken from Kanatzidis et ah, 1984). The resulting energy map, a function of the two variable side chain angles highlighted, a and b, is shown on the right-hand side of the figure, as both a contour map and a three-dimensional histogram. A variety of maxima and minima are readily apparent in the...

While most of the tetrapyrrole pigments in the RC (FeS) type function as light harvesting, six central tetrapyrroles and three iron-sulphur complexes appear to constitute the photochemical heart of the reaction center. 

Both iron(II) and iron(III) can be determined spectrophotometrically the reddish-orange iron(II) complex absorbs at 515 nm, and both the iron(II) and the yellow iron(III) complex have identical absorption at 396 nm, the amount being additive. The solution, slightly acid with sulphuric acid, is treated with... 

All the complexes consist of several subunits (Table 2) complex I has a flavin mononucleotide (FMN) prosthetic group and complex II a flavin adenine dinucleotide (FAD) prosthetic group. Complexes I, II, and III contain iron-sulphur (FeS) centers. These centers contain either two, three, or four Fe atoms linked to the sulphydryl groups of peptide cysteine residues and they also contain acid-labile sulphur atoms. Each center can accept or donate reversibly a single electron. 

Complex II contains four peptides, the two largest form succinate dehydrogenase, the largest has covalently boiuid flavin adenine dinucleotide (FAD) which reacts with succinate, and the other has three iron-sulphur centers. Smaller subunits anchor the two larger subunits to the membrane and form the UQ binding site. Ubiquinone is the electron acceptor but complex II does not pump protons (see below). 

The nuclear-encoded proteins are inserted into both inner and outer mitochondrial membranes, the intermembrane space, and the matrix and there are several different mechanisms involved. As mentioned above there is no apparent requirement for a presequence on proteins which insert specifically into the mitochondrial outer membrane. For proteins destined for the inner mitochondrial membrane, a stop-transfer mechanism is proposed. Thus some information in the peptide must stop the complete transfer of the protein into the mitochondrial matrix, enabling the protein to remain in the inner mitochondrial membrane. For some proteins in the intermembrane space (for example the Rieske iron-sulphur protein associated with the outer face of complex III), a particularly complicated import pathway... 

Butler, A. R., Glidewell, C., Hyde, A. R., and Walton, J. C. (1985). Formation of paramagnetic mononuclear iron nitrosyl complexes from diamagnetic di- and tetranuclear iron-sulphur nitrosyls Characterization by EPR spectroscopy and study of thiolate and nitrosyl ligand exchange reactions. Polyhedron 4, 797-809. 

The chemistry of cluster complexes, e.g. of the sort [FeitSi, (SR) i,] 2, is of particular interest since such complexes are known to be close representations or synthetic analogues of the redox centres present in various iron-sulphur proteins. It is important to know whether the valence electrons are localized or delocalized in such complexes - in fact several studies by e.s.r., n.m.r., and, more recently, resonance Raman spectroscopy have shown that such clusters are delocalized rather than trapped-valence species. This result is linked with the most important biophysical property of iron-sulphur proteins, viz. that of electron transfer. Rapid electron transfer is possible if any consequential geometric rearrangements around the metal atom sites are small, as implied by many resonance Raman results on such cluster complexes (cf. the small-displacement approximation, which provides a basis for enhancement to fundamental but not to overtone bands) (22). Initial studies of [MSi,]2- ions (M = Mo or W) (23,24) have since been supplemented by studies of dinuclear species e.g. [(PhS)2FeS2MS2]2 (25) and cluster species... 

Glidewell, C. and Glidewell, S.M. (1993) The fate of nitrite in food processing isolation of dinuclear and tetranuclear iron-sulphur nitrosyl complexes from cysteine and methionine sources. Food Chem., 46, 225-230. 

Kapazoglou A, Mould RM, Gray JC (2000) Assembly of the Rieske iron-sulphur protein into the cytochrome bf complex in thylakoid membranes of isolated pea chloroplasts. Eur J Biochem 267 352-360... 

Numerous transition metals ions form cluster complexes with chalcogenide anions [42-52], Iron and sulphur are unique elements in the sense that no two other elements can generate such a large diversity of cluster structures. This is the consequence of two stable oxidation states of iron ions and strong Fe-S bonds of significantly covalent character [53], Moreover, numerous structures are stable in several oxidation states, so these clusters serve as electron reservoirs in biological systems [51], This is why iron-sulphur proteins usually catalyze redox reactions. 

Abbreviations AA, antimycin BAL, British Anti-Lewisite (2,3-dimercaptopropanol) DCCD, dicyclo-hexylcarbodiimide DTNB, 5,5 -dithiobis(2-nitrobenzoate) oxidoreduction potential relative to the Normal Hydrogen Electrode midpoint oxidoreduction potential E midpoint oxidoreduction potential at pH = x FeS, iron-sulphur (centre or protein) FMN, flavin mononucleotide HMHQQ, 7-( n-heptadecyl)mercapto-6-hydroxy-5,8-quinolinequinone HOQNO, 2-/i-heptyl-4-hydroxyquinoline N-oxide Lb, leghaemoglobin MX, myxothiazol NEM, 7V-ethylmaIeimide pmf, protonmotive force, electrochemical proton gradient Q, ubiquinone Qj i, ubiquinone bound to Complex I SQ, ubise-miquinone SQ , ubisemiquinone anion UHDBT, 5- -undecyl-6-hydroxy-4,7-dioxobenzothiazol. 

After suggestions on an iron-sulphur protein in the 6c, complex [215], this subunit was isolated and characterised by Rieske et al. [216,218], but in a form that was not active in reconstitution. Isolation in a reconstitutively active form was pioneered by Racker et al. [219], who showed that a soluble oxidation factor was required for activity. Subsequently, Trumpower and Edwards [220] purified oxidation factor and identified it as a form of the FeS protein that is active in reconstitution. An excellent review on the structure and function of the FeS protein is available [221]. 

Treatment of Complex I with chaotropic agents dissociates two water-soluble subcomplexes from the enzyme. Both contain iron, and one also contains FMN. These are called iron-sulphur protein (ISP) and flavoprotein or NADH dehydrogenase (Type II), respectively [285]. The hydrophobic residue left behind still contains six or seven of the original 22-23 Fe atoms per FMN, while ISP contains 9.3, and the dehydrogenase 6.2 [296]. The ISP fraction may be further dissociated into three parts with trichloroacetate, each containing an FeS centre. Two of these... 

Fig. 3.13. Complex I. A, the map shows how Complex I can be dissected into subcomplexes. Treatment with chaotropic agents splits the Complex into three fractions an iron-sulphur protein (ISP), a flavoprotein (NADH dehydrogenase) complex, and a hydrophobic residue. The figure shows the polypeptide composition and the flavin and iron content of these subcomplexes. ISP can be further split into three FeS-containing fractions by trichloroacetate. See text for details and references. B, the topography of the components mapped in A has been studied by Ragan et al. [290,296]. The NADH-binding site is in the flavoprotein fraction [297]. This, as well as the FeS protein complex (ISP), are possibly buried in the membrane and shielded from phospholipids by the hydrophobic residue (HR). Adapted from Refs. 290, 296.

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