Sulphur Proteins

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  

Association constants, K, for the parsley (998 M ) and spinach (993 M ) proteins are very similar with [Co(NH3)b] + as oxidant and the value for [Co(en)3] + is 597 with parsley at 25 °C. A more complex alternative mechanism in which association produces an inactive complex cannot be ruled out but is considered unlikely since changing the ionic strength does not affect the limiting rate. Of a series of cobalt(iii) complexes, only those with 3 + and 2+ charges exhibit association behaviour in the available concentration ranges but the thermodynamic parameters for overall electron transfer, Kket (Table 3) show a trend with the charge on the complex, suggesting that electrostatic and solvation effects are important. 

Redox-inactive [Cr(NH3)e] + binds to reduced parsley ferredoxin with an association constant of476 at the same site as [Co(NH3)6] , [Co(NH3)5Cl] +, 

The rate of reduction of cytochrome c by the cobalt-substituted analogue of rubredoxin is a factor of 2.25 lower than the rate with rubredoxin itself. Both proteins mediate the reduction of cytochrome c in the presence of NADH and the decrease in the rate is attributed to the decreased efficiency in oxidation of cobalt(ii) compared with iron(ii). Reduction of cytochrome c by NADPH is catalysed by an adrenodoxin reductase-adrenodoxin complex in which the rate-determining step is electron transfer from the flavin (FAD) of the reductase to the FeaS2 centre of adrenodoxin. The pH dependence of the rate shows a pATa of 6.75, with the high-pH form 27.5 times more reactive than the low-pH form. Both NADPH reduction of the complex and cytochrome c oxidation of the complex were faster than the catalytic rate. Catalytic roles of four iron-sulphur centres in trimethylene dihydrogenase and ferredoxin nitrite reductase have also been examined. Synthetic analogues of four iron ferredoxins have also attracted much attention. -  

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Synthetic approaches to the active sites of iron-sulphur proteins. R. H. Holm, Acc. Chem. Res.,... 

Iron-sulphur proteins structural chemistry of their chromophores and related systems. R. Mason and J. A. Zubieta, Angew. Chem., Int. Ed. Engl., 1973,12, 390-399 (101). 

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... 

Figure 15.6 The Wood-Ljungdahl pathway. One molecule of C02 (blue) is converted to formate and then reduced to a methyl group, which is then transferred to the corrinoid-iron-sulphur protein CFeSP. CFeSP transfers the methyl group to the A-cluster of the bifunctional CODH/ACS. The other molecule of C02 (red) is reduced to CO by the C-cluster of the CODH subunit. The CO is then transferred to the A-cluster through a long channel, some 70 A long, where with the methyl group and CoA it forms acetylCoA. (From Drennan et al., 2004. With kind permission of Springer Science and Business Media.)...

With an iron-sulphur protein as acceptor Acting on single donors with incorporation of molecular oxygen (oxygenases) 1.18 1.18.1 Acting on reduced ferredoxin as donor With NAD+ or NADP+ as acceptor... 

Figure 18.3 Competition for iron between proliferating bacteria, erythropoiesis in the bone marrow and proliferating lymphocytes in the lymph nodes. The iron ion is required for synthesis of haemoglobin, cytochromes and iron-sulphur proteins, and for maintenance of the structure of DNA.

Active Site Structure of Rubredoxin There are several non-heme iron-sulphur proteins that are involved in electron transfer. They contain distinct iron-sulphur clusters composed of iron atoms, sulphydryl groups from cysteine residues and inorganic or labile sulphur atoms or sulphide ions. The labile sulphur is readily removed by washing with acid. The cysteine moieties are incorporated within the protein chain and are thus not labile. The simplest type of cluster is bacteria rubredoxin, (Cys-S)4 Fe (often abbreviated FelSO where S stands for inorganic sulphur), and contains only non labile sulphur. It is a bacterial protein of uncertain function with a molecular weight of 6000. The single iron atom is at the centre of a tetrahedron of four cysteine ligands (Fig.). 

Balk J, Pierik AJ, Aguilar Netz DJ, Miihlenhoff U, Lill R (2005) Narlp, a conserved eukaryotic protein with similarity to Fe-only hydrogenases, functions in cytosolic iron-sulphur protein biogenesis. Biochem Soc Trans 33 86-89 Barton RM, Worman HJ (1999) Prenylated prelamin A interacts with Narf, a novel nuclear protein. J Biol Chem 274 30008-30018... 

Tovar J, Leon-Avila G, Sanchez LB, Sutak R, Tachezy J, van der Giezen M, Hernandez M, Muller M, Lucocq JM (2003) Mitochondrial remnant organelles of Giardia function in iron-sulphur protein maturation. Nature 426 172-176... 

Kispal G, Sipos K, Lange H, Fekete Z, Bedekovics T, Janaky T, Bassler J, Guilar Netz DJ, Balk J, Rotte C, Lill R (2005) Biogenesis of cylosolic ribosomes requires the essential iron-sulphur protein Rlilp and mitochondria. EMBO J 24 589-598... 

Spiro, T. G., ed., Iron-Sulphur Proteins. Wiley (Interscience), New York, 1982. 

As is known in the reaction centres of PSl, there are at least two molecules of iron-sulphur protein which can be reduced photochemically [46-48]. Observations of the simultaneous decay at low temperatures of P700 and reduced molecules of iron-sulphur protein with a half-wave potential of 530 mV indicate that, at least at such temperatures, it is this iron-sulphur protein that serves as the stable primary acceptor A [40,49]. This conclusion is also corroborated by the results of EPR and optical experiments which show the yield of P700+ to decrease upon preliminary (previous to illumination) redox titration of subchloroplasts leading to the... 

Parry, D. A. D., Fraser, R. D. B., and MacRae, T. P. (1979). Repeating patterns of amino acid residues in the sequences of some high-sulphur proteins from a-keratin. Int. J. Biol. Macromol. 1, 17-22. 

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... 

The kinetics of the decay of P700+ centres due to their recombination with acceptor centres, which most likely are the primary acceptors A", i.e. iron-sulphur proteins,... 

Once cholesterol is transferred to the inner mitochondrial membrane of steroidogenic tissues such as adrenals, ovaries and testes, it encounters the enzyme system known as the cholesterol SCC system. This probably comprises 20- and 22-hydroxylases and a C-20,22-lyase, all tightly bound to the inner face of the membrane and associated with a specific cytochrome />-450scc. In addition, molecular 02 is necessary together with NADPH reductase and non-haem iron sulphur protein, which are called adrenodoxin reductase and adrenodoxin, respectively, in the adrenal [24] (Fig. 3). 

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... 

Iron Sulphur Proteins Lovenberg, W., Ed. Academic New York and London, 1977 pp. 15-59. 

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