Iron-sulphur centres

Oxidation-reduction and substitution reactions of iron sulphur centres. F. Armstrong, Adv. Inorg. Bioinorg. Mech., 1982,1, 65-120 (167). 

Johnson MK, Morningstar JE, Cecchini G, Ackrell BAC. 1985. Detection of a tetranuclear iron sulphur centre in fumarate reductase from Escherichia coli by EPR. Arch Microbiol 131 756-62. 

P700 is the primary electron donor, A may be a chlorophyll-a, A a quinone and A2 (also called X) a specialised iron-sulphur centre. A and B are bound iron-sulphur centres characterised by absorption at 430nm. 

Low potential electron relays accept electrons from excited P700 of PSI either directly or via X, the membrane-bound iron-sulphur centre. The effect of the following electron carriers on photocurrent generated by PSI-coated electrodes were examined. 

Figure 10.3 Crystallographic structures of the most important iron-sulphur centres in proteins (a) dimeric centre of ferrodoxin from Spinacia oleracea [54], (b) trimeric centre of ferredoxin from Bacillus schlegelii [55], (c) cubane cluster of nitrogenase reductase from Azotobacter vinelandii [56], (d) nitrogenase octameric cluster from Azotobacter vinelandii [57], (e) nitrogenase octameric cluster from Clostridium pasteurianum [58], (f) MoFe cluster of nitrogenase from Azotobacter vinelandii [59], and (g) active centre of sulphite reductase from Escherichia coli [60]...

Fernandez VM, Hatchikian EC, Patil DS, Cammack R (1986) ESR-detectable nickel and iron-sulphur centres in relation to the reversible activation of Desulfovibrio gigas hydrogenase. Biochim. 

Fritsch F, Scheerer P, Frielingsdorf S, et al. The crystal structure of an oxygen-tolerant hydrogenase unvocers a novel iron-sulphur centre. Nature. 2011 479(7372) 249-52. 

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. 

The iron-sulphur centre is probably of the 2Fe-2S type [191,223]. It is a one-electron donor/acceptor with of approx. 280 mV in mitochondria (pH independent below pH 8 [224,225]). It exhibits an EPR spectrum in the reduced state that is somewhat anomalous for 2Fe-2S clusters (see Ref. 221). This, as well as the high midpoint redox potential, suggest that the iron ligands may be less electronegative than the four cysteine sulphurs of the plant ferredoxin model (see Ref. 226). The EPR spectrum of the FeS cluster is affected by the redox state of ubiquinone... 

Five proteins containing molybdenum are known nitrate reductase, nit-rogenase, xanthine oxidase, aldehyde oxidase and sulphite oxidase. They also contain iron, and the first four are best classified as multi-enzyme systems. Early studies on xanthine oxidase used a number of important ESR techniques, particularly rapid freeze kinetic methods and isotopic substitution in metalloproteins. This work has been reviewed [38, 39], Nitrogenase is the subject of considerable recent interest since it contains detectable iron-sulphur centres but as there is some disagreement at present concerning the interpretations of the results readers are referred to the original literature [40-42]. 

Reaction [9] is inhibited by 0.4 mM morin or dicoiunarol to 80 and 60%, respectively. The same concentration of dicoiunarol inhibits Reaction [9a] by less than 5 % (17). These facts and other results (20) are indicative of three binding domains of enoate reductase one for NADH which can be blocked by dicoiunarol or morin, another for enoates which can be occupied by hunarate, and a third one for reduced methylviologen. For studies on the iron/sulphur centres see (21). 

Fig. 4.23 The complete set of electron carriers of the respiratory chain. In site 1, there are at least five different iron-sulphur centres. In site 2, there are two different forms of cytochrome b, with different light-absorption jjeaks, as well as an iron-sulphur centre distinct from those in site 1. In site 3, there are two copper ions in addition to cytochromes a and a. The precise sequence and function of all the redox centres is not known with certainty.

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. - ... 

The more typical iron-sulphur centre contains two iron atoms, two sulphides and four cysteine sulphurs . It is believed that each iron is surrounded by four sulphur ligands in an approximate tetrahedral array. Every iron is coordinated by two sulphurs from cysteine and two from sulphide, with each of the sulphides binding both irons. The general structure of the two-iron redox centre is depicted below. 

NADH dehydrogenase, by other name NADH cytochrome c reductase (EC 1.6.99.3 formerly EC 1.6.2.1), a flavoprotein containing iron-sulphur centres, was detected in pituicytes of the rat (Bock and Goslar 1969) using 2,2 ,5,5 -tetra-p-nitrophenyl-3,3 -(3,3 -dimethoxy-4,4 -biphenylene)-ditetrazolium chloride according to Rosa and Tsou (1961). 

This suggested that the Complex I generator as in the case of rats and pigeons (Herrero and Barja 1997), can be the Complex I iron-sulphur centres. Complex III also generated free radicals in the three species studied by Herrero and Barja (1998). 

The iron-sulphur centres then pass electrons to ubiquinone (UQ). Ubiquinone (otherwise known as Coenzyme Q) is a substituted quinone which can be readily oxidized and reduced ... 

Ubiquinone is a hydrogen atom acceptor and 2H are therefore taken up during its reduction by the reduced iron-sulphur centres ... 

Cammack R, Patil DS, Hatchikian EC, Fernandez VM. 1987. Nickel and iron-sulphur centres in Desulfovibrio gigas hydrogenase ESR spectra, redox properties and interaction. Biochim Biophys Acta 912 98-109. 

Citrate is converted into another tricarboxylate, isocitrate, because the position of the hydroxyl group prevents its direct oxidation. Aconitate hyd-ratase, the enzyme that catalyses this reaction, contains an iron-sulphur centre (Section 13.2) and... 

FeS = iron-sulphur centre t QP = a specific ubiquinone apoprotein which is required for the binding of ubiquinone 1 FeS = Rieske iron-sulphur binuclear centre, nanti after its discoverer ... 

FIGURE 13.1 Iron-sulphur centres, (a) Mononuclear, (b) Binuclear (2Fe-2S). (c) Tetranuclear (4Fe-4S)... 

Electron transport operates by sequential oxidation-reduction reactions involving FAD, FMN, ubiquinone (Figure 8.7b), iron-sulphur centres, haems and protein-bound copper atoms. However, each enzyme can only catalyse the transfer of electrons between specific carriers so that electron transport is a highly ordered event with carriers organized in a defined order within the complexes which are precisely positioned within the membrane. 

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