Iron-sulphur redox proteins

In mitochondria (Fig. lb), the electron acceptor protein is also a flavoprotein termed NADPH-adrenodoxin reductase (MW 50 kDa) because it was discovered in the adrenal cortex and because it donates its electrons not directly to the P450 but to the smaller redox protein adrenodoxin (MW 12.5 kDa). The two iron-sulphur clusters of this protein serve as electron shuttle between the flavoprotein and the mitochondrial P450. 

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... 

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... 

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. 

In the past ten years a large number of novel iron-sulphur proteins have been discovered, largely as a result of ESR measurements. Indeed ESR is the technique of choice for their identification and has made important contributions to knowledge of their structure and function. These proteins are now known to occur widely in animals, plants and bacteria and play important roles in respiration, photosynthesis, nitrogen fixation, hormone synthesis and sulphur and carbon metabolism. They function as electron carriers and most, though not all, have negative mid-point redox potentials at pH 7. 

A simplified representation of the mitochondrial respiratory chain in terms of the oxidation of NADH and succinate by oxygen is illustrated in Figure 4.9 together with Table 4.2 showing the twelve iron sulphur proteins identified, their g values and mid-point redox potentials. The g values found are consistent from a variety of different preparations, though some changes in line share are found. The mid-point potentials are variable, with values from -20 mV to -265 mV for centre N-2. 

Another important group of redox proteins are the iron-sulphur proteins. These are proteins of relatively low molecular weight containing one or more iron atoms... 

In this section we review applications of electroactive polymers as electrode materials for use with redox cofactors, primarily NAD/NADH, and with components of redox proteins and redox enzymes, such as iron-sulphur clusters or flavins. We begin by considering the NAD/NADH couple. 

Rieske protain/centar an iron-sulphur protein first isolat from Complex III of the mitochondrial electron transport chain, in which it occurs with cytochromes b and C) [J.S, Rieske el al. Biochem. Biophys Res Commun. IS (1%4) 338-344], but which has now been found in the equivalent cytochrome be complexes in the bacterial plasma membrane and the chloroplast thylakoid membrane. The latter, known as the cytochrome bff complex, partidpates in cyclic and noncyclic electron flow in the light phase of photosynthesis (see Photosynthesis). All Rieske proteins are one-electron redox systems with a standanl redox potential in the + 0.2 to + 0.3V range and have a (2Fe-2S] center, a single membrane-spanning a-helix, and a characteristic electron spin resonance (ESR) spectrum. The chloroplastidic R.p/c, with a M, of - 20,000, is smaller than that of the mitochondnon. It is encoded in the nucleus, synthesized in the cytoplasm and translocated to the chloroplast, where it is inserted into the thylakoid membrane. Within the thylakoid membrane its [2Fe-2S] redox centre (near to its C-terminus) can readily pass electrons to cytochrome /, a c-type cytochrome that projects from the luminal surface cytochrome / then passes electrons to plastocyanin (see) dissolved in the aqueous milieu of the thylakoid lumen. 

In addition to the monograph on the iron-sulphur proteins, there have been two reviews on this important class of macromolecule, and the lUPAC-IUB commission on biochemical nomenclature has published recommendations on their nomenclature. A method has been described for the interpretation of e.p.r. spectra of reduced dinuclear iron-sulphur proteins which will allow both the symmetry and the extent of covalency at the paramagnetic site to be parameterized. The parameters can then be related to the chemical composition of the paramagnetic centre, the protein-dependent charge delocalization of the unpaired electron, and the geometrical arrangement at the reduced iron atom - an analysis which, it is hoped, will ultimately be useful in rationalizing the redox behaviour of these important metalloproteins. 

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