Iron-sulfur proteins oxidation states

Table I reports the observed NMR linewidths for the H/3 protons of the coordinating cysteines in a series of iron-sulfur proteins with increasing nuclearity of the cluster, and in different oxidation states. We have attempted to rationalize the linewidths on the basis of the equations describing the Solomon and Curie contributions to the nuclear transverse relaxation rate [Eqs. (1) and (2)]. When dealing with polymetallic systems, the S value of the ground state has been used in the equations. When the ground state had S = 0, reference was made to the S of the first excited state and the results were scaled for the partial population of the state. In addition, in polymetallic systems it is also important to account for the fact that the orbitals of each iron atom contribute differently to the populated levels. For each level, the enhancement of nuclear relaxation induced by each iron is proportional to the square of the contribution of its orbitals (54). In practice, one has to calculate the following coefficient for each iron atom ...

Iron-sulfur proteins can be observed by EPR spectroscopy, either in their oxidized or in their reduced state. As a method of observing iron-sulfur clusters, EPR is discriminating but not particularly sensitive lack of a detectable EPR signal cannot be taken as evidence of absence. However, a positive EPR signal is good evidence for the intactness of an iron-sulfur cluster in a protein. Moreover, EPR can be used to follow reduction of the clusters and, by use of mediated electrochemical titrations, to estimate redox potentials. 

A condition where metal ions within a coordination complex or cluster are present in more than one oxidation state. In such systems, there is often complete delocalization of the valence electrons over the entire complex or cluster, and this is thought to facilitate electron-transfer reactions. Mixed valency has been observed in iron-sulfur proteins. Other terms for this behavior include mixed oxidation state and nonintegral oxidation state. 

Iron-sulfur proteins belong to the class of electron-transport proteins [29]. They contain an iron sulfur cluster, e.g. [4Fe-4S], which shuttles between different oxidation states. The structure of the cluster is quite consistent among a series of these proteins, but their redox potentials vary widely. Synthetic models of iron-sulfur proteins have been designed [30] to investigate the factors that determine the reduction potential of the core and to mimic other biologically... 

Some detailed comparisons of the protein environments around the HiPIP and Fd clusters have been made.769,770 It is noteworthy that the HiPIP cluster is more deeply buried (about 4.5 A) than is the case for the clusters in the other iron-sulfur proteins. All iron-sulfur proteins for which structural data are available, with the exception of the three-iron protein from Azotobacter vinelandii, have hydrogen bonding between the cysteine sulfur in the iron-sulfur cluster and the backbone peptide link. It appears that there is an approximate correlation between the number of NH S hydrogen bonds in the environment of a cluster and its redox potential. In HiPIP, these hydrogen bonds become more linear and shorten on reduction of the cluster. It is possible, therefore, that the oxidation states of the cluster may be controlled by the geometries of the hydrogen bonds.770... 

This cluster formally contains three iron(III) and one iron(E). It is present in a class of proteins called high potential iron-sulfur proteins (HiPIP). It has also been prepared through oxidation of [(RS)4Fe4S4]2 model compounds [57]. Both in the model compound at low temperatures and in proteins there is electron delocalization on one mixed valence pair [58-62]. Therefore, the polymetallic center is constituted by two iron ions at the oxidation state +2.5 and two iron ions at the oxidation state +3. Hamiltonian (6.20), or a more complicated one [40, 41,43], can be used to describe the electronic structure. Indeed, a delocalization operator is sometimes needed in the Hamiltonian [40,41,43]. Consistently with magnetic Mossbauer data the S M subspin involving the mixed valence pair is 9/2, whereas the S n subspin involving the iron(IH) ions is 4. Mossbauer and EPR data do not exclude % and 3, respectively, for the two pairs [57] in any case, the... 

The electron-transport chain contains a number of iron-sulfur proteins (also known as nonheme iron proteins). The iron atoms are bound to the proteins via cysteine —S— groups and sulfide ions one such 4-Fe cluster is shown in Fig. 14-1. These proteins mediate electron transport by direct electron transfer changes in oxidation state of the iron in iron-sulfur proteins can be monitored by electron spin resonance spectroscopy (ESR). 

Apply the crossover theorem, with reference to the figure in Example 14.5. The proportions of the iron-sulfur proteins in complexes I, II, and III with iron atoms in the Fe3+ and Fe2+ states can, in principle, be estimated from the electron spin resonance signals of the preparation of mitochondria. In the case of mitochondria oxidizing NADH in the presence of rotenone, application of the crossover theorem reveals that the iron atoms in the iron-sulfur proteins in complex I will become more reduced, while those in complex III will become more oxidized, in comparison to their states in the absence of rotenone. In the case of KCN-treated mitochondria, both sets of iron-sulfur proteins will become more... 

The [4Fe 4S] cluster is exposed to solvent and is symmetrically bridged between the two subunits of the Fe protein by the side chains of Cys97 and Cys 132 (Figure 3). The cluster hgands are located near the N termini of a-hehces that are directed toward the cluster, permitting favorable electrostatic interactions between the terminal amide groups of the helices and the anionic cluster. The [4Fe 4S] cluster of the Fe protein can exist in three different oxidation states, that is, [4Fe 4S] +, [4Fe-4S] +, and [4Fe 4S] see Iron-Sulfur Proteins). The physiological relevance of the various oxidation states of... 

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