Protein-bound clusters

The fourth state with [Fe4S4]° shown in Table 6.1 was recently described as the most reduced form possible for the Fe-protein s [Fe4S4] cluster.16 Usually, only two oxidation states for a given metal-sulfur cluster are stable. Therefore a stable [Fe4S4]° state in Fe-protein s iron-sulfur cluster (as appears likely from experimental evidence presented in reference 16) would be unique because the cluster would then have three stable oxidation states, [Fe4S4]2+/1+/0. It appears also that the all-ferrous state is only stable in the protein-bound cluster and not for model... 

The [Fe4 84] + state is isoelectronic with HPqx cores. EPR and Mossbauer studies of the sole isolated synthetic cluster, [Fe484(8tibt)4] , indicate that it closely resembles HPqx in electronic structure. The redox couple exhibited by this and other model compounds, however, show significantly more negative potentials than the protein-bound cluster. 

Oarter has reviewed the comparative crystallography of oxidized and reduced C. vinosum HiPIP (1), and the dimensional changes of the iron-sulfur cube following oxidation or reduction have also been extensively tabulated and discussed for both model complexes and protein-bound clusters (118). In spite of the low sequence homology in HiPIPs, there is a remarkable similarity in tertiary structure, especially around the cluster (114). No significant secondary structure is observed in the HiPIPs, with only two short a-helical segments, three strands of antiparallel /3-pleated sheet, and one small helix near the N terminus (Fig. 1). The 4Fe-4S cluster is buried in the protein interior and is inaccessible to solvent (Fig. 2). This feature has been pro-... 

Spectra and spin states are generally similar between synthetic analogs and protein-bound clusters however, both exhibit considerable variability from species to species. 

Structure of the MoFe Protein. Extensive spectroscopic studies of the MoEe proteia, the appHcation of cluster extmsion techniques (84,151), x-ray anomalous scattering, and x-ray diffraction (10,135—137,152) have shown that the MoEe proteia contains two types of prosthetic groups, ie, protein-bound metal clusters, each of which contains about 50% of the Ee and content. Sixteen of the 30 Ee atoms and 14—16 of the 32—34... 

When induced in macrophages, iNOS produces large amounts of NO which represents a major cytotoxic principle of those cells. Due to its affinity to protein-bound iron, NO can inhibit a number of key enzymes that contain iron in their catalytic centers. These include ribonucleotide reductase (rate-limiting in DNA replication), iron-sulfur cluster-dependent enzymes (complex I and II) involved in mitochondrial electron transport and cis-aconitase in the citric acid cycle. In addition, higher concentrations of NO,... 

These studies of protein-bound heterometallic cubanes have amply demonstrated that the heterometal site is redox active and able to bind small molecules. Although they have yet to be identified as intrinsic components of any protein or enzyme (except as part of the nitrogenase FeMo cofactor cluster (254)), they are clearly attractive candidates for the active sites of redox enzymes. 

The systematic development of the chemistry of synthetic [MFe3S4] clusters is largely the work of Holm and co-workers and has occurred in parallel and synergistically with the development of the protein-bound analogs. A comprehensive review of this work up to 1992 can... 

Three core oxidation states are known for protein-bound [Fe4-S4(S.Cys)4]3+ clusters as illustrated in Figure 2.9. Native proteins exhibit either the [Fe4-S4]2+ + or the [Fe4-S4]3+,2+ redox couple, with proteins involved in the latter couple being referred to historically as HiPIP (high-potential iron protein). The three oxidation states have not been traversed in one protein unless its tertiary structure is significantly perturbed. 

Another enzyme, cysteine desulfurase, converts L-cysteine to L-alanine and a sulfane sulfur - a chain of divalent sulfur atoms. A protein-bound cysteine persulfide is formed on a conserved cysteine residue. Such enzymes have important roles in the biosynthesis of Fe-S clusters and sulfur-containing cofactors.9... 

The so-called midpoint potential, Em, of protein-bound [Fe-S] clusters controls both the kinetics and thermodynamics of their reactions. Em may depend on the protein chain s polarity in the vicinity of the metal-sulfur cluster and also upon the bulk solvent accessibility at the site. It is known that nucleotide binding to nitrogenase s Fe-protein, for instance, results in a lowering of the redox potential of its [4Fe-4S] cluster by over 100 mV. This is thought to be essential for electron transfer to MoFe-protein for substrate reduction.11 3... 

Proteins containing iron-sulfur clusters are ubiquitous in nature, due primarily to their involvement in biological electron transfer reactions. In addition to functioning as simple reagents for electron transfer, protein-bound iron-sulfur clusters also function in catalysis of numerous redox reactions (e.g., H2 oxidation, N2 reduction) and, in some cases, of reactions that involve the addition or elimination of water to or from specific substrates (e.g., aconitase in the tricarboxylic acid cycle) (1). 

A third type of trinuclear cluster duplicates the Fe3S stoichiometry that is most accepted for protein-bound 3Fe clusters and in fact appears to be a structural isomer of the 3Fe cluster present in aconitase at least. The [Fe3S (SR) ]ion (3) is obtained by reaction of [Fe(SR) ] " with 1.4 equivalents of sulfur in MeCN, as indicated above (13, 16). It contains a central Fe(III) ligated tetrahedrally by four sulfides that connect it to two terminal Fe(IIl)(SR)2 units. The spectroscopic and magnetic properties of 3 are virtually identical with those of the 3Fe form of aconitase at high pH, but substantially different from those of aconitase under normal physiological conditions (23). Possible structures for the latter that are consistent with available data are shown in Figure 7. 

Since a common route to 3Fe derivatives of proteins containing 4Fe-4S clusters is via oxidation (34), this implies that it might be possible to prepare a synthetic 3Fe-4S cluster by simple oxidation of an [Fe S L ] " cluster. A recent attempt to implement this strategy examined oxidation of [Fe SA(S-t-Bu)4] by ferricyanide in DMF/H2O solution at T < -40°C (35). Although a species with EPR and Mossbauer parameters similar to those of protein-bound 3Fe centers was generated (in < 30% yield), it was... 

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