Iron-sulfur proteins centers

Three protein complexes have been isolated, including the flavoprotein (FP), iron-sulfur protein (IP), and hydrophobic protein (HP). FP contains three peptides (of mass 51, 24, and 10 kD) and bound FMN and has 2 Fe-S centers (a 2Fe-2S center and a 4Fe-4S center). IP contains six peptides and at least 3 Fe-S centers. HP contains at least seven peptides and one Fe-S center. 

Resonance Raman (RR) spectroscopy provides information about the vibrational characteristics of a chromophore, for example, a metal center, within the complex environment of a protein. In RR spectra, those vibrational transitions are observed selectively that are coupled to electronic transitions. In iron sulfur proteins, this technique has been used to resolve the complex electronic absorption spectra and to identify both vibrational and electronic transitions. 

In addition to the standard constraints introduced previously, structural constraints obtainable from the effects of the paramagnetic center(s) on the NMR properties of the nuclei of the protein can be used (24, 103). In iron-sulfur proteins, both nuclear relaxation rates and hyperfine shifts can be employed for this purpose. The paramagnetic enhancement of nuclear relaxation rates [Eqs. (1) and (2)] depends on the sixth power of the nucleus-metal distance (note that this is analogous to the case of NOEs, where there is a dependence on the sixth power of the nucleus-nucleus distance). It is thus possible to estimate such distances from nuclear relaxation rate measurements, which can be converted into upper (and lower) distance limits. When there is more than one metal ion, the individual contributions of all metal ions must be summed up (101, 104-108). If all the metal ions are equivalent (as in reduced HiPIPs), the global paramagnetic contribution to the 7th nuclear relaxation rate is given by... 

It has always been assumed that these simple proteins act as electron-transfer proteins. This is also a fair conclusion if we take in account that different proteins were isolated in which the Fe(RS)4 center is in association with other non-heme, non-iron-sulfur centers. In these proteins the Fe(RS)4 center may serve as electron donor/ac-ceptor to the catalytic site, as in other iron-sulfur proteins where [2Fe-2S], [3Fe-4S], and [4Fe-4S] clusters are proposed to be involved in the intramolecular electron transfer pathway (see the following examples). 

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

Pereira MM, Carita JN, Teixeira M. 1999. Membrane-bound electron transfer chain of the ther-mohalophilic bacterium Rhodothermus marinus Characterization of the iron- sulfur centers from the dehydrogenases and investigation of the high-potential iron- sulfur protein function by in vitro reconstitution of the respiratory chain. Biochemistry 38 1276. 

Iron-Sulfur Proteins (Examples FeS4 - and Fe4S4 - Centers)... 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

The following description of the electron transfer-proton transport scheme is illustrated in Figure 7.26. First, an electron is transferred from doubly reduced dihydroplastoquinone (PQFI2) to a high potential electron transfer chain that consists of the Reiske iron-sulfur protein and the cytochrome protein containing heme f. Rappaport,Lavergne and co-workers have reported a midpoint potential at pH 7.0 of +355 mV for heme f. These two centers reside on the electropositive (lumen or p) side of the membrane, exterior to the membrane. As a result, two protons are transferred to the aqueous lumen phase. A second electron is transferred from PQH2 sequentially to heme bp. 

Iron-sulfur proteins serve predominantly as electron carriers (28,29). The best understood examples are those proteins with IFe, 2Fe, and 4Fe centers. The environment of the mononuclear iron center, rubredoxin, is shown in structure C (17). It consists of a distorted tetrahedral array of sulfur atoms from cysteine residues at nearly equal distances from the iron atom. Crystal structures are available for 2Fe-2S ferredoxins from Spindina plantensis (19) and Aphanothece sacrum (20). A representation of the geometry of this site is given in structure D. The 2Fe-2S core is anchored to the polypeptide by ligation to 4 cysteine sulfur atoms, yielding distorted tetrahedral geometry for both iron atoms. Crystal... 

A newly emerging class of iron sulfur proteins are those with 3Fe-XS centers. Aerobically isolated, inactive beef heart aconitase has a 3Fe-4S center which has... 

A designation for a two iron, two labile-sulfur cluster present in a number of iron-sulfur proteins. The [2Fe-2S] center consists of two iron atoms bridged by two sulfur atoms (the sulfido-bridges). The oxidation levels of the clusters are usually indicated by adding the charges on the iron and sulfide atoms (e.g., [2Fe-2S] +). 

A class of iron-sulfur proteins lacking acid-labile sulfur, but similar in function to the ferredoxins. The iron center is coordinated by four sulfur-containing hgands, usually from cysteinyl residues. Where known, these proteins function as electron carriers. 

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