Electron transport chain iron-sulfur proteins

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

Most mechanisms which control biological functions, such as cell respiration and photosynthesis (already discussed in Chapter 5, Section 3.1), are based on redox processes. In particular, as shown again in Figure 1, it is evident that, based on their physiological redox potentials, in photosynthesis a chain of electron carriers (e.g. iron-sulfur proteins, cytochromes and blue copper proteins) provides a means of electron transport which is triggered by the absorption of light. 

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. 

A second group of electron carriers in mitochondrial membranes are the iron-sulfur [Fe-S] clusters which are also bound to proteins. Iron-sulfur proteins release Fe3+ or Fe2+ plus H2S when acidified. The "inorganic clusters" bound into the proteins have characteristic compositions such as Fe2S2 and Fe4S4. The sulfur atoms of the clusters can be regarded as sulfide ions bound to the iron ions. The iron atoms are also attached to other sulfur atoms from cysteine side chains from the proteins. The Fe-S proteins are often tightly associated with other components of the electron transport chain. For example, the flavoproteins Flavin 1, Flavin 2, and Flavin 3 shown in Fig. 10-5 all contain Fe-S clusters as does the Q-cytochrome b complex. All of these Fe-S clusters seem to be one-electron carriers. 

Cytochromes b, a, and o. Protoheme-containing cytochromes b are widely distributed.127,128 There are at least five of them in E. coli. Whether in bacteria, mitochondria, or chloroplasts, the cytochromes b function within electron transport chains, often gathering electrons from dehydrogenases and passing them on to c-type cytochromes or to iron-sulfur proteins. Most cytochromes b are bound to or embedded within membranes of bacteria, mitochondria, chloroplasts, or endoplasmic reticulum (ER). For example, cyto-... 

Functions of iron-sulfur enzymes. Numerous iron-sulfur clusters are present within the membrane-bound electron transport chains discussed in Chapter 18. Of special interest is the Fe2S2 cluster present in a protein isolated from the cytochrome be complex (complex III) of mitochondria. First purified by Rieske et al.,307 this protein is often called the Rieske iron-sulfur protein 308 Similar proteins are found in cytochrome be complexes of chloroplasts.125 300 309 310 In... 

Heme coenzymes participate in a variety of electron-transfer reactions, including reactions of peroxides and 02. Iron-sulfur clusters, composed of Fe and S in equal numbers with cysteinyl side chains of proteins, mediate other electron-transfer processes, including the reduction of N2 to 2 NH3. Nicotinamide, flavin, and heme coenzymes act cooperatively with iron-sulfur proteins in multienzyme systems that catalyze hydroxylations of hydrocarbons and also in the transport of electrons from foodstuffs... 

The b cytochromes and cytochrome c, fit into this scheme between reducing substrates and cytochrome c. The idea thus developed that the respiratory apparatus includes a chain of cytochromes that operate in a defined sequence. The next question was whether the cytochromes are all bound together in a giant complex, or whether they diffuse independently in the membrane. Before we address this point, we need to consider three other types of electron carriers that participate in the electron-transport chain flavo-proteins, iron-sulfur proteins, and ubiquinone. 

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). 

In the chemiosmotic model, as first developed by Mitchell in the early 1960 s, proton translocation arises from transfer of electrons from an (H + + e ) carrier (such as FMNH2) to an electron carrier (such as an iron-sulfur protein), with expulsion of protons to the outer compartment of the inner mitochondrial membrane. This process is followed by electron transfer to an (H+ + e ) carrier, with uptake of protons from the matrix. In this model, the electron-transport chain is organized into three such loops, as shown in Fig. 14-5. 

Fe3+ in iron-sulfur proteins has an electron spin resonance (esr) signal, while Fe2+ does not. Assume that you have a preparation of mitochondria that are able to synthesize ATP via oxidation of NADH supplies of rotenone, antimycin A, and KCN and access to an esr spectrometer. How could you establish which of the complexes of the electron-transport chain contain iron-sulfur proteins ... 

The Cyt f complex lying between PS II and PS I in the electron-transport system resembles the Cyt be complex of mitochondria and photosynthetic bacteria. These cytochrome complexes possess one Rieske iron-sulfur protein R-FeS (a [2Fe-2S] protein discovered by John Rieske) and a so-called subunit IV. The two fc-hemes of Cyt b(, and the subunit IV span the thylakoid membrane, while the R-FeS and Cyt/ are located near the lumen side. As previously noted, the placement of the i>-hemes across the thylakoid membrane helps form a redox chain across the membrane. The function of the Cyt complex in green-plant thylakoids is to oxidize the plastohydroquinone formed by PS II and to transfer these electrons to plastocyanin. Accordingly, the Cyt ig/ complex has therefore also been called the plastohydroquinone-plastocyanin-oxidoreductase. ... 

T Ohnishi and JC Salerno (1982) Iron-sulfur clusters in the mitochondrial electron-transport chain in Iron-Sulfur Proteins Vol. 4 (TGSpiro ed) pp. 285-327. Wiley... 

This chapter deals with another electron carrier on the reducing side of the electron-transfer chain of photosystem I, namely, the iron-sulfur center FeS-X (also abbreviated as Fx in the literature). As described below, studies indicate that it is a [4Fe 4S] cluster that is uniquely in being coordinated to both the two major polypeptide subunits PsaA and PsaB that make up the protein heterodimer of the PS-I reaction center. For reference, we show in Fig. 1 a model of the location of this electron carrier in the photosystem-I reaction center, in terms of both its physical locale (A) and its position in the electron-transport chain (B). 

The electron-transfer pathway involving FeS-A and FeS-B and their location relative to FeS-X have been discussed in detail in Chapter 29. As FeS-X precedes FeS-A/FeS-B in the electron-transport chain, its role as an electron donor will now be considered. The problem is inherently difficult, first because not only are their spectra quite similar to one another but they also overlap, making direct spectral differentiation among them difficult, in spite of the experimentally adequate time resolution available in optical spectroscopy. Second, even though the three iron-sulfur proteins have characteristically different EPR spectra, the EPR technique itself does not have adequate time resolution to obtain sufficiently precise kinetic information and, furthermore, is not usable at physiological temperatures. 

The Rieske iron-sulfur protein is an ubiquitous electron-transfer component common to many redox systems, including all Cyt-fcc, and Cyt-bJcomplexes. The major function of R-[2Fe 2S] in the chloro-plast electron-transport chain is to facilitate the oxidation of plastoquinol by Cyt/. Like other iron-sulfur proteins, it does not have any prominent oxidized-minus-reduced difference absorbance bands in the visible region. In the oxidized state it consists of two antiferromagnetically coupled high-spin ferric... 

Electron transport chain Present in the mitochondrial membrane, this linear array of redox active electron carriers consists of NADH dehydrogenase, coenzyme Q, cytochrome c reductase, cytochrome c, and cytochrome oxidase as well as ancillary iron sulfur proteins. The electron carriers are arrayed in order of decreasing reduction potential such that the last carrier has the most positive reduction potential and transfers electrons to oxygen. 

The components of the electron transport chain have various cofactors. Complex I, NADH dehydrogenase, contains a flavin cofactor and iron sulfur centers, whereas complex ID, cytochrome reductase, contains cytochromes b and Cj. Complex IV, cytochrome oxidase, which transfers electrons to oxygen, contains copper ions as well as cytochromes a and a. The general structure of the cytochrome cofactors is shown in Figure 16-2. Each of the cytochromes has a heme cofactor but they vary slightly. The b-type cytochromes have protoporphyrin IX, which is identical to the heme in hemoglobin. The c-type cytochromes are covalently bound to cysteine residue 10 in the protein. The a-type... 

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