Electron transfer components

Photon absorption in the PS II/OEC leads to charge separation in the PS II reaction center to generate the oxidized reaction center, P-680 (Ref. 18 Chapter 4, this volume.) The simplest scheme for subsequent electron transfer steps involves only the intermediate carrier, Z, and the Mn ensemble at the water-splitting site  

The questions of branched pathways and of additional intermediates are often raised (Section 3.1), but only the components noted above have been detected directly as entities with distinct functional and spectroscopic properties (Table 2). 

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Over the years, there have been numerous reports of oxidase preparations that contain polypeptide components, additional to those described above. As yet no molecular probes are available for these, and so their true association with the oxidase is unconfirmed. There are many reports in the literature describing the role of ubiquinone as an electron transfer component of the oxidase, but its involvement is controversial. Quinones (ubiquinone-10) have reportedly been detected in some neutrophil membrane preparations, but other reports have shown that neither plasma membranes, specific granules nor most oxidase preparations contain appreciable amounts of quinone, although some is found in either tertiary granules or mitochondria. Still other reports suggest that ubiquinone, flavoprotein and cytochrome b are present in active oxidase preparations. Thus, the role of ubiquinone and other quinones in oxidase activity is in doubt, but the available evidence weighs against their involvement. Indeed, the refinement of the cell-free activation system described above obviates the requirement for any other redox carriers for oxidase function. 

The first of these new, electron transferring components was coenzyme Q (CoQ). Festenstein in R.A. Morton s laboratory in Liverpool had isolated crude preparations from intestinal mucosa in 1955. Purer material was obtained the next year from rat liver by Morton. The material was lipid soluble, widely distributed, and had the properties of a quinone and so was initially called ubiquinone. Its function was unclear. At the same time Crane, Hatefi and Lester in Wisconsin were trying to identify the substances in the electron transport chain acting between NADH and cytochrome b. Using lipid extractants they isolated a new quininoid coenzyme which showed redox changes in respiration. They called it coenzyme Q (CoQ). CoQ was later shown to be identical to ubiquinone. 

Studies of medium effects on hexacyanoferrate(II) reductions have included those of dioxygen,iodate, peroxodisulfate, - [Co(NH3)5(DMSO)] +, and [Co(en)2Br2]+. Rate constants for reaction with dioxygen depended strongly on the electron-donor properties of the organic cosolvent. Rate constants for reduction of peroxodisulfate in several binary aqueous media were analyzed into their ion association and subsequent electron transfer components. Rate constants for reduction of [Co(en)2Br2] in methanol water and dioxan water mixtures were analyzed by a variety of correlatory equations (dielectric constant Grunwald-Winstein Swain Kamlet-Taft). 

An abbreviation for Ifigh-potential iron-sulfur protein, which is now regarded as a ferredoxin. In its role as a bacterial electron transfer component, this [4Fe-4S] cluster protein can undergo interconversion to the [4Fe-4S] and [4Fe-4S] states. 

Another source of rubredoxins was found in an aerobic bacterium, Pseudomonas oleovorans, utilizing n-hexane as a carbon source (10). This particular rubredoxin differs from those commonly found in anaerobic bacteria in some of its properties it has a molecular weight of 19,000, and one iron form of the protein is readily converted to a two-iron form (11). The rubredoxin of P. oleovorans functions as a terminal electron transfer component in an enzyme system which participates in the ( -hydroxylation of fatty acids and hydrocarbons. The hydrocarbon-oxidizing... 

Iron-Sulfur Proteins as an Electron Transfer Component to Cytochrome... 

Putidaredoxin. Cushman et al. (36) isolated a low molecular iron-sulfur protein from camphor-grown Pseudomonas putida. This protein, putidaredoxin, is similar to the plant type ferredoxins with two irons attached to two acid-labile sulfur atoms (37). It has a molecular weight of 12,000 and shows absorption maxima at 327, 425 and 455 nm. Putidaredoxin functions as an electron transfer component of a methylene hydroxylase system involved in camphor hydroxylation by P. putida. This enzyme system consists of putidaredoxin, flavoprotein and cytochrome P.cQ (38). The electron transport from flavoprotein to cytochrome P.cq is Smilar to that of the mammalian mixed-function oxidase, but requires NADH as a primary electron donor as shown in Fig. 4. In this bacterial mixed-function oxidase system, reduced putidaredoxin donates an electron to substrate-bound cytochrome P. g, and the reduced cytochrome P. g binds to molecular oxygen. One oxygen atom is then used for substrate oxidation, and the other one is reduced to water (39, 40). 

The entire iron-porphyrin-protein complex is called a cytochrome and such proteins are important electron-transfer components of cells. Generally, access to the macromolecular region in which the oxidation reactions occur is via a hydrophobic channel through the protein (Mueller et al., 1995). As a result, organic substrates are transferred from aqueous solution into the enzyme s active site primarily due to their hydrophobicity and are limited by their size. This important feature seems very appropriate hydrophobic molecules are selected to associate with this enzyme, and these are precisely the ones that are most difficult for organisms to avoid accumulating from a surrounding aquatic environment. 

Tripathi GNR (1998) Electron-transfer component in hydroxyl radical reactions observed by time resolved resonance Raman spectroscopy. J Am Chem Soc 120 4161-4166 TsaiT, Strauss R, Rosen GM (1999) Evaluation of various spin traps for the in vivo in situ detection of hydroxyl radical. J Chem Soc Perkin Trans 2 1759-1763 Tsay L-Y, Lee K-T, Liu T-Z (1998) Evidence for accelerated generation of OH radicals in experimental obstructive jaundice of rats. Free Rad Biol Med 24 732-737 Ulanski P, von Sonntag C (2000) Stability constants and decay of aqua-copper(lll) - a study by pulse radiolysis with conductometric detection. Eur J Inorg Chem 1211-1217 Veltwisch D, Janata E, Asmus K-D (1980) Primary processes in the reactions of OH radicals with sul-phoxides. J Chem Soc Perkin Trans 2 146-153... 

The enzymes appear to be cytochrome Ph50 mixed-function oxygenases with electron transfer components similar to those found in liver (19,20). 

Hasson, E.P., West, C.A. "Properties of the system for the mixed function oxidation of kaurene and kaurene derivatives in microsomes of the immature seed of Marah macrocarpus. Electron transfer components." Plant Physiol., 1976, 58, U79-U8U. 

Williams-Smith, D.L., Heathcote, P., Sihra, C.K. and Evans, M.C.W. 1978. Quantitative EPR measurements of the electron-transfer components of the photosystem I reaction centre. Biochem. J., 170. 365-371. 

Peters, A.L.J., Wielink, J.E.V., Sang, H.W.W.F., De Vries, S. and Kraayenhof, R. 1983. Studies on well coupled photosystem I-enriched subchloroplast vesicles content and redox properties of electron-transfer components. Biochim. Biophys. Acta, 722.460-470. 

Mechanism of the 2 reaction (which possibly has an electron transfer component see e.g. Bilevich and Okhlobystin, 1968 Bank and Noyd, 1973). 

Figure 5-19. Schematic representation of reactions occurring at the photosystems and certain electron transfer components, emphasizing the vectorial or unidirectional flows developed in the thylakoids of a chloroplast. Outwardly directed election movements occur in the two photosystems (PS I and PS II), where the election donors are on the inner side of the membrane and the election acceptors are on the outer side. Light-harvesting complexes (LHC) act as antennae for these photosystems. The plastoquinone pool (PQ) and the Cyt b(f complex occur in the membrane, whereas plastocyanin (PC) occurs on the lumen side and ferredoxin-NADP+ oxidoreductase (FNR), which catalyzes electron flow from ferredoxin (FD) to NADP+, occurs on the stromal side of the thylakoids. Protons (H+) are produced in the lumen by the oxidation of water and also are transported into the lumen accompanying electron (e ) movement along the electron transfer chain.

Many organic compounds involved in photosynthesis accept or donate electrons (see Table 5-3). The negatively charged electrons spontaneously flow toward more positive electrical potentials (A > 0), which are termed redox potentials for the components involved with electron flow in chlo-roplast lamellae (Fig. 1-10) or the inner membranes of mitochondria (Fig. 1-9). Redox potentials are a measure of the relative chemical potential of electrons accepted or donated by a particular type of molecule. The oxidized form plus the reduced form of each electron transfer component can be regarded as an electrode, or half-cell. Such a half-cell can interact with other electron-accepting and electron-donating molecules in the membrane, in which case the electrons spontaneously move toward the component with the higher redox potential. 

Dutton, P. L., and Jackson, J. B., 1972, Thermodynamic and kinetic characterization of electron transfer components in situ in Rhodopseudomonas spheroides and Rhodospirillum rubrum, Eur. J. Biochem. 30 495n510. 

Early studies by Stuehr and Nathan and colleagues demonstrated a speciflc stimulation of NOS activity in crude subcellular fractions by BH4, which is true for all purifled NOS enzymes to date. BH4 is important both as a stabilizer of protein conformation and also as an electron transfer component.The activity of the enzyme intracellularly can be affected by modulations in cellular BH4 levels, and this may have important consequences under physiological and pathophysiological conditions. " ... 

M Sabaty, J Japp0,J Olive and A Vermeglio (1994) Organization of electron transfer components in Rhodobacter sphaeroides forma sp. dentrificans whole cells. Biochim Biophys Acta 1187 313-323... 

Fig. 7. Redox-titration ourves of the reaction centers in (A) Rb. sphaeroides, (B) Cf. aurantiacus, (C) Rp. viridis and (D) Chromatium. See text for other details. Figure sources (A) Dutton and Jackson (1972) Thermodynamic and kinetic characterization of electron-transfer components in situ in Rhodopseudomonas spheroides and Rhodospiriiium rubrum. Eur J Biochem. 39 500 (B) Bruce, Fuiler and Biankenship (1982) Primary photochemistry in the facultatively aerobic green photosynthetic bacterium Chloroflexus aurantiacus. Proc Nat Acad, USA. 79 6533 (C) Prince, Leigh and Dutton (1976) Thermodynamic properties ofthe reaction center of Rhodopseudomonas viridis. Biochim Blophys Acta. 440 625 (D) Cusanovich, Bartsch and Kamen (1968) Light-induced electron transport In Chromatium. II. Light-induced absorbance changes in Chromatium chromatophores. Biochim Biophys Acta 153 408.

Table II. Number of cA-Chl-a molecules at various distances from electron-transfer components...

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... 

Le Gall, J., DerVartanian, D.V., Peck, H.D., Jr. In Flavoproteins, Iron-Proteins, and Hemo-proteins as Electron Transfer Components of the Sulfate-Reducing Bacteria (ed. Sanadi, R.), New York, Academic Press 1979, Current Topics in Bioenergetics 9, 237... 

B b 50 Electron transfer component. Contains 2 heme B molecules and occurs as cytochrome bCi... 

C c 12.4 (N. winogradskyi) Electron transfer component. Contains 1 heme C molecule... 

C3 12.3 (D. vulgaris) Electron transfer component. Contains 4 heme C molecules... 

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