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Mobile electron carrier

The final step of the reaction involves the transfer of two electrons from iron-sulfur clusters to coenzyme Q. Coenzyme Q is a mobile electron carrier. Its isoprenoid tail makes it highly hydrophobic, and it diffuses freely in the hydrophobic core of the inner mitochondrial membrane. As a result, it shuttles electrons from Complexes I and II to Complex III. The redox cycle of UQ is shown in Figure 21.5, and the overall scheme is shown schematically in Figure 21.6. [Pg.682]

Cytochrome c, like UQ is a mobile electron carrier. It associates loosely with the inner mitochondrial membrane (in the intermembrane space on the cytosolic side of the inner membrane) to acquire electrons from the Fe-S-cyt C aggregate of Complex 111, and then it migrates along the membrane surface in the reduced state, carrying electrons to cytochrome c oxidase, the fourth complex of the electron transport chain. [Pg.688]

FIGURE 21.21 A model for the electron transport pathway in the mitochondrial inner membrane. UQ/UQH9 and cytochrome e are mobile electron carriers and function by transferring electrons between the complexes. The proton transport driven by Complexes I, III, and IV is indicated. [Pg.692]

ORR catalysis by Fe or Co porphyrins in Nation [Shi and Anson, 1990 Anson et al., 1985 Buttry and Anson, 1984], polyp5rrolidone [Wan et al., 1984], a surfactant [Shi et al., 1995] or lipid films [CoUman and Boulatov, 2002] on electrode surfaces has been studied. The major advantages of diluting a metalloporphyrin in an inert film include the abUity to study the catalytic properties of isolated molecules and the potentially higher surface loading of the catalyst without mass transport Umit-ations. StabUity of catalysts may also improve upon incorporating them into a polymer. However, this setup requires that the catalyst have a reasonable mobUity in the matrix, and/or that a mobile electron carrier be incorporated in the film [Andrieux and Saveant, 1992]. The latter limits the accessible electrochemical potentials to that of the electron carrier. [Pg.652]

The most recent model of this membrane, the fluid mosaic model [13] is pictured in cartoon fashion in Fig. 2. In this model, the transduction proteins (complexes I-IV) are randomly dispersed in the membrane and redox equivalents are delivered from one complex to another via the mobile electron carriers cytochrome c and ubiquinone. It is necessary that cytochrome c be able to move relatively facilely from one complex to another. Thus the binding constants cannot be too high without making the associated OS rates too slow. Conversely, to prevent unproductive short circuits via cytochrome c from complex I directly to IV, there must exist molecular recognition which favors selective binding of cytochrome c to hcj and cytochrome oxidase (and perhaps disfavors binding to complex 1 or II). [Pg.163]

Cytochromes serve as electron donors and electron acceptors in biological electron transfer chains, and with >75,000 members (53) they provide the bulk of natural heme proteins in biology. Cytochromes may be fixed into place within an extended electron transfer chain, such as the membrane-bound 6l and 6h of the cytochrome bci complex, or may be soluble and act as mobile electron carriers between proteins, for example, cytochrome c (54). In either role, the cytochrome may be classified by the peripheral architecture of the porphyrin macrocycle. Figure 1 shows the dominant heme types in biological systems, which are hemes a, b, c, and d, with cytochomes b and c being most prevalent. The self-association of a protein with heme via two axial ligands is a... [Pg.412]

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. [Pg.693]

The inner mitochondrial membrane can be disrupted into five sepa rate enzyme complexes, called complexes I, II, III, IV, and V. Complexes I to IV each contain part of the electron transport chain (Figure 6.8), whereas complex V catalyzes ATP synthesis (see p. 78). Each complex accepts or donates electrons to relatively mobile electron carriers, such as coenzyme Q and cytochrome c. Each car rier in the electron transport chain can receive electrons from an electron donor, and can subsequently donate electrons to the next carrier in the chain. The electrons ultimately combine with oxygen and protons to form water. This requirement for oxygen makes the electron transport process the respiratory chain, which accounts for the greatest portion of the body s use of oxygen. [Pg.74]

If the reaction centers of photosystem I and photosystem II are segregated into separate regions of the thylakoid membrane, how can electrons move from photosystem I to photosystem II Evidently the plastoquinone that is reduced in photosystem II can diffuse rapidly in the membrane, just as ubiquinone does in the mitochondrial inner membrane. Plastoquinone thus carries electrons from photosystem II to the cytochrome b6f complex. Plastocyanin acts similarly as a mobile electron carrier from the cytochrome b f complex to the reaction center of photosystem I, just as cytochrome c carries electrons from the mitochondrial cytochrome bct complex to cytochrome oxidase and as a c-type cytochrome provides electrons to the reaction centers of purple bacteria (see fig. 15.13). [Pg.344]

Electron channels, as transmembrane wires, represent the channel-type counterpart to the mobile electron carriers discussed above and will be considered in Section 8.3.2. Anion channels may also be envisaged. [Pg.79]

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. [Pg.114]

Reduction of plastoquinone Qb by QA- and protonation at the acceptor side of PSII. The Qa is tightly bound to the protein, acting as a one electron acceptor. It passes electrons to a second plastoquinone, Qb, which can accept two electrons and two protons and acts as a mobile electron carrier connecting PSII to the next complex of the photosynthetic apparatus (i.e. the cytochrome b(f complex). After two electron-reductions and two protonation events, QbH2 leaves the reaction center and is replaced by an oxidized quinone from the pool in the membrane. [Pg.189]

Fig. 1. A schematic illustration of the mitochondrial (A) and Escherichia coli (B) respiratory chains. Respiratory enzymes perform a series of oxidation-reduction reactions by transferring electrons (dashed lines) through mobile electron carriers. Electron transfer is coupled to the pumping of protons (thick black arrows) from the A/-side (negative side) to the P-side (positive side) generating a proton gradient that ultimately drives the conversion of ADP to ATP. Fig. 1. A schematic illustration of the mitochondrial (A) and Escherichia coli (B) respiratory chains. Respiratory enzymes perform a series of oxidation-reduction reactions by transferring electrons (dashed lines) through mobile electron carriers. Electron transfer is coupled to the pumping of protons (thick black arrows) from the A/-side (negative side) to the P-side (positive side) generating a proton gradient that ultimately drives the conversion of ADP to ATP.
The third protein complex in this electron-transfer chain (complex 111) is ubiquinol cytochrome c oxidoreductase (E.C. 1.10.2.2), or commonly known as cytochrome be, complex named after the its b-type and c-type cytochrome subunits. Probably the best-understood one among the complexes, be, complex catalyses electron transfers between two mobile electron carriers the hydrophobic molecule ubiquinone (Q) and the small soluble haem-containing protein cytochrome c. Two protons are translocated across the membrane per quinol oxidized (Hinkel, 1991 Crofts, 1985 Mitchell, 1976). [Pg.542]

If the model proposed by Andersson and Anderson [109] of total separation of PS I and PS II in the granal chloroplasts were to be accepted, electron transport from the PS II acceptors to P-700 would require a mobile electron carrier(s) which should diffuse laterally in the membrane fast enough to account for the observed electron transport rate. Plastoquinone [112] and plastocyanin are the candidates of choice for this role. The former has been shown to be present at approximately the same activity in the partitions and in the stroma-exposed membranes [43], while PC is known to be located in the intrathylakoid space [113],... [Pg.13]

Because of the rapid decay of the electronic coupling with distance, very long-distance (>40 A) ET reactions rarely occur in a single step. Instead, extremely long range ET involves an array of multiple redox centers, mobile electron carriers, or large-scale motion of redox-active domains (46). All intermolecular ET reactions require either protein-protein docking or formation of an encounter complex in which the two protein cofactors are... [Pg.376]

As is apparent in Fig. 3, considerable similarity exists in the arrangement of the electron transfer cofactors in PS I and PS n. The main differences between the two systems are as follows 1) PS I has three Pe4S4 iron-sulfur clusters. Ex, Ea, and Eb, located on the stromal side of the complex 2) In PS I the primary acceptor is a chlorophyll, not pheophytin and 3) the distance between the primary acceptor (Aqa3 ) and phylloquinone (Aia,b) in PS I is significantly shorter than the corresponding distance between PheoA,B and Qa.b in PS II and Type II reaction centers. These structural differences correlate with functional differences between the two types of reaction centers. In PS II, the mobile electron carrier on the stromal side of the complex is Qb, which is a lipid-soluble, two-electron acceptor. In contrast, the mobile electron carrier in PS I is ferredoxin, which is a water-soluble, one-electron acceptor. The three iron-sulfur clusters in PS I provide a chaimel by which electrons are funneled out of the reaction center to ferredoxin. On the donor side of the complex, plastocyanin, the reductant that replenishes electrons removed from P700, is also a water-soluble protein and is a one-electron donor. Thus, each photon absorbed by the PS I complex leads to the transfer of one electron from plastocyanin to ferredoxin. In Fig. 2, it is apparent that the midpoint potentials of the acceptors in PS I are about 500 to 700 mV more negative than those in PS II, and the... [Pg.1490]

Mobile Electron Carriers Plastocyanin and Ferredoxin, and Ferredoxin-NADP""-Reductase... [Pg.605]

Up to this point, we have considered the election carriers bound to the photosystem-I thylakoid membrane. They include the primary electron donor P700 and the series ofelectron acceptors Aq (Chi a). A, (phylloquinone), FeS-X, and FeS-A/B. We now turn to two mobile electron carriers around photosystem I called plastocyanin (PC), a copper protein and ferredoxin (Fd) a [2Fe-2S] irons-sulfur protein, plus the enzyme that catalyzes the reduction of NADP by ferredoxin and called the ferredoxin-NADP -reductase and abbreviated as FNR. [Pg.605]

These mobile electron carriers are relatively low-molecular-weight proteins and are both linked to the PS-1 reaction-center complex by electrostatic forces. Plastocyanin is located on the lumen side and is the electron donor to P700, while ferredoxin is located on the stroma side and receives electrons from the terminal, membrane-bound iron-sulfur proteins FeS-A/B. Water is the ultimate source of electrons for plastocyanin reduction. The electron from water is generated by its oxidation by photosystem 11, and transferred by way of cytochrome/in the cytochrome b(f complex. Reduction of NADP by ferredoxin requires mediation by the enzyme ferredoxin-NADP -reductase orFNR. Figure 1, right is a schematic representation of the relationship of the PS-1 reaction center to the peripheral electron carriers plastocyanin and ferredoxin as well as the protein catalyst ferredoxin-NADP -reductase. [Pg.606]

Chapter 34 Mobile Electron Carriers and Ferredoxin NADP Reductase... [Pg.607]

Chapter 34 Mobile Electron Carriers and Ferredoxin-NADP Reductase II.B. Reduction of Ferredoxin Kinetic-Spectrophotometric Measurement... [Pg.623]

The answer is c. (Murray, pp 123-148. Scriver, pp 2367-2424. Sack, pp 159-175. Wilson, pp 287-317.) The electron transport chain shown contains three proton pumps linked by two mobile electron carriers. At each of these three sites (NADH-Q reductase, cytochrome reductase, and cytochrome oxidase) the transfer of electrons down the chain powers the pumping of protons across the inner mitochondrial membrane. The blockage of electron transfers by specific point inhibitors leads to a buildup of highly reduced carriers behind the block because of the inability to transfer electrons across the block. In the scheme shown, rotenone blocks step A, antimycin A blocks step B, and carbon monoxide (as well as cyanide and azide) blocks step E. Therefore a carbon monoxide inhibition leads to a highly reduced state of all of the carriers of the chain. Puromycin and chloramphenicol are inhibitors of protein synthesis and have no direct effect upon the electron transport chain. [Pg.185]

Ionic limitation or 0 < X < 1 and excess of mobile electronic carriers... [Pg.388]


See other pages where Mobile electron carrier is mentioned: [Pg.688]    [Pg.141]    [Pg.189]    [Pg.197]    [Pg.568]    [Pg.128]    [Pg.250]    [Pg.2311]    [Pg.294]    [Pg.250]    [Pg.40]    [Pg.511]    [Pg.666]   
See also in sourсe #XX -- [ Pg.226 ]




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