Proton translocation oxidation

Fillingame, R. H., 1980. The proton-translocating pump of oxidative pho.sphorylation. Annual Review of Biochemistry 49 1079-1113. 

Vance, J.E., eds.), pp. 116-142, Benjamin/Cummings Publishing Co., Menlo Park, California. Senior, A.E. (1988). ATP synthesis by oxidative phosphorylation. Physiological Rev. 68, 177-230. Senior, A.E. (1990). The proton-translocating ATPase of Esherichia colt. Ann. Rev. Biophys. Chem. 19,7- 1. 

Figure 12-8. Principles of the chemiosmotic theory of oxidative phosphorylation. The main proton circuit is created by the coupling of oxidation in the respiratory chain to proton translocation from the inside to the outside of the membrane, driven by the respiratory chain complexes I, III, and IV, each of which acts as a protonpump. Q, ubiquinone C, cytochrome c F Fq, protein subunits which utilize energy from the proton gradient to promote phosphorylation. Uncoupling agents such as dinitrophenol allow leakage of H" across the membrane, thus collapsing the electrochemical proton gradient. Oligomycin specifically blocks conduction of H" through Fq.

The proton-motive Q-cycle model, put forward by Mitchell (references 80 and 81) and by Trumpower and co-workers, is invoked in the following manner (1) One electron is transferred from ubiquinol (ubiquinol oxidized to ubisemi-quinone see Figure 7.27) to the Rieske [2Fe-2S] center at the Qo site, the site nearest the intermembrane space or p side (2) this electron can leave the bci complex via an attached cytochrome c or be transferred to cytochrome Ci (3) the reactive ubisemiquinone reduces the low-potential heme bL located closer to the membrane s intermembrane (p) side (4) reduced heme bL quickly transfers an electron to high-potential heme bn near the membrane s matrix side and (5) ubiquinone or ubisemiquinone oxidizes the reduced bn at the Qi site nearest the matrix or n side. Proton translocation results from the deprotonation of ubiquinol at the Qo site and protonation of ubisemiquinone at the Qi site. Ubiquinol generated at the Qi site is reoxidized at the Qo site (see Figure 7.27). Additional protons are transported across the membrane from the matrix (see Figure 7.26 illustrating a similar process for cytochrome b(6)f). The overall reaction can be written... 

A third, clearer explanation of the electron transfer, proton translocation cycle is given by Saratse. Each ubiquinol (QH2) molecule can donate two electrons. A hrst QH2 electron is transferred along a high-potential chain to the [2Fe-2S] center of the ISP and then to cytochrome Ci. From the cytochrome Cl site, the electron is delivered to the attached, soluble cytochrome c in the intermembrane space. A second QH2 electron is transferred to the Qi site via the cytochrome b hemes, bL and bn. This is an electrogenic step driven by the potential difference between the two b hemes. This step creates part of the proton-motive force. After two QH2 molecules are oxidized at the Qo site, two electrons have been transferred to the Qi site (where one ubiquinone (Qio) can now be reduced, requiring two protons to be translocated from the matrix space). The net effect is a translocation of two protons for each electron transferred to cytochrome c. Each explanation of the cytochrome bci Q cycle has its merits and its proponents. The reader should consult the literature for updates in this ongoing research area. 

Boogerd, F. C., Van Verseveld, H. W., and Stouthamer, A. H. (1981). Respiration-driven proton translocation with nitrate and nitrous oxide in Paracoccus denitrificans. Biochim. Biophys. Acca 638, 181-191. 

Garber, E. A. E., Castignetti, D., and Hollocher, T. C. (1982). Proton translocation and proline uptake associated with reduction of nitric oxide by denitrifying Paracoccus denicrificans. Biochem. Biophys. Res. Commun. 107, 1504-1507. 

Shapleigh, j. P., and Payne, W. j. (1985b). Nitric oxide-dependent proton translocation in various denitrifiers. J. Bacterial. 163, 837-840. 

The Q cycle accommodates the switch between the two-electron carrier ubiquinone and the one-electron carriers—cytochromes b562, b566, clt and c—and explains the measured stoichiometry of four protons translocated per pair of electrons passing through the Complex III to cytochrome c. Although the path of electrons through this segment of the respiratory chain is complicated, the net effect of the transfer is simple QH2 is oxidized to Q and two molecules of cytochrome c are reduced. 

This basic idea accounts well for the proton translocation that occurs in complex III. In the Q cycle (see fig. 14.11), UQH2 is oxidized to UQ on the outside of the inner membrane, and UQ is reduced to UQH2 on the matrix side. Hydrogen atoms move across the membrane by the diffusion of UQH2 from one of these catalytic sites to the other. The net transfer of two electrons from UQH2 to cytochrome c results in the uptake of two protons from the matrix and release of four protons to the intermembrane space. 

The combined effect of exchanging extramitochon-drial ADP-3 and H2P04 for mitochondrial ATP-4 and OH is to move one proton into the mitochondrial matrix for every molecule of ATP that the mitochondrion releases into the cytosol. This proton translocation must be considered, along with the movement of protons through the ATP synthase, to account for the P-to-O ratio of oxidative phosphorylation. If three protons pass through the ATP synthase, and the adenine nucleotide and Pj transport systems move one additional proton, then four protons in total move into the matrix for each ATP molecule provided to the cytosol. 

Transport of two electrons from photosystem II through the cytochrome bhf complex to photosystem I results in the movement of four protons from the chloroplast stroma to the thylakoid lumen. The proton translocation probably occurs in a Q cycle resembling that illustrated in figure 14.11. Two more protons are released in the lumen for each molecule of H20 that is oxidized to 02, and one additional proton is removed from the stroma for each molecule of... 

Electron flow through the cytochrome b6f complex results in proton translocation from the stroma to the thylakoid lumen. In addition, protons are released in the lumen when H20 is oxidized and are taken up from the stromal space when NADP+ is reduced. Protons move from the thylakoid lumen back to the stroma through an ATP-synthase, driving the formation of ATP. 

Fig. 5.3. The major components involved in mitochondrial NADH oxidation in facultative anaerobic mitochondria. In anaerobically functioning mitochondria, NADH is oxidized either by soluble enzymes (left) or by membrane-bound complexes of the electron-transport chain (middle). Under aerobic conditions, a classic respiratory chain is used to oxidize NADH (right). Proton translocation is indicated by H with arrows. Ovals represent the electron transporters RQ, UQ and cytochrome c (cyt. c), and electron transport is indicated by dashed arrows. The vertical bar represents a scale for the standard redox potentials in millivolts. Fum fumarate, NADH-DH NADH dehydrogenase, NADH-ECR soluble NADH enoyl-CoA reductase, RQH2 rhodoquinol, Succ succinate, UQH2 ubiquinol...

The idea that oxidative phosphorylation and photophosphorylation systems are coupled with the transfer of a proton through the membrane was introduced by Mitchell (1966) and is now widely accepted. H+-ATPase (ATP synthase, F,Fo-ATPase) catalyzes ATP synthesis coupled to an electrochemical gradient and ATP hydrolysis driven by proton translocation in mitochondrial or bacterial membranes. (Boyer, 2001 Babcock and Wikstroem, 1992 Abraham et al., 1994 Allison, 1998 Ogilvie et al. 1997 Musser and Theg, 2000 Backgren et al., 2000 Arechada and Jones, 2001 Gibbsons et. al., 2000 and references therein). The enzyme from Escherichia coli consists of two parts, a water-... 

In the binuclear haem-copper centre of cytochrome oxidases there is no cation radical formed at the active site. Instead the extra positive charge is held by the copper atom as it converts from cuprous (Cu1+) to cupric (Cu2+). In fact there is growing evidence to support the model of Mitchell [56] that it is the protonation steps associated with oxidation state changes in this copper atom (Cub) that provide the link between the electron transfer and proton translocation activities of this enzyme. 

Cytochrome oxidases. Mitochondrial cytochrome c oxidase uses the energy involved in the oxidation of cytochrome c and reduction of water to generate a proton electrochemical gradient across the inner mitochondrial membrane [57], As stated above, a ferryl state is an essential intermediate in this process. Similar intermediates are to be expected in all similar proton-translocating cytochrome oxidases that contain a binuclear haem-copper centre,... 

Oxidative phosphorylation occurs in the mitochondria of all animal and plant tissues, and is a coupled process between the oxidation of substrates and production of ATP. As the TCA cycle runs, hydrogen ions (or electrons) are carried by the two carrier molecules NAD or FAD to the electron transport pumps. Energy released by the electron transfer processes pumps the protons to the intermembrane region, where they accumulate in a high enough concentration to phosphorylate the ADP to ATP. The overall process is called oxidative phosphorylation. The cristae have the major coupling factors F, (a hydrophilic protein) and F0 (a hydrophobic lipoprotein complex). F, and F0 together comprise the ATPase (also called ATP synthase) complex activated by Mg2+. F0 forms a proton translocation pathway and Fj... 

The coupling of electron transfer and proton translocation is described by the proton-motive Q-cycle mechanism first proposed by Peter Mitchell [1], In the bc complex, hydroquinone is oxidized at a reaction site which is at the positive side of the membrane,... 

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