Oxidative phosphorylation

22: 9530: 1948:". In this model, the various complexes exist as organized sets of interacting enzymes. These associations might allow channeling of substrates between the various enzyme complexes, increasing the rate and efficiency of electron transfer. Within such mammalian supercomplexes, some components would be present in higher amounts than others, with some data suggesting a ratio between complexes I/II/III/IV and the ATP synthase of approximately 1:1:3:7:4. However, the debate over this supercomplex hypothesis is not completely resolved, as some data do not appear to fit with this model. 484: 10763: 2571:, which can be shifted by altering the proton-motive force. In the absence of a proton-motive force, the ATP synthase reaction will run from right to left, hydrolyzing ATP and pumping protons out of the matrix across the membrane. However, when the proton-motive force is high, the reaction is forced to run in the opposite direction; it proceeds from left to right, allowing protons to flow down their concentration gradient and turning ADP into ATP. Indeed, in the closely related 10793: 10781: 1389: 10873: 10857: 10841: 10829: 10817: 10805: 649:; in other words, they will release a large amount of energy upon oxidation. However, the cell does not release this energy all at once, as this would be an uncontrollable reaction. Instead, the electrons are removed from NADH and passed to oxygen through a series of enzymes that each release a small amount of the energy. This set of enzymes, consisting of complexes I through IV, is called the electron transport chain and is found in the 1675: 1102: 886: 2379:, there are two different types of ubiquinol oxidase using oxygen as an electron acceptor. Under highly aerobic conditions, the cell uses an oxidase with a low affinity for oxygen that can transport two protons per electron. However, if levels of oxygen fall, they switch to an oxidase that transfers only one proton per electron, but has a high affinity for oxygen. 2330:

succinate/fumarate pair is unusual, as its midpoint potential is close to zero. Succinate can therefore be oxidized to fumarate if a strong oxidizing agent such as oxygen is available, or fumarate can be reduced to succinate using a strong reducing agent such as formate. These alternative reactions are catalyzed by

2983:

level can alter ATP production rates. Under anoxic conditions, ATP-synthase will commit 'cellular treason' and run in reverse, forcing protons from the matrix back into the inner membrane space, using up ATP in the process. The proton motive force and ATP production can be maintained by intracellular

2594:

portion contains six proteins of two different kinds (three α subunits and three β subunits), whereas the "stalk" consists of one protein: the γ subunit, with the tip of the stalk extending into the ball of α and β subunits. Both the α and β subunits bind nucleotides, but only the β subunits catalyze

1970:

possess a large variety of electron-transfer enzymes. These use an equally wide set of chemicals as substrates. In common with eukaryotes, prokaryotic electron transport uses the energy released from the oxidation of a substrate to pump ions across a membrane and generate an electrochemical gradient.

1655:

As coenzyme Q is reduced to ubiquinol on the inner side of the membrane and oxidized to ubiquinone on the other, a net transfer of protons across the membrane occurs, adding to the proton gradient. The rather complex two-step mechanism by which this occurs is important, as it increases the efficiency

503:

The electron transport chain carries both protons and electrons, passing electrons from donors to acceptors, and transporting protons across a membrane. These processes use both soluble and protein-bound transfer molecules. In mitochondria, electrons are transferred within the intermembrane space by

413:

ATP synthase releases this stored energy by completing the circuit and allowing protons to flow down the electrochemical gradient, back to the N-side of the membrane. The electrochemical gradient drives the rotation of part of the enzyme's structure and couples this motion to the synthesis of ATP.

1084:

in complex I that cause the protein to bind protons on the N-side of the membrane and release them on the P-side of the membrane. Finally, the electrons are transferred from the chain of iron–sulfur clusters to a ubiquinone molecule in the membrane. Reduction of ubiquinone also contributes to the

3009:

inhibits ATP synthase, protons cannot pass back into the mitochondrion. As a result, the proton pumps are unable to operate, as the gradient becomes too strong for them to overcome. NADH is then no longer oxidized and the citric acid cycle ceases to operate because the concentration of NAD falls

2931:

in complex III, as a highly reactive ubisemiquinone free radical is formed as an intermediate in the Q cycle. This unstable species can lead to electron "leakage" when electrons transfer directly to oxygen, forming superoxide. As the production of reactive oxygen species by these proton-pumping

2641:

and involves the active site of a β subunit cycling between three states. In the "open" state, ADP and phosphate enter the active site (shown in brown in the diagram). The protein then closes up around the molecules and binds them loosely – the "loose" state (shown in red). The enzyme then

680:

across the membrane. The energy stored in this potential is then used by ATP synthase to produce ATP. Oxidative phosphorylation in the eukaryotic mitochondrion is the best-understood example of this process. The mitochondrion is present in almost all eukaryotes, with the exception of anaerobic

2363:

Prokaryotes control their use of these electron donors and acceptors by varying which enzymes are produced, in response to environmental conditions. This flexibility is possible because different oxidases and reductases use the same ubiquinone pool. This allows many combinations of enzymes to

2329:

can grow with reducing agents such as formate, hydrogen, or lactate as electron donors, and nitrate, DMSO, or oxygen as acceptors. The larger the difference in midpoint potential between an oxidizing and reducing agent, the more energy is released when they react. Out of these compounds, the

1898:

have alternative NADH oxidases, which oxidize NADH in the cytosol rather than in the mitochondrial matrix, and pass these electrons to the ubiquinone pool. These enzymes do not transport protons, and, therefore, reduce ubiquinone without altering the electrochemical gradient across the inner

596:. Metal ion cofactors undergo redox reactions without binding or releasing protons, so in the electron transport chain they serve solely to transport electrons through proteins. Electrons move quite long distances through proteins by hopping along chains of these cofactors. This occurs by 2397:, is the final enzyme in the oxidative phosphorylation pathway. This enzyme is found in all forms of life and functions in the same way in both prokaryotes and eukaryotes. The enzyme uses the energy stored in a proton gradient across a membrane to drive the synthesis of ATP from ADP and 2926:

The cytochrome c oxidase complex is highly efficient at reducing oxygen to water, and it releases very few partly reduced intermediates; however small amounts of superoxide anion and peroxide are produced by the electron transport chain. Particularly important is the reduction of

3013:

Many site-specific inhibitors of the electron transport chain have contributed to the present knowledge of mitochondrial respiration. Synthesis of ATP is also dependent on the electron transport chain, so all site-specific inhibitors also inhibit ATP formation. The fish poison

387:

process, which requires an input of energy. Both the electron transport chain and the ATP synthase are embedded in a membrane, and energy is transferred from the electron transport chain to the ATP synthase by movements of protons across this membrane, in a process called

2626: 3280: 2932:

complexes is greatest at high membrane potentials, it has been proposed that mitochondria regulate their activity to maintain the membrane potential within a narrow range that balances ATP production against oxidant generation. For instance, oxidants can activate

2881: 1712:

oxygen is reduced to water in this step. Both the direct pumping of protons and the consumption of matrix protons in the reduction of oxygen contribute to the proton gradient. The reaction catalyzed is the oxidation of cytochrome c and the reduction of oxygen:

1943:

The original model for how the respiratory chain complexes are organized was that they diffuse freely and independently in the mitochondrial membrane. However, recent data suggest that the complexes might form higher-order structures called supercomplexes or

1980:

The main difference between eukaryotic and prokaryotic oxidative phosphorylation is that bacteria and archaea use many different substances to donate or accept electrons. This allows prokaryotes to grow under a wide variety of environmental conditions. In

2740:

because it is a strong oxidizing agent. The reduction of oxygen does involve potentially harmful intermediates. Although the transfer of four electrons and four protons reduces oxygen to water, which is harmless, transfer of one or two electrons produces

916:(kDa). The structure is known in detail only from a bacterium; in most organisms the complex resembles a boot with a large "ball" poking out from the membrane into the mitochondrion. The genes that encode the individual proteins are contained in both the 1595: 1871: 1124:, is a second entry point to the electron transport chain. It is unusual because it is the only enzyme that is part of both the citric acid cycle and the electron transport chain. Complex II consists of four protein subunits and contains a bound 2717:

So we can conclude that when NADH is oxidized, about 42% of energy is conserved in the form of three ATPs and the remaining (58%) energy is lost as heat (unless the chemical energy of ATP under physiological conditions was underestimated).

3288:

For another twenty years, the mechanism by which ATP is generated remained mysterious, with scientists searching for an elusive "high-energy intermediate" that would link oxidation and phosphorylation reactions. This puzzle was solved by

1693:, is the final protein complex in the electron transport chain. The mammalian enzyme has an extremely complicated structure and contains 13 subunits, two heme groups, as well as multiple metal ion cofactors – in all, three atoms of 2545: 1224:, an enzyme similar to complex II, fumarate reductase (menaquinol:fumarate oxidoreductase, or QFR), operates in reverse to oxidize ubiquinol and reduce fumarate. This allows the worm to survive in the anaerobic environment of the 1047: 7461:

Tsubaki M (January 1993). "Fourier-transform infrared study of cyanide binding to the Fea3-CuB binuclear site of bovine heart cytochrome c oxidase: implication of the redox-linked conformational change at the binuclear site".

3029:

Carbon monoxide, cyanide, hydrogen sulphide and azide effectively inhibit cytochrome oxidase. Carbon monoxide reacts with the reduced form of the cytochrome while cyanide and azide react with the oxidised form. An antibiotic,

3004:

that inhibit oxidative phosphorylation. Although any one of these toxins inhibits only one enzyme in the electron transport chain, inhibition of any step in this process will halt the rest of the process. For example, if

2351:

oxidize nitrite to nitrate, donating the electrons to oxygen. The small amount of energy released in this reaction is enough to pump protons and generate ATP, but not enough to produce NADH or NADPH directly for use in

6777:"Catalytic site cooperativity of beef heart mitochondrial F1 adenosine triphosphatase. Correlations of initial velocity, bound intermediate, and oxygen exchange measurements with an alternating three-site model" 2622:. This movement of the tip of the γ subunit within the ball of α and β subunits provides the energy for the active sites in the β subunits to undergo a cycle of movements that produces and then releases ATP. 1342: 1197: 2663:

synthase, a form of the enzyme that contains additional proteins with little similarity in sequence to other bacterial and eukaryotic ATP synthase subunits. It is possible that, in some species, the A

394:. A current of protons is driven from the negative N-side of the membrane to the positive P-side through the proton-pumping enzymes of the electron transport chain. The movement of protons creates an 1931:, and infection by pathogens, as well as other factors that inhibit the full electron transport chain. Alternative pathways might, therefore, enhance an organism's resistance to injury, by reducing 1140:

and reduces ubiquinone. As this reaction releases less energy than the oxidation of NADH, complex II does not transport protons across the membrane and does not contribute to the proton gradient.

7390:

Joshi S, Huang YG (August 1991). "ATP synthase complex from bovine heart mitochondria: the oligomycin sensitivity conferring protein is essential for dicyclohexyl carbodiimide-sensitive ATPase".

1077:. The electrons are then transferred through a series of iron–sulfur clusters: the second kind of prosthetic group present in the complex. There are both and iron–sulfur clusters in complex I. 6355:

Van Walraven HS, Strotmann H, Schwarz O, Rumberg B (February 1996). "The H+/ATP coupling ratio of the ATP synthase from thiol-modulated chloroplasts and two cyanobacterial strains is four".

453:

produces only 2 ATP molecules, but somewhere between 30 and 36 ATPs are produced by the oxidative phosphorylation of the 10 NADH and 2 succinate molecules made by converting one molecule of

2757: 4414:

Yankovskaya V, Horsefield R, Törnroth S, Luna-Chavez C, Miyoshi H, Léger C, et al. (January 2003). "Architecture of succinate dehydrogenase and reactive oxygen species generation".

1065:

The start of the reaction, and indeed of the entire electron chain, is the binding of a NADH molecule to complex I and the donation of two electrons. The electrons enter complex I via a

7587:

Dervartanian DV, Veeger C (November 1964). "STUDIES ON SUCCINATE DEHYDROGENASE. I. SPECTRAL PROPERTIES OF THE PURIFIED ENZYME AND FORMATION OF ENZYME-COMPETITIVE INHIBITOR COMPLEXES".

1927:

yields than the full pathway. The advantages produced by a shortened pathway are not entirely clear. However, the alternative oxidase is produced in response to stresses such as cold,

1708:

This enzyme mediates the final reaction in the electron transport chain and transfers electrons to oxygen and hydrogen (protons), while pumping protons across the membrane. The final

5125:

Tsukihara T, Aoyama H, Yamashita E, Tomizaki T, Yamaguchi H, Shinzawa-Itoh K, et al. (May 1996). "The whole structure of the 13-subunit oxidized cytochrome c oxidase at 2.8 A".

1617:

donor to a cytochrome c acceptor at a time, the reaction mechanism of complex III is more elaborate than those of the other respiratory complexes, and occurs in two steps called the

1443:

group. The iron atoms inside complex III's heme groups alternate between a reduced ferrous (+2) and oxidized ferric (+3) state as the electrons are transferred through the protein.

2642:

changes shape again and forces these molecules together, with the active site in the resulting "tight" state (shown in pink) binding the newly produced ATP molecule with very high

584:

and cytochromes. There are several types of iron–sulfur cluster. The simplest kind found in the electron transfer chain consists of two iron atoms joined by two atoms of inorganic

2405:). Estimates of the number of protons required to synthesize one ATP have ranged from three to four, with some suggesting cells can vary this ratio, to suit different conditions. 1462: 588:; these are called clusters. The second kind, called , contains a cube of four iron atoms and four sulfur atoms. Each iron atom in these clusters is coordinated by an additional 9885: 569:

oxidized on the other, ubiquinone will couple these reactions and shuttle protons across the membrane. Some bacterial electron transport chains use different quinones, such as

367:. This means one cannot occur without the other. The chain of redox reactions driving the flow of electrons through the electron transport chain, from electron donors such as 1080:

As the electrons pass through this complex, four protons are pumped from the matrix into the intermembrane space. Exactly how this occurs is unclear, but it seems to involve

8442: 8430: 5453:

Ito Y, Saisho D, Nakazono M, Tsutsumi N, Hirai A (December 1997). "Transcript levels of tandem-arranged alternative oxidase genes in rice are increased by low temperature".

1989:

of a chemical measures how much energy is released when it is oxidized or reduced, with reducing agents having negative potentials and oxidizing agents positive potentials.

8454: 2988:

can freely diffuse across the mitochondrial outer-membrane and acidify the inter-membrane space, hence directly contributing to the proton motive force and ATP production.

1721: 8516: 893:. The abbreviations are discussed in the text. In all diagrams of respiratory complexes in this article, the matrix is at the bottom, with the intermembrane space above. 4702:"Separation and properties of five distinct acyl-CoA dehydrogenases from rat liver mitochondria. Identification of a new 2-methyl branched chain acyl-CoA dehydrogenase" 257:. In eukaryotes, five main protein complexes are involved, whereas in prokaryotes many different enzymes are present, using a variety of electron donors and acceptors. 286:

across this membrane. This store of energy is tapped when protons flow back across the membrane and down the potential energy gradient, through a large enzyme called

9312: 8093:"Partial resolution of the enzymes catalyzing oxidative phosphorylation. I. Purification and properties of soluble dinitrophenol-stimulated adenosine triphosphatase" 4939:

Iwata S, Lee JW, Okada K, Lee JK, Iwata M, Rasmussen B, et al. (July 1998). "Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex".

2649:

In some bacteria and archaea, ATP synthesis is driven by the movement of sodium ions through the cell membrane, rather than the movement of protons. Archaea such as

1132:

group that does not participate in electron transfer to coenzyme Q, but is believed to be important in decreasing production of reactive oxygen species. It oxidizes

6915:

Müller V (February 2004). "An exceptional variability in the motor of archael A1A0 ATPases: from multimeric to monomeric rotors comprising 6-13 ion binding sites".

230:, which is used throughout the cell whenever energy is needed. During oxidative phosphorylation, electrons are transferred from the electron donors to a series of 3313:, by his development in 1973 of the "binding change" mechanism, followed by his radical proposal of rotational catalysis in 1982. More recent work has included 8067: 6868:"Delta mu Na+ drives the synthesis of ATP via an delta mu Na(+)-translocating F1F0-ATP synthase in membrane vesicles of the archaeon Methanosarcina mazei Gö1" 2413: 7226:

Echtay KS, Roussel D, St-Pierre J, Jekabsons MB, Cadenas S, Stuart JA, et al. (January 2002). "Superoxide activates mitochondrial uncoupling proteins".

7191:

Kadenbach B, Ramzan R, Wen L, Vogt S (March 2010). "New extension of the Mitchell Theory for oxidative phosphorylation in mitochondria of living organisms".

4268:

Baranova EA, Holt PJ, Sazanov LA (February 2007). "Projection structure of the membrane domain of Escherichia coli respiratory complex I at 8 A resolution".

2578:

ATP synthase is a massive protein complex with a mushroom-like shape. The mammalian enzyme complex contains 16 subunits and has a mass of approximately 600

7070:

Valko M, Leibfritz D, Moncol J, Cronin MT, Mazur M, Telser J (2007). "Free radicals and antioxidants in normal physiological functions and human disease".

3275: 4000:

Boxma B, de Graaf RM, van der Staay GW, van Alen TA, Ricard G, Gabaldón T, et al. (March 2005). "An anaerobic mitochondrion that produces hydrogen".

951: 1228:, carrying out anaerobic oxidative phosphorylation with fumarate as the electron acceptor. Another unconventional function of complex II is seen in the 3358:

in the 1930s but was ultimately discontinued due to its dangerous side effects. However, illicit use of the drug for this purpose continues today. See

1985:, for example, oxidative phosphorylation can be driven by a large number of pairs of reducing agents and oxidizing agents, which are listed below. The 1660:

were used to directly reduce two molecules of cytochrome c, the efficiency would be halved, with only one proton transferred per cytochrome c reduced.

4541:

Painter HJ, Morrisey JM, Mather MW, Vaidya AB (March 2007). "Specific role of mitochondrial electron transport in blood-stage Plasmodium falciparum".

465:

yields about 14 ATPs. These ATP yields are theoretical maximum values; in practice, some protons leak across the membrane, lowering the yield of ATP.

3258:

were known to be involved. However, in the early 1940s, the link between the oxidation of sugars and the generation of ATP was firmly established by

1644:

is bound and again passes its first electron to a cytochrome c acceptor. The second electron is passed to the bound ubisemiquinone, reducing it to QH

1454:, a heme protein loosely associated with the mitochondrion. Unlike coenzyme Q, which carries two electrons, cytochrome c carries only one electron. 8855: 5790:"Relationship between lateral diffusion, collision frequency, and electron transfer of mitochondrial inner membrane oxidation-reduction components" 1250: 1380:

dehydrogenases. In plants, ETF-Q oxidoreductase is also important in the metabolic responses that allow survival in extended periods of darkness.

8509: 3914:

Page CC, Moser CC, Chen X, Dutton PL (November 1999). "Natural engineering principles of electron tunnelling in biological oxidation-reduction".

2708:

When one NADH is oxidized through the electron transfer chain, three ATPs are produced, which is equivalent to 7.3 kcal/mol x 3 = 21.9 kcal/mol.

2618:(the γ subunit stalk) within the α and β subunits. The α and β subunits are prevented from rotating themselves by the side-arm, which acts as a 8403: 8199: 1265:

and a cluster, but, unlike the other respiratory complexes, it attaches to the surface of the membrane and does not cross the lipid bilayer.

1238:. Here, the reversed action of complex II as an oxidase is important in regenerating ubiquinol, which the parasite uses in an unusual form of 8789: 8381: 9028: 5745:"The ratio of oxidative phosphorylation complexes I-V in bovine heart mitochondria and the composition of respiratory chain supercomplexes" 3390: 1894:

Many eukaryotic organisms have electron transport chains that differ from the much-studied mammalian enzymes described above. For example,

4887: 7338:"Acidosis Maintains the Function of Brain Mitochondria in Hypoxia-Tolerant Triplefin Fish: A Strategy to Survive Acute Hypoxic Exposure?" 3220:

can uncouple respiration from ATP synthesis. This rapid respiration produces heat, and is particularly important as a way of maintaining

7540:"Inhibitors of the quinone-binding site allow rapid superoxide production from mitochondrial NADH:ubiquinone oxidoreductase (complex I)" 2701:

The potential difference between these two redox pairs is 1.14 volt, which is equivalent to -52 kcal/mol or -2600 kJ per 6 mol of O

1640:. The first two substrates are released, but this ubisemiquinone intermediate remains bound. In the second step, a second molecule of QH 8502: 3301:

in 1978. Subsequent research concentrated on purifying and characterizing the enzymes involved, with major contributions being made by

2614:

interactions that propel the ring of c subunits past the proton channel. This rotating ring in turn drives the rotation of the central

4984:"The protonmotive Q cycle. Energy transduction by coupling of proton translocation to electron transfer by the cytochrome bc1 complex" 2646:. Finally, the active site cycles back to the open state, releasing ATP and binding more ADP and phosphate, ready for the next cycle. 6451:"Effects of carbon source on expression of F0 genes and on the stoichiometry of the c subunit in the F1F0 ATPase of Escherichia coli" 3116: 10777:

Single lines: pathways common to most lifeforms. Double lines: pathways not in humans (occurs in e.g. plants, fungi, prokaryotes).

8824: 8819: 6997: 6004:"Alternative respiratory pathways of Escherichia coli: energetics and transcriptional regulation in response to electron acceptors" 5383:

Moore AL, Siedow JN (August 1991). "The regulation and nature of the cyanide-resistant alternative oxidase of plant mitochondria".

3862: 429:

operate mainly on ΔpH. However, they also require a small membrane potential for the kinetics of ATP synthesis. In the case of the

8138:"A new concept for energy coupling in oxidative phosphorylation based on a molecular explanation of the oxygen exchange reactions" 4643:"Structure of electron transfer flavoprotein-ubiquinone oxidoreductase and electron transfer to the mitochondrial ubiquinone pool" 2360:

to produce enough proton-motive force to run part of the electron transport chain in reverse, causing complex I to generate NADH.

2010: 1986: 708: 9500: 8760: 5344:"Alternative oxidase in the branched mitochondrial respiratory network: an overview on structure, function, regulation, and role" 3195: 1273: 1148: 1625:, which is then oxidized, with one electron being passed to the second substrate, cytochrome c. The two protons released from QH 8482: 8439: 8427: 7981:

Mitchell P (July 1961). "Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism".

7667:

Borecký J, Vercesi AE (2005). "Plant uncoupling mitochondrial protein and alternative oxidase: energy metabolism and stress".

5774: 8487: 8477: 8451: 7514: 5016: 2599:

portion and back into the membrane is a long rod-like subunit that anchors the α and β subunits into the base of the enzyme.

25:

Oxidative phosphorylation is made up of two closely connected components: the electron transport chain and chemiosmosis. The

8472: 7825:"Esterification of inorganic phosphate coupled to electron transport between dihydrodiphosphopyridine nucleotide and oxygen" 6527: 4779: 3504: 8892: 6300: 3879:

Johnson DC, Dean DR, Smith AD, Johnson MK (2005). "Structure, function, and formation of biological iron-sulfur clusters".

4798:"The critical role of Arabidopsis electron-transfer flavoprotein:ubiquinone oxidoreductase during dark-induced starvation" 10906: 6806: 3270:

proved that the coenzyme NADH linked metabolic pathways such as the citric acid cycle and the synthesis of ATP. The term

1400: 1393: 765: 650: 8122: 8642: 8561: 8395: 4731: 3038:, an antidote used against chemical weapons, are the two important inhibitors of the site between cytochrome B and C1. 2876:{\displaystyle {\ce {O2->{\underset {Superoxide}{O2^{\underline {\bullet }}}}->{\underset {Peroxide}{O2^{2-}}}}}} 2168: 2078: 1258: 7854: 4502:"Role of complex II in anaerobic respiration of the parasite mitochondria from Ascaris suum and Plasmodium falciparum" 8365: 8346: 8327: 8308: 8282: 8260: 8241: 7807: 2068: 2064: 928: 728: 724: 547:, so it diffuses freely within the membrane. When Q accepts two electrons and two protons, it becomes reduced to the 5303:"Branched mitochondrial electron transport in the Animalia: presence of alternative oxidase in several animal phyla" 3965:

Leys D, Scrutton NS (December 2004). "Electrical circuitry in biology: emerging principles from protein structure".

3605:"Mitochondrial proton conductance and H+/O ratio are independent of electron transport rate in isolated hepatocytes" 8056: 5090:

Calhoun MW, Thomas JW, Gennis RB (August 1994). "The cytochrome oxidase superfamily of redox-driven proton pumps".

2331: 2282: 2187: 1113: 1106: 192:

carry out oxidative phosphorylation. This pathway is so pervasive because it releases more energy than alternative

9827: 9023: 3297:

in 1961. At first, this proposal was highly controversial, but it was slowly accepted and Mitchell was awarded a

1962:

In contrast to the general similarity in structure and function of the electron transport chains in eukaryotes,

8799: 8629: 8617: 8566: 7437: 6260:"Energetic efficiency of Escherichia coli: effects of mutations in components of the aerobic respiratory chain" 5254:"Purification and characterization of a 43-kDa rotenone-insensitive NADH dehydrogenase from plant mitochondria" 4747:"A new iron-sulfur flavoprotein of the respiratory chain. A component of the fatty acid beta oxidation pathway" 2097: 9100: 6688:

Capaldi RA, Aggeler R (March 2002). "Mechanism of the F(1)F(0)-type ATP synthase, a biological rotary motor".

1590:{\displaystyle {\ce {QH2{}+ 2 Cyt\, c_{ox}{}+ 2H+_{matrix}-> Q{}+ 2 Cyt\, c_{red}{}+ 4H+_{intermembrane}}}} 10901: 9426: 8638: 2586:

and contains a ring of c subunits and the proton channel. The stalk and the ball-shaped headpiece is called F

6406:

Yoshida M, Muneyuki E, Hisabori T (September 2001). "ATP synthase--a marvellous rotary engine of the cell".

2575:, the hydrolysis reaction is used to acidify cellular compartments, by pumping protons and hydrolysing ATP. 425:

bacteria the electrical energy even has to compensate for a counteracting inverse pH difference. Inversely,

363:-releasing chemical reactions to drive energy-requiring reactions. The two sets of reactions are said to be 10533: 9529: 8625: 8466: 5348:

Brazilian Journal of Medical and Biological Research = Revista Brasileira de Pesquisas Medicas e Biologicas

4594:"Reactions of electron-transfer flavoprotein and electron-transfer flavoprotein: ubiquinone oxidoreductase" 3241: 2920: 646: 265: 9673: 9662: 7870:"50 years of biological research--from oxidative phosphorylation to energy requiring transport regulation" 6119:"Genome sequence of the chemolithoautotrophic nitrite-oxidizing bacterium Nitrobacter winogradskyi Nb-255" 2364:

function together, linked by the common ubiquinol intermediate. These respiratory chains therefore have a

1629:

pass into the intermembrane space. The third substrate is Q, which accepts the second electron from the QH

565:(Q) form. As a result, if two enzymes are arranged so that Q is reduced on one side of the membrane and QH 9493: 7497:

Heytler PG (1979). "Uncouplers of oxidative phosphorylation". In Sidney Fleischer, Lester Packer (eds.).

4078:

Hirst J (June 2005). "Energy transduction by respiratory complex I--an evaluation of current knowledge".

3251: 3156:

Prevents the transfer of electrons from complex I to ubiquinone by blocking the ubiquinone-binding site.

2671:

form of the enzyme is a specialized sodium-driven ATP synthase, but this might not be true in all cases.

1125: 747: 445:

The amount of energy released by oxidative phosphorylation is high, compared with the amount produced by

193: 6215:

Iuchi S, Lin EC (July 1993). "Adaptation of Escherichia coli to redox environments by gene expression".

253:, these proteins are located in the cell's outer membrane. These linked sets of proteins are called the 10707: 10582: 10109: 8933: 4065:

Medical CHEMISTRY Compendium. By Anders Overgaard Pedersen and Henning Nielsen. Aarhus University. 2008

3834:"Microbial ubiquinones: multiple roles in respiration, gene regulation and oxidative stress management" 3309:

on the ATP synthase. A critical step towards solving the mechanism of the ATP synthase was provided by

2341:

Some prokaryotes use redox pairs that have only a small difference in midpoint potential. For example,

2139: 2083: 1866:{\displaystyle {\ce {4Cyt\,c_{red}{}+O2{}+8H+_{matrix}->4Cyt\,c_{ox}{}+2H2O{}+4H+_{intermembrane}}}} 260:

The energy transferred by electrons flowing through this electron transport chain is used to transport

6117:

Starkenburg SR, Chain PS, Sayavedra-Soto LA, Hauser L, Land ML, Larimer FW, et al. (March 2006).

5217:

Rasmusson AG, Soole KL, Elthon TE (2004). "Alternative NAD(P)H dehydrogenases of plant mitochondria".

3228:

animals, although these proteins may also have a more general function in cells' responses to stress.

3138:

proton pumping from ATP synthesis because it carries protons across the inner mitochondrial membrane.

1923:

The electron transport pathways produced by these alternative NADH and ubiquinone oxidases have lower

1257:, is a third entry point to the electron transport chain. It is an enzyme that accepts electrons from 21: 10612: 10293: 9842: 9565: 9297: 9219: 337:. The enzymes carrying out this metabolic pathway are also the target of many drugs and poisons that 7017: 4866: 3789:

Mitchell P (December 1979). "Keilin's respiratory chain concept and its chemiosmotic consequences".

520:

group in its structure. Cytochrome c is also found in some bacteria, where it is located within the

10270: 9980: 9767: 9317: 9038: 8979: 8885: 8721: 8681: 8556: 1261:

in the mitochondrial matrix, and uses these electrons to reduce ubiquinone. This enzyme contains a

677: 657:

is also oxidized by the electron transport chain, but feeds into the pathway at a different point.

613: 395: 254: 26: 5176:

Yoshikawa S, Muramoto K, Shinzawa-Itoh K, Aoyama H, Tsukihara T, Shimokata K, et al. (2006).

10861: 10849: 10785: 10507: 10252: 9411: 9160: 9155: 9141: 9116: 8746: 8662: 8573: 3359: 3355: 3237: 2900: 1928: 1220: 1085:

generation of a proton gradient, as two protons are taken up from the matrix as it is reduced to

738: 639: 581: 478: 434: 310: 3382: 1920:, and possibly some animals. This enzyme transfers electrons directly from ubiquinol to oxygen. 10735: 9952: 9750: 9486: 9446: 9380: 9266: 9261: 8966: 8705: 8621: 8195: 7140:

Finkel T, Holbrook NJ (November 2000). "Oxidants, oxidative stress and the biology of ageing".

7012: 5847:

Nealson KH (January 1999). "Post-Viking microbiology: new approaches, new data, new insights".

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Raha S, Robinson BH (October 2000). "Mitochondria, oxygen free radicals, disease and ageing".

1423:, with each subunit complex containing 11 protein subunits, an iron–sulfur cluster and three 512:. This carries only electrons, and these are transferred by the reduction and oxidation of an 10833: 10363: 10137: 9999: 9451: 9416: 9372: 9136: 8995: 8599: 5230: 4917: 4392: 4217:

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Mitchell P, Moyle J (January 1967). "Chemiosmotic hypothesis of oxidative phosphorylation".

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Horsefield R, Iwata S, Byrne B (April 2004). "Complex II from a structural perspective".

3530: 3294: 3267: 2540:{\displaystyle {\ce {ADP + P_i + 4H+_{intermembrane}<=> ATP + H2O + 4H+_{matrix}}}} 1904: 673: 421:

equivalent: In mitochondria, the largest part of energy is provided by the potential; in

399: 322: 8494: 8153: 7994: 7943: 7886: 7869: 7294: 7239: 7153: 7046: 6725:"Catalytic and mechanical cycles in F-ATP synthases. Fourth in the Cycles Review Series" 6596:

Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences

6368: 6134: 5860: 5805: 5564: 5505: 5490:"The alternative oxidase lowers mitochondrial reactive oxygen production in plant cells" 5138: 5050: 4952: 4658: 4554: 4427: 4230: 4177: 4162:"Structure of the hydrophilic domain of respiratory complex I from Thermus thermophilus" 4013: 3927: 3833: 3802: 3706: 3434: 3411:

Voet, D.; Voet, J. G. (2004). "Biochemistry", 3rd ed., p. 804, Wiley.ISBN 0-471-19350-X.

10865: 10775:. Click any text (name of pathway or metabolites) to link to the corresponding article. 10169: 10130: 10076: 9939: 9458: 9131: 9076: 9063: 8741: 8230: 8014: 7963: 7799: 7768: 7692: 7644: 7623: 7569: 7364: 7337: 7259: 7173: 7038: 6940: 6845: 6749: 6724: 6670: 6616: 6591: 6431: 6388: 6240: 6228: 6192: 6175: 6151: 6118: 6102: 6085: 5880: 5635: 5586: 5418:

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that disrupt the proton gradient by carrying protons across a membrane. This ionophore

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is understood in most detail, while archaeal systems are at present poorly understood.

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across the inner membrane of the mitochondrion. This causes protons to build up in the

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Crofts AR (2004). "The cytochrome bc1 complex: function in the context of structure".

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Gupte S, Wu ES, Hoechli L, Hoechli M, Jacobson K, Sowers AE, et al. (May 1984).

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Hirst J (December 2009). "Towards the molecular mechanism of respiratory complex I".

4326: 4285: 4242: 4191: 4139: 4095: 4025: 3982: 3939: 3896: 3854: 3814: 3763: 3720: 3715: 3690: 3671: 3666: 3649: 3630: 3585: 3550: 3496: 3446: 3290: 3120: 2737: 2633:. ATP is shown in red, ADP and phosphate in pink and the rotating γ subunit in black. 2244: 2225: 2206: 1709: 1439:. A cytochrome is a kind of electron-transferring protein that contains at least one 1042:{\displaystyle {\ce {NADH + Q + 5H+_{matrix}-> NAD+ + QH2 + 4H+_{intermembrane}}}} 631: 521: 372: 318: 208: 158: 34: 8870: 7930:

Slater EC (November 1953). "Mechanism of phosphorylation in the respiratory chain".

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Ishizaki K, Larson TR, Schauer N, Fernie AR, Graham IA, Leaver CJ (September 2005).

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Cecchini G (2003). "Function and structure of complex II of the respiratory chain".

3775: 3546: 3477:"Crucial role of the membrane potential for ATP synthesis by F(1)F(o) ATP synthases" 2602:

As protons cross the membrane through the channel in the base of ATP synthase, the F

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with the mammalian complex I having 46 subunits and a molecular mass of about 1,000

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Lipmann F (1941). "Metabolic generation and utilization of phosphate bond energy".

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Biochimica et Biophysica Acta (BBA) - Specialized Section on Enzymological Subjects

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Rich PR (December 2003). "The molecular machinery of Keilin's respiratory chain".

3262:, confirming the central role of ATP in energy transfer that had been proposed by 2679:

The transport of electrons from redox pair NAD/ NADH to the final redox pair 1/2 O

33:

is the site of oxidative phosphorylation. The NADH and succinate generated in the

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Inhibit the electron transport chain by binding more strongly than oxygen to the

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Kalckar HM (November 1974). "Origins of the concept oxidative phosphorylation".

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Devaux JB, Hedges CP, Birch N, Herbert N, Renshaw GM, Hickey AJ (January 2019).

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Proceedings of the National Academy of Sciences of the United States of America

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Proceedings of the National Academy of Sciences of the United States of America

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Proceedings of the National Academy of Sciences of the United States of America

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Proceedings of the National Academy of Sciences of the United States of America

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Proceedings of the National Academy of Sciences of the United States of America

3531:"Structures and proton-pumping strategies of mitochondrial respiratory enzymes" 3318: 3302: 3259: 2365: 2295: 2263: 2106: 1361: 1216: 927:

The reaction that is catalyzed by this enzyme is the two electron oxidation of

635: 458: 430: 235: 212: 162: 30: 8408: 7680: 7026: 6776: 5868: 5670: 5655:"Supercomplexes in the respiratory chains of yeast and mammalian mitochondria" 4281: 3978: 664:, the enzymes in this electron transport system use the energy released from O 402:. It has two components: a difference in proton concentration (a H gradient, Δ 309:

Although oxidative phosphorylation is a vital part of metabolism, it produces

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and is the site of ATP synthesis. The ball-shaped complex at the end of the F

2371:

In addition to this metabolic diversity, prokaryotes also possess a range of

2342: 2314: 2196: 2173: 1420: 1133: 1073:(FMN). The addition of electrons to FMN converts it to its reduced form, FMNH 908:, is the first protein in the electron transport chain. Complex I is a giant 688: 654: 418: 354: 8392: 8162: 8057:"David Keilin's Respiratory Chain Concept and Its Chemiosmotic Consequences" 8041: 7447: 5514: 4701: 4667: 4435: 4186: 4161: 687:

that instead reduce protons to hydrogen in a remnant mitochondrion called a

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acidosis. Cytosolic protons that have accumulated with ATP hydrolysis and

2711:

The conservation of the energy can be calculated by the following formula

1446:

The reaction catalyzed by complex III is the oxidation of one molecule of

10772: 10638: 10575: 10526: 10220: 10204: 10190: 10088: 10083: 10012: 9900: 9870: 9795: 9362: 9224: 7524: 6592:"Structural model of F1-ATPase and the implications for rotary catalysis" 6515: 6498:

Nelson N, Perzov N, Cohen A, Hagai K, Padler V, Nelson H (January 2000).

4775: 3818: 3526: 3492: 3298: 3225: 3181: 3035: 3031: 3019: 2970: 2964: 2940: 2731: 2347: 2129: 2102: 2040: 2026: 1945: 1634: 1436: 1396:. After each step, Q (in the upper part of the figure) leaves the enzyme. 426: 422: 7475: 6543:"Structure of the mitochondrial ATP synthase by electron cryomicroscopy" 4562: 4413: 4238: 4021: 3246:

The field of oxidative phosphorylation began with the report in 1906 by

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proton-driven motor rotates. Rotation might be caused by changes in the

561:

releases two electrons and two protons, it becomes oxidized back to the

306:

of a part of the enzyme. The ATP synthase is a rotary mechanical motor.

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radical, are very harmful to cells, as they oxidize proteins and cause

2742: 2607: 2579: 2155: 1428: 1424: 1388: 1377: 1369: 1365: 1239: 627: 589: 544: 492: 488: 462: 450: 384: 334: 314: 250: 181: 7624:"The uncoupling protein homologues: UCP1, UCP2, UCP3, StUCP and AtUCP" 6258:

Calhoun MW, Oden KL, Gennis RB, de Mattos MJ, Neijssel OM (May 1993).

6045:"Succinate dehydrogenase and fumarate reductase from Escherichia coli" 4609: 3616: 383:

process – it releases energy, whereas the synthesis of ATP is an

302:

reaction. The reaction is driven by the proton flow, which forces the

10645: 10630: 10621: 10499: 10489: 10477: 10464: 10450: 10439: 10435: 10404: 10399: 10392: 10351: 9913: 9358: 9165: 8909: 8551: 8270: 8002: 7951: 7161: 6500:"The cellular biology of proton-motive force generation by V-ATPases" 6419: 4116:

Lenaz G, Fato R, Genova ML, Bergamini C, Bianchi C, Biondi A (2006).

3442: 3203: 3199: 3186: 3148: 3135: 3131: 2952: 2948: 2398: 2353: 1698: 1447: 1086: 661: 623: 570: 380: 242: 7247: 6116: 5175: 5035:"Protonmotive pathways and mechanisms in the cytochrome bc1 complex" 3089:

center in cytochrome c oxidase, preventing the reduction of oxygen.

2967:, which detoxify the reactive species, limiting damage to the cell. 2939:

To counteract these reactive oxygen species, cells contain numerous

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ending in oxygen, whose reaction releases half of the total energy.

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Davies KJ (1995). "Oxidative stress: the paradox of aerobic life".

6354: 6320:

Boyer PD (1997). "The ATP synthase--a splendid molecular machine".

3746:

Crane FL (December 2001). "Biochemical functions of coenzyme Q10".

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Inhibits ATP synthase by blocking the flow of protons through the F

3015: 2960: 2908: 2904: 2746: 2572: 2049: 1963: 593: 474: 303: 173: 6998:"Theories of biological aging: genes, proteins, and free radicals" 6043:

Cecchini G, Schröder I, Gunsalus RP, Maklashina E (January 2002).

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of proton transfer. If, instead of the Q cycle, one molecule of QH

1245: 249:

within the inner membrane of the cell's mitochondria, whereas, in

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Kita K, Hirawake H, Miyadera H, Amino H, Takeo S (January 2002).

4305:"The gross structure of the respiratory complex I: a Lego System" 3058: 2944: 2916: 2372: 2253: 2249: 2234: 2230: 2177: 2151: 2147: 2030: 1967: 1917: 1621:. In the first step, the enzyme binds three substrates, first, QH 1618: 1373: 1229: 940: 505: 454: 326: 204: 10762: 5124: 3176:

Competitive inhibitors of succinate dehydrogenase (complex II).

924:, as is the case for many enzymes present in the mitochondrion. 10318: 10279: 10060: 8532: 7225: 5704:"Respiratory chain supercomplexes of mitochondria and bacteria" 5178:"Proton pumping mechanism of bovine heart cytochrome c oxidase" 3212:

Not all inhibitors of oxidative phosphorylation are toxins. In

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Another example of a divergent electron transport chain is the

1694: 909: 669: 585: 376: 360: 261: 166: 4844: 1648:

as it gains two protons from the mitochondrial matrix. This QH

1383: 8834: 8829: 7499:

Biomembranes Part F: Bioenergetics: Oxidative Phosphorylation

6820:

Dimroth P (1994). "Bacterial sodium ion-coupled energetics".

5420:

Annual Review of Plant Physiology and Plant Molecular Biology

3321:, with Walker and Boyer being awarded a Nobel Prize in 1997. 3305:

on the complexes of the electron-transport chain, as well as

3066: 3001: 2979:

is fundamental for oxidative phosphorylation, a shortage in O

2215: 2005: 1909: 1895: 1674: 1101: 880: 703: 536: 528: 330: 170: 108: 8091:

Pullman ME, Penefsky HS, Datta A, Racker E (November 1960).

7072:

The International Journal of Biochemistry & Cell Biology

5603: 4118:"Mitochondrial Complex I: structural and functional aspects" 2595:

the ATP synthesis reaction. Reaching along the side of the F

1337:{\displaystyle {\ce {ETF_{red}{}+ Q -> ETF_{ox}{}+ QH2}}} 1192:{\displaystyle {\ce {{Succinate}+ Q -> {Fumarate}+ QH2}}} 885: 10428: 8814: 8809: 8804: 8794: 8784: 8577: 7279:"Mitochondria as ATP consumers: cellular treason in anoxia" 5604:

Heinemeyer J, Braun HP, Boekema EJ, Kouril R (April 2007).

5549:"A critical appraisal of the mitochondrial coenzyme Q pool" 4795: 4540: 3082: 2997: 2615: 1702: 1613:

As only one of the electrons can be transferred from the QH

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it drives the counter-rotation of subunits a and c of the F

368: 219: 215: 129: 79: 8090: 7069: 6448: 6257: 6176:"The nitrite oxidizing system of Nitrobacter winogradskyi" 1096: 939:(represented as Q in the equation below), a lipid-soluble 695:

Typical respiratory enzymes and substrates in eukaryotes.

58: 10700: 8524: 8275:

Power, Sex, Suicide: Mitochondria and the Meaning of Life

7501:. Methods in Enzymology. Vol. 55. pp. 462–472. 6540: 6449:

Schemidt RA, Qu J, Williams JR, Brusilow WS (June 1998).

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Animated diagrams illustrating oxidative phosphorylation

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Berry EA, Guergova-Kuras M, Huang LS, Crofts AR (2000).

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on the enzymes involved in oxidative phosphorylation by

1951: 1251:

Electron transfer flavoprotein-ubiquinone oxidoreductase

176:, thereby releasing chemical energy in order to produce 7276: 6541:

Rubinstein JL, Walker JE, Henderson R (December 2003).

6405: 4216: 3878: 3383:"oxidative Meaning in the Cambridge English Dictionary" 607: 403: 279: 9103:(amino acid→pyruvate, acetyl CoA, or TCA intermediate) 7193:

Biochimica et Biophysica Acta (BBA) - General Subjects

7190: 6497: 5897: 3650:"The structure, function and evolution of cytochromes" 3535:

Annual Review of Biophysics and Biomolecular Structure

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Oxidative phosphorylation in hypoxic/anoxic conditions

2582:. The portion embedded within the membrane is called F 2473: 2368:, with easily interchangeable sets of enzyme systems. 1889: 222:. Oxidative phosphorylation uses these molecules and O 8900: 5787: 4591: 2760: 2416: 1724: 1465: 1276: 1151: 954: 245:, these redox reactions are catalyzed by a series of 123: 117: 111: 67: 61: 9520: 5417: 5300: 5216: 4699: 4464: 3010:

below the concentration that these enzymes can use.

1663: 645:. This coenzyme contains electrons that have a high 141: 135: 126: 91: 85: 73: 8358:

Biophysical and Structural Aspects of Bioenergetics

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Into the Cool: Energy Flow, Thermodynamics and Life

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