4 Обмен энергией (1160073), страница 15
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The determination of its molecular structure should yield important insights into light-dependentenergy transduction and the action of the proton pumps that functionin respiration and photosynthesis.Figure 18-48 Light-driven proton currents in Halobacterium halobium provide the proton-motiveforce for ATP synthesis. Illumination of thephototransducing protein bacteriorhodopsin resultsin outward proton movement, generating a protonmotive force. Reentry of protons through the F ^complex provides the energy for ATP synthesis.Light^H+Light\ H+Bacteriorhodopsin \ Ain purple patches VTCytosolBacterialplasma membraneChapter 18 Oxidative Phosphorylation and Photophosphorylation591Photosynthesis Uses the Energy in Light Very EfficientlyThe standard free-energy change for the synthesis of glucose from CO2and H 2 0 during photosynthesis by the reaction6CO2 + 6H2OC 6 H 1 2 O 6 + 6O 2(18-9)is 2,840 kJ/mol. (Recall that oxidation of glucose by the reverse of thisequation proceeds with a decrease of 2,840 kJ/mol.) Now let us comparethis energy requirement with the energy yielded by the light reactionsof plant photosynthesis.Recall that two photons must be absorbed, one by each photosystem, to cause flow of one electron from H2O to NADP + .
To generate onemolecule of O2, four electrons must be transferred. Therefore, production of six molecules of O2 (as in Eqn 18-9) requires the absorption anduse of 48 photons: (2 photons/e~)(4e"/O2)(6O2) = 48 photons. Becausethe energy of one einstein may range from 300 kJ at 400 nm to about170 kJ at 700 nm (Fig. 18-35), anywhere from 8,160 to 14,400 kJ (depending upon the wavelength of the absorbed light) is required understandard conditions to make 1 mol of glucose "costing" 2,840 kJ.In the next chapter we turn to a consideration of the carbon fixation reactions by which photosynthetic organisms use the ATP andNADPH produced in the light reactions to carry out the reduction ofCO2 to carbohydrates.SummaryChemiosmotic theory provides the intellectualframework for understanding many biological energy transductions, including the processes of oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts.
The mechanism ofenergy coupling is similar in both cases. The conservation of free energy involves the passage ofelectrons through a chain of membrane-bound oxidation-reduction (redox) carriers and the concomitant pumping of protons across the membrane,producing an electrochemical gradient, the protonmotive force. This force drives the synthesis of ATPby membrane-bound enzyme complexes throughwhich protons flow back across the membrane,down their electrochemical gradient. Protonmotive force also drives other energy-requiringprocesses of cells.In mitochondria, H atoms removed from substrates by the action of NAD-linked dehydrogenases donate their electrons to the respiratory (electron transfer) chain, which transfers them tomolecular O2, reducing it to H2O.
Shuttle systemsconvey reducing equivalents from cytosolic NADHto mitochondrial NADH. Reducing eauivalentsfrom all NAD-linked dehydrogenations are transferred to mitochondrial NADH dehydrogenase(Complex I), which contains FMN as its prostheticgroup. They are then passed via a series of Fe-Scenters to ubiquinone, which transfers the electrons to cytochrome 6, the first carrier inComplex III. In this complex, electrons passthrough two b-type cytochromes and cytochrome C\before reaching an Fe-S center.
The Fe-S centerpasses electrons, one at a time, through cytochrome c and into Complex IV, cytochrome oxidase.This copper-containing enzyme, which also contains cytochromes a and a3, accumulates electrons,then passes them to O2, reducing it to H2O.There are alternative paths of entry of electronsinto this chain of carriers. Succinate, for example,is oxidizedby succinatedehydrogenase(Complex II), which contains a flavoprotein (withFAD) that passes electrons through several Fe-Scenters and into the chain at the level of ubiquinone. Electrons derived from the oxidation of fattyacids pass into ubiquinone via the electron-transferring flavoprotein (ETFP).592Part III Bioenergetics and MetabolismThe flow of electrons through Complexes I, III,and IV results in the pumping of protons across themitochondrial inner membrane, making the matrix alkaline relative to the extramitochondrialspace.
This proton gradient provides the energy(proton-motive force) for ATP synthesis from ADPand Pi by an inner-membrane protein complex,ATP synthase, also called F0Fi ATPase. The detailsof this ATP-synthesizing mechanism are stillunder investigation. Bacteria carry out oxidativephosphorylation by essentially the same mechanism, using electron carriers and an ATP synthasein the plasma membrane. Oxidative phosphorylation produces most of the ATP required by aerobiccells; it is regulated by cellular energy demands.
Inbrown fat tissue, which is specialized for the production of metabolic heat, electron transfer is uncoupled from ATP synthesis; the energy of fattyacid oxidation is therefore dissipated as heat.Photophosphorylation in the chloroplasts ofgreen plants and in cyanobacteria also involveselectron flow through a series of membrane-boundcarriers. In the light reactions of plants, the absorption of a photon excites chlorophyll moleculesand other (accessory) pigments that funnel theenergy into reaction centers in the thylakoid membranes of chloroplasts. At the reaction centers,photoexcitation results in a charge separation thatproduces one chemical species that is a good electron donor (reducing agent) and another that is agood electron acceptor.
In chloroplasts there aretwo different photoreaction centers, which functiontogether. Photosystem I passes electrons from itsexcited reaction center, P700, through a series ofcarriers to ferredoxin, which then reduces NADP+to NADPH. The reaction center, P680, of photosystem II passes electrons to plastoquinone, reducingit to the quinol form. The electrons lost from P680are replaced by electrons abstracted from H2O (hydrogen donors other than H2O are used in otherorganisms). This light-driven splitting of H2O iscatalyzed by a Mn-containing protein complex; O2is produced. Reduced plastoquinone carries electrons from photosystem II to the cytochrome bfcomplex; these electrons pass to the soluble proteinplastocyanin, and then to P700 to replace thoselost during its photoexcitation.
Electron flowthrough the cytochrome bf complex is accompaniedby proton pumping across the thylakoid membrane, and the proton-motive force thus createddrives ATP synthesis by a CF0CF! complex closelysimilar to the FoFx complex of mitochondria. Thisflow of electrons through photosystems II and Ithus produces both NADPH and ATP.
A secondtype of electron flow (cyclic flow) produces ATPonly.Both mitochondria and chloroplasts containtheir own genomes and are believed to have originated from prokaryotic endosymbionts of earlyeukaryotic cells. Oxidative phosphorylation in aerobic bacteria and photophosphorylation in photosynthetic bacteria are closely similar, in machineryand mechanism, to the homologous processes inmitochondria and chloroplasts.History and BackgroundKeilin, D. (1966) The History of Cell Respirationand Cytochrome, Cambridge University Press,London.An authoritative and absorbing account of the discovery of cytochromes and of their roles in respiration, written by the discoverer of cytochromes.Further ReadingArnon, D.I. (1984) The discovery of photosyntheticphosphorylation.
Trends Biochem. Sci. 9, 258-262.Harold, F.M. (1986) The Vital Force: A Study inBioenergetics, W.H. Freeman and Company, NewYork.A very readable synthesis of the principles of bioenergetics and their application to energy transductions.Kalckar, H.M. (1991) 50 years of biological research—from oxidative phosphorylation to energyrequiring transport regulation. Annu. Rev. Biochem. 60, 1-37.A delightful autobiographical account by one of thepioneers in the field.Lehninger, A.L. (1964) The Mitochondrion: Molecular Basis of Structure and Function, The Benjamin Co., Inc., New York.A classic description of early work on mitochondria.Mitchell, P. (1979) Keilin's respiratory chain concept and its chemiosmotic consequences.
Science206, 1148-1159.The author's Nobel lecture, outlining the evolutionof the chemiosmotic hypothesis.Chapter 18 Oxidative Phosphorylation and PhotophosphorylationSkulachev, V.P. (1992) The laws of cell energetics.Eur. J. Biochem. 208, 203-209.On the interconvertibility ofATP and ion gradients.Slater, E.C.
(1987) The mechanism of the conservation of energy of biological oxidations. Eur. J.Biochem. 166, 489-504.A clear and critical account of the evolution of thechemiosmotic model.Staehelin, L.A. & Arntzen, C.J. (eds) (1986) Photosynthesis III: Photosynthetic Membranes andLight Harvesting Systems, Encyclopedia of PlantPhysiology, Vol. 19, Springer-Verlag, Berlin.Authoritative reviews of many aspects of photosynthesis.Respiratory Electron FlowBabcock, G.T. & Wickstrom, M. (1992) Oxygenactivation and the conservation of energy in cellrespiration. Nature 356, 301-309.An advanced discussion of the reduction of waterand pumping of protons by cytochrome oxidase.Coupling ATP Synthesis toElectron Flow593RespiratoryBoyer, P.D.
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