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Chapter 19 Oxidative phosphorylation and photophosphorylationGeneration of ATP by using a across-membrane proton gradient,which is generated from electron flow through a chain of carriersPhotosynthesis(光合作用光合作用)The chemical process by which green plants and other phototrophs synthesize organic compounds from carbon dioxide and water in the presence of sunlight.light CO2 +H2O （CH2O）+O219.Photosynthetic organisms generate ATP(and NADPH)via photophosphorylationThe molecular mechanism of photophosphorylation is remarkably similar to that of oxidative phosphorylation:also mediated via a across-membrane proton gradient generated using energy released from stepwise electron flow through a series of electron carriers,located on the thylakoid membranes of chloroplasts or plasma membrane of bacteria.Schematic diagram of a chloroplastElectron micrograph of a chloroplastSolar energy as the ultimate source of all biological energyThe electron donor in photophosphorylation,H2O,is first charged by using light energy to provide electrons of high potential energy.The excess energy-rich ATP and NADPH generated by photophosphorylation are further stored in stable energy-rich carbohydrates through the carbon-assimilation(fixation)reactions occurring in the stroma of chloroplasts.1.Light-dependent reactions or light reactions Produces energy from solar power in the form of ATP and NADPH.2.Light-independent reactions or carbon-assimilation reactions Uses energy(ATP and NADPH)from light reaction to make carbohydrates.Two processes of photosynthesisThylakoidStromaBoth ATP and NADPH are generated by the light reactions of photosynthesis 20.It took a long time for human beings to understand the chemical process of photosynthesisO2 is produced by plants(1780).Light is needed for plants to produce O2(1786).CO2 is taken up by plants(1790s).H2O is taken up during CO2 fixation because the sum of weights of organic matter and O2 is much more than the weight of CO2 consumed,water is the only other substance present(1790s).Plants convert solar energy into chemical free energy(1842).Experiment with leaf extract containing chloroplasts revealed that absorbed light energy causes electrons to flow from H2O to an artificial electron acceptor(NADP+was found to be the acceptor in chloroplasts later);CO2 is not required for this process;therefore O2 could not be produced from CO2(1930s,Hill).Hill reaction:2H2O+2A 2AH2+O2Radio isotope tracer experiments revealed that CO2 is added to ribulose-1,5-bisphosphate in a cyclic pathway before it is used for glucose synthesis (1950s,Calvin).light21.The major light absorbing pigments on thylakoid membrane were revealed to be chlorophyllsChlorophylls(叶绿素叶绿素)(a and b)were found to resemble the heme group of hemoglobin,being polycyclic planar polytenes,except that the central Fe2+is replaced by a Mg2+;a 21-carbon alcohol called phytol is attached to a carboxyl group on the protoporphyrin ring;there exists an extra non-pyrrole ring.The light absorbing pigments in algae and photosynthetic bacteria(named as bacteriochlorophyll)are very similar to that of higher plants.Chlorophyll a and b and bacteriochlorophyll are the primary gatherers of light energy.Chlorophyll is always associated with specific proteins to form light-harvesting complexes(LHCs).Carotenoids(类胡萝卜素类胡萝卜素),absorbing light at wavelengths distinct from chlorophylls,act as accessory pigments on thylakoid membranes.Cyanobacteria and red algae use open-chain tetrapyrroles,called phycobilins,to absorb light at wavelengths between 520-630 nm.The absorption spectra of chlorophyll a and b overlap with the action spectrum of photosynthesis in chloroplasts.Carotene and lutein act as accessory pigments in plants(absorbing light at wavelengths distinct from chlorophyll)A light-harvesting complex(LHCII)Phycobilins,light absorbing pigments in cyanobacteria and red algae,are open-chain tetraparrole polytenes,absorbing light at wavelengths between 520-630 nm.A phycobilisome-the primary light-harvesting structures in cyanobacteria and red algaeElectromagnetic radiationThe combination of chlorophylls(a and b)and accessory pigments enable plants to harvest most of the energy available in sunlight The action spectrumof photosynthesis in algaoverlaps with theabsorption spectraof chlorophyll a and b.22.Photons absorbed by many chlorophylls funnel into one reaction center via exciton transfer The light-absorbing pigments are arranged in functional arrays called photosystem.Photons absorbed by many antenna pigments funnel to one reaction center having a specially localized pair of chlorophyll a molecules via exciton transfer.The light-absorbing pigmentsof thylakoid or bacterialmembranes are arrangedin functional arrays called photosystems.Charge separation at the reaction center may be caused by the absorption of one photon from one chlorophyll molecule.23.Two types of photochemical reaction centers have been revealed in bacteriaType II in purple bacteria:a cyclic electron flow pathway;electrons activated from the reaction center chlorophylls(P870)are first accepted by pheophytins(chlorophylls lacking the central Mg2+)(脱镁叶绿素脱镁叶绿素)causing charge separation;then to a quinone,before being transferred back to P870 via cytochrome bc1 complex and Cyt c2.The cyclic and noncyclic electron transfer path found in photosynthetic bacteria(pheophytin)Photoreaction center of the purple bacteriumRhodopseudomonas viridisA deduced path of electron flow on purple bacterial plasma membraneType I in green sulfur bacteria:both a cyclic electron pathway corresponding to the one in purple bacteria and a linear pathway leading to NADH formation using ferredoxin(a 2Fe-2S)and Fd:NAD reductase.The cytochrome bc1 complexes,being similar to the complex III in mitochondria,pumps protons across the plasma membranes.24.Two photosystems(II and I)work in tandem to move electrons from H2O to NADP+in higher plantsPSI and PSII were revealed by quantum efficiency studies(“red drop”and“enhancement”phenomena for chloroplasts)and bleaching studies(a temporary decrease in absorption of light at a specific wavelength).The electrons are charged twice(at P680 and P700).The“Z scheme”to show the electron flow fromPSII to PSIPheophytin acts as the first electron acceptor for the excited chlorophyll molecules(the“special pair”)in PSII resulting in charge separation.Plastoquinone(质体醌质体醌),structurally similar to ubiquinone,carries electrons from PSII to cytochrome b6f complex.The cytochrome b6f complex(similar to the cytochrome bc1 complex for oxidative phosphorylation)pumps H+across the thylakoid membrane.Photosystem II of the cyanobacteriumSynechococcus elongates Plastocyanin(质体蓝素质体蓝素),a Cu-containing soluble protein,carries electrons from the cytochrome b6f complex to P700 of PSI(playing a similar role as cytochrome c in oxidative phosphorylation).PSII are often found only in the stacked regions of the thylakoid membrane,with PSI and ATP synthase often only in the unstacked region.The cytochrome b6f complex and cytochrome c6 act in both oxidative phosphorylation and photophosphorylation in cyanobacteria.The supramolecular complex of PSIand its associated chlorophylls Essential proteins and cofactors in a single unit of PSILocation of PSI and PSII in thylakoid membranesLocation of PSI and PSII in thylakoid membranesH+Accumulation of reduced plastoquinone stimulates the kinase,leading to conversion of stacked to unstacked membranes.The cytochrome b6f complexThe cofactors of the cytochrome b6f complex that are involved in electron transfersFlow of electrons from PQH2 to plastocyanin via cytochrome b6f complexElectron and proton flow through the cytochrome b6f complexThe cytochrome b6f complexand cytochrome c6 act in bothoxidative phosphorylation and photophosphorylation incyanobacteria.25.P680+in PSII extracts electrons from H2O to form O2 via a Mn-containing oxygen-evolving complexP680+first accepts electrons from a Tyr residue(often designated as Z)of the D1 subunits of PSII,producing a Tyrosyl radical(Tyr*).Tyr*then accepts electrons from the Mn complex in the oxygen-evolving complex.The Mn complex is believe to serve as a charge accumulator that enables O2 to be formed(from 2 splitting H2O)without generating hazardous partly reduced intermediates,however with mechanism yet to be elucidated.The Mn complex releases H+to the thylakoid lumen while transferring electrons from H2O to Tyr via amechanism yet to be revealed26.Photosynthetic organisms use diverse hydrogen donorsUse H2O 2H2O+CO2 (CH2O)+O2Use H2S(green sulfur bacteria)2H2S+CO2 (CH2O)+2SUse lactate(some photosynthetic bacteria)2 Lactate+CO2 (CH2O)+2 pyruvate General form:2H2D+CO2 (CH2O)+2Dlightlightlightlight2H2O+8 photons+2NADP+3ADP+3Pi O2+3ATP+2NADPH Proton and electron circuits in thylakoids*Proton and electron circuits during photophosphorylationCyclic electron flow is not accompanied by net formation of NADPH or evolutionof O2(involves only PSI).Comparison of the topology of proton movement and ATP synthase orientation in the membranes of mitochondria,chloroplasts and bacteriumOxidative phosphorylation and photophosphorylation are mechanistically similar1).Both involve the flow of electrons through a chain of membrane-bound carriers.2).The energy released from“downhill”electron flow is first used for“uphill”pumping of protons to produce a proton gradient(thus a transmembrane electrochemical potential)cross a biomembrane.3).ATP is then synthesized by a“downhill”transmembrane flow of protons through a specific protein machinery(ATP synthase).ATP is synthesized using the same strategy in oxidative phosphorylation and photophosphorylationOxidative phosphorylation is the process in which ATP is generated as a result of electron flow from NADH or FADH2 to O2 via a series of membrane-bound electron carriers called the respiratory chain(reducing O2 to H2O at the end)-氧化磷氧化磷酸化酸化Photophosphorylation is the process in which ATP(and NADPH)is synthesized as a result of electron flow from H2O to NADP+via a series of membrane-bound electron carriers(oxidizing H2O to O2 at the beginning)-光合磷酸化光合磷酸化Differences between oxidative phosphorylation and photophosphorylation1).The organelle in which the process occurs:mitochondrion vs.chloroplast2).The initial source of electrons:NADH or FADH2 vs.H2O(or H2S,or organic hydrogen donors)3).Source of energy for the electron donors:fuel molecules vs.photons4).Final electron acceptor:O2 vs.NADP+KeywordsPhotophosphorylation -generates both ATP and NADPHOxidative phosphorylation and photophosphorylation -similarities and differencesWords of the Weeksubstrate-level phosphorylationoxidative phosphorylationphotophosphorylation SummaryATP is synthesized using the same strategy in oxidative phosphorylation and photophosphorylation.Photosynthetic organisms generate ATP(and NADPH)via photophosphorylation.The major light absorbing pigments on thylakoid membrane are chlorophylls.Photons absorbed by many chlorophylls funnel into one reaction center via exciton transfer.Two types of photochemical reaction centers have been revealed in bacteria.Two photosystems(II and I)work in tandem to move electrons from H2O to NADP+in higher plants.P680+in PSII extracts electrons from H2O to form O2 via a Mn-containing oxygen-evolving complex.
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