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PC, plastocyanin.How can light energy captured by chloroplasts induce electrons to flowenergetically "uphill"? Excitation briefly creates a chemical species ofvery low standard reduction potential—an excellent electron donor.Excited P680, designated P680*, within picoseconds transfers an electron to pheophytin (a chlorophyll-like accessory pigment lackingMg 2+ ), giving it a negative charge (designated Ph~) (Fig. 18-41a). Withthe loss of its electron, P680* is transformed into a radical cation, designated P680 + .
Thus excitation, in creating Ph" and P680 + , causescharge separation. Ph~ very rapidly passes its extra electron to a protein-bound plastoquinone, QA, which in turn passes its electron toanother, more loosely bound quinone, QB (Fig. 18-42; see also Fig. 1844). When QB has acquired two electrons in two such transfers from QAand two protons from the solvent water, it is in its fully reduced quinolform, QBH 2 . This molecule dissociates from its protein and diffusesaway from the photochemical reaction center, carrying in its chemicalbonds some of the energy of the photons that originally excited P680.The overall reaction initiated by light in photosystem II is therefore4 P680 + 4H + + 2Q B + light (4 photons)4 P680 + + 2Q B H 2 (18-4)QBH2OI—CH=C—CH 2 ), ? —HPlastoquinone(oxidized form)(a)Figure 18-42 (a) Plastoquinone (QA).
(b) The herbicide DCMU (3-(3,4-dichlorophenyl)-l,ldimethylurea), which displaces QB from its bindingsite in photosystem II and blocks electron transfer—C—N/,CH3CH,DCMUPSII(b)from photosystem II to photosystem I. (c) The roleof QA and QB in transferring electrons away fromphotosystem II. QBH2 carries some of the energy oflight absorbed by PSII.(c)584Figure 18-45 Proton and electron circuits in chloroplast thylakoids.
Electrons (blue arrows) movefrom H2O through photosystem II, the intermediatechain of carriers, photosystem I, and finally toNADP+. Protons (red arrows) are pumped into thethylakoid lumen by the flow of electrons throughthe chain of carriers between photosystem II andphotosystem I, and reenter the stroma through proton channels formed by the Fo portion of the ATPsynthase, designated CF0 in the chloroplast enzyme.
The Fx subunit (CFX) catalyzes synthesis ofATP.ThylakoidmembraneADPATPthis complex involves a Q cycle (see Fig. 18-10) in which electrons passfrom QBH 2 to cytochrome b one at a time. As in the mitochondrialComplex III, this cycle results in the pumping of protons across themembrane; in chloroplasts, the direction of proton movement is fromthe stromal compartment to the thylakoid lumen. The result is theproduction of a proton gradient across the thylakoid membrane as electrons pass from photosystem II to photosystem I (Fig.
18-45). Becausethe volume of the flattened thylakoid lumen is small, the influx of asmall number of protons has a relatively large effect on lumenal pH.The measured difference in pH between the stroma (pH 8) and thethylakoid lumen (pH 4.5) represents a 3,000-fold difference in protonconcentration—a powerful driving force for ATP synthesis.Coupling ATP Synthesis to Light-DrivenElectron FlowDaniel ArnonWe have now seen how one of the two energy-rich products formed inthe light reactions, NADPH, is generated by photosynthetic electrontransfer from H2O to NADP+. What about the other energy-rich product, ATP?In 1954 Daniel Arnon and his colleagues discovered that ATP isgenerated from ADP and Pi during photosynthetic electron transfer inilluminated spinach chloroplasts.
Support for thesefindingscame fromthe work of Albert Frenkel who detected light-dependent ATP production in membranous pigment-containing structures called chromatophores, derived from photosynthetic bacteria. They concluded thatsome of the light energy captured by the photosynthetic systems ofthese organisms is transformed into the phosphate bond energy ofATP. This process is called photophosphorylation or photosyn-Chapter 18 Oxidative Phosphorylation and Photophosphorylationthetic phosphorylation, to distinguish it from oxidative phosphorylation in respiring mitochondria.Recall that oxidative phosphorylation of ADP to ATP in mitochondria occurs at the expense of the free energy released as high-energyelectrons flow downhill along the electron transfer chain from substrates to O2. In a similar way, photophosphorylation of ADP to ATP iscoupled to the energy released as high-energy electrons flow down thephotosynthetic electron transfer chain from excited photosystem II tothe electron-deficient photosystem I.
The direct effect of electron flowis the formation of a proton gradient, which then provides the energyfor ATP synthesis by an ATP synthase.ATP synthesis in chloroplasts can be coupled to two types of electron flow—cyclic and noncyclic—as we shall see. We will also turn ourattention to photophosphorylation in organisms other than greenplants, and to the possible bacterial origins of chloroplasts.
The chapter concludes with the development of an overall equation for photosynthesis in plants.A Proton Gradient Couples Electron Flow andPhosphorylationSeveral properties of photosynthetic electron transfer and photophosphorylation in chloroplasts show a role for a proton gradient as in mitochondrial oxidative phosphorylation: (1) the reaction centers, electroncarriers, and ATP-forming enzymes are located in a membrane—thethylakoid membrane; (2) photophosphorylation requires intact thylakoid membranes; (3) the thylakoid membrane is impermeable to protons; (4) photophosphorylation can be uncoupled from electron flow byreagents that promote the passage of protons through the thylakoidmembrane; (5) photophosphorylation can be blocked by venturicidinand similar agents that inhibit the formation of ATP from ADP and Piby ATP synthase of mitochondria (see Fig.
18-13); and (6) ATP synthesis is catalyzed by FQFX complexes, located on the outer surface of thethylakoid membranes, that are very similar in structure and functionto the FOF! complexes of mitochondria.Electron-transferring molecules in the connecting chain betweenphotosystem II and photosystem I are oriented asymmetrically in thethylakoid membrane, so that photoinduced electron flow results in thenet movement of protons across the membrane, from the outside of thethylakoid membrane to the inner compartment (Fig. 18-45).In 1966 Andre Jagendorf showed that a pH gradient across thethylakoid membrane (alkaline outside) could furnish the driving forceto generate ATP.
Jagendorf's early observations provided some of themost important experimental evidence in support of Mitchell's chemiosmotic hypothesis. In the dark, he soaked chloroplasts in a pH 4buffer, which slowly penetrated into the inner compartment of the thylakoids, lowering their internal pH.
He added ADP and Pi to the darksuspension of chloroplasts and then suddenly raised the pH of theouter medium to 8, momentarily creating a large pH gradient acrossthe membrane. As protons moved out of the thylakoids into the medium, ATP was generated from ADP and Pi. Because the formation ofATP occurred in the dark (with no input of energy from light), thisexperiment showed that a pH gradient across the membrane is a highenergy state that can, as in mitochondrial oxidative phosphorylation,mediate the transduction of energy from electron transfer into thechemical energy of ATP.Andre Jagendorf586Part III Bioenergetics and MetabolismThe stoichiometry for this process (protons transported per electron) is not well established.
Electron transfer between photosystemsII and I through the cytochrome bf complex contributes to the protongradient an amount of proton-motive force roughly equivalent to one totwo ATP formed per pair of electrons.The ATP Synthase of Chloroplasts IsLike That of MitochondriaThe enzyme responsible for ATP synthesis in chloroplasts is a largecomplex with two functional components, CFO and CFi (the C denotingchloroplast origin). CFO is a transmembrane proton pore composed ofseveral integral membrane proteins and is homologous with mitochondrial F o .
CFi is a peripheral membrane protein complex very similar insubunit composition, structure, and function to mitochondrial Fi(Table 18-8). Together these proteins constitute the ATP synthase ofchloroplasts. (Bacteria also contain ATP synthases remarkably similarin structure and function to those of chloroplasts and mitochondria(Table 18-8).)Table 18-8 Equivalent subunits in ATP synthase of mitochondria, chloroplasts, and bacteria (E. coli)*Portion ofATPsynthaseFiSubunitaPFoChloroplastsMitochondriayOSCP8eabcNumber331111116-12SubunitaE. coliNumber33111Py8€a (IV)b and b' (I and II)c (III)21+16-12SubunitaNumber€33111abc1210-12Py8Source: Data primarily from Walker, J.E., Lutter, R., Dupuis, A., & Runswick, M.J.
(1991) Identification of the subunits of FiFo-ATPase frombovine heart mitochondria. Biochemistry 30, 5369-5378.* Subunits on the same horizontal line are structurally relatedhere, the mitochondrial enzyme complex contains at least fiveand are believed to be functionally homologous. Chloroplastsmore proteins with no apparent homologs in the chloroplast orcontain two nonidentical subunits, b and b', that are togetherbacterium: F6, inhibitor protein, A6L, d, and e. These subunitsthe homologs of mitochondrial b; E. coli has two identical bare released as soluble proteins when F1 and Fo are dissocisubunits. OSCP is oligomycin sensitivity-conferring protein.ated, but they are believed to be part of the intact, functioningAlternative nomenclatures for the four chloroplast Fo subunitscomplex in mitochondria,are shown in parentheses.
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