Biochemistry, 4th Edition P68 ppsx

21.2 How Is Solar Energy Captured by Chlorophyll? 633 CO 2 by a series of enzymatic reactions found in the stroma (see Equation 21.3, which follows). Water Is the Ultimate e ؊ Donor for Photosynthetic NADP ؉ Reduction In green plants, water serves as the ultimate electron donor for the photosynthetic generation of reducing equivalents. The reaction sequence nh␷ 2 H 2 O ϩ 2 NADP ϩ ϩ x ADP ϩ x P i ⎯⎯→ O 2 ϩ 2 NADPH ϩ 2 H ϩ ϩ x ATP ϩ x H 2 O (21.2) describes the process, where nh␷ symbolizes light energy (n is some number of pho- tons of energy h␷, where h is Planck’s constant and ␷ is the frequency of the light). Light energy is necessary to make the unfavorable reduction of NADP ϩ by H 2 O (⌬Ᏹ o ЈϭϪ1.136 V; ⌬G°Јϭϩ219 kJ/mol NADP ϩ ) thermodynamically favorable. Thus, the light energy input, nh␷, must exceed 219 kJ/mol NADP ϩ . The stoi- chiometry of ATP formation depends on the pattern of photophosphorylation op- erating in the cell at the time and on the ATP yield in terms of the chemiosmotic ratio, ATP/H ϩ , as we will see later. Nevertheless, the stoichiometry of the metabolic pathway of CO 2 fixation is certain: 12 NADPH ϩ 12 H ϩ ϩ 18 ATP ϩ 6 CO 2 ϩ 12 H 2 O ⎯⎯→ C 6 H 12 O 6 ϩ 12 NADP ϩ ϩ 18 ADP ϩ 18 P i (21.3) A More Generalized Equation for Photosynthesis In 1931, comparative study of photosynthesis in bacteria led van Niel to a more general formulation of the over- all reaction: Light CO 2 ϩ 2 H 2 A ⎯⎯→ (CH 2 O) ϩ 2A ϩ H 2 O (21.4) Hydrogen Hydrogen Reduced Oxidized acceptor donor acceptor donor In photosynthetic bacteria, H 2 A is variously H 2 S (photosynthetic green and pur- ple sulfur bacteria), isopropanol, or some similar oxidizable substrate. [(CH 2 O) sym- bolizes a carbohydrate unit.] CO 2 ϩ 2 H 2 S ⎯⎯→ (CH 2 O) ϩ H 2 O ϩ 2 S In cyanobacteria and the eukaryotic photosynthetic cells of algae and higher plants, H 2 A is H 2 O, as implied earlier, and 2 A is O 2 . The accumulation of O 2 to constitute 21% of the earth’s atmosphere is the direct result of eons of global oxy- genic photosynthesis. 21.2 How Is Solar Energy Captured by Chlorophyll? Photosynthesis depends on the photoreactivity of chlorophyll. Chlorophylls are magnesium-containing substituted tetrapyrroles whose basic structure is reminis- cent of heme, the iron-containing porphyrin (see Chapters 5 and 20). Chloro- phylls differ from heme in a number of properties: Magnesium instead of iron is coordinated in the center of the planar conjugated ring structure; a long-chain alcohol, phytol, is esterified to a pyrrole ring substituent; and the methine bridge linking pyrroles III and IV is substituted and crosslinked to ring III, leading to the formation of a fifth five-membered ring. The structures of chlorophyll a and b are shown in Figure 21.5a. Chlorophylls are excellent light absorbers because of their aromaticity. That is, they possess delocalized ␲ electrons above and below the planar ring structure. The (CH 2 O) H 2 O 8 n ϩ 2 CH 3 CH 3 C O ϩCO 2 2 CH 3 CH 3 CHOHϩ 634 Chapter 21 Photosynthesis energy differences between electronic states in these ␲ orbitals correspond to the energies of visible light photons. When light energy is absorbed, an electron is pro- moted to a higher orbital, enhancing the potential for transfer of this electron to a suitable acceptor. Loss of such a photoexcited electron to an acceptor is an oxidation–reduction reaction. The net result is the transduction of light energy into the chemical energy of a redox reaction. Chlorophylls and Accessory Light-Harvesting Pigments Absorb Light of Different Wavelengths The absorption spectra of chlorophylls a and b (Figure 21.5b) differ somewhat. Plants that possess both chlorophylls can harvest a wider spectrum of incident energy. Other pigments in photosynthetic organisms, so-called accessory light-harvesting pigments (Figure 21.6), increase the possibility for absorption of incident light of wavelengths not absorbed by the chlorophylls. Carotenoids and phycocyanobilins, like chloro- phyll, possess many conjugated double bonds and thus absorb visible light. Carotenoids have two primary roles in photosynthesis—light harvesting and photo- protection through destruction of reactive oxygen species that arise as by-products of photoexcitation. The Light Energy Absorbed by Photosynthetic Pigments Has Several Possible Fates Each photon represents a quantum of light energy. A quantum of light energy ab- sorbed by a photosynthetic pigment has four possible fates (Figure 21.7): 1. Loss as heat. The energy can be dissipated as heat through redistribution into atomic vibrations within the pigment molecule. CH 2 CH 3 H CH 3 H CHH 2 C (a) CH 3 H H CH 3 CH 2 H H CH 2 C O O O H C O OCH 3 Mg II III V IIV NN NN H 2 C C CCH 3 H 2 C CH 2 H 2 C CH H 2 C CH 2 H 2 C CH H 2 C CH 2 H 2 C CH H 3 C CH 3 CH 3 CH 3 R Hydrophobic phytyl side chain R= Chlorophyll a —CH 3 Chlorophyll b —CHO (b) Absorbance 400 500 Wavelength (nm) 600 700 a b b a FIGURE 21.5 Structures (a) and absorption spectra (b) of chlorophyll a and b. The phytyl side chain of ring IV pro- vides a hydrophobic tail to anchor the chlorophyll in membrane protein complexes. 21.2 How Is Solar Energy Captured by Chlorophyll? 635 H 3 C O H CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 H 3 C O H N H N N H N CH 3 CH CH 3 CH 2 CH 2 CO OH CH 2 CH 2 CO OH CH 3 CH 3 CH 2 CH 3 H 3 C CH 3 (a) (b) ␤-Carotene Phycocyanobilin FIGURE 21.6 Structures of representative accessory light-harvesting pigments in photosynthetic cells. (a) ␤-Carotene, an accessory light-harvesting pigment in leaves. (b) Phycocyanobilin, a blue pigment found in cyanobacteria. e – e – e – e – e – e – e – e – Heat +++ + Photon of fluorescence Thermal dissipation Fluorescence Energy transfer Transfer Oxidized P (P + ) Energy transfer to neighboring P molecule + P* h␯ + + Pigment molecule (P) Light energy (hv) Excited state (P*) Q – red Q ox FIGURE 21.7 Possible fates of the quantum of light energy absorbed by photosynthetic pigments. 636 Chapter 21 Photosynthesis 2. Loss as light. Energy of excitation reappears as fluorescence (light emission); a photon of fluorescence is emitted as the e Ϫ returns to a lower orbital. This fate is common only in saturating light intensities. For thermodynamic reasons, the photon of fluorescence has a longer wavelength and hence lower energy than the quantum of excitation. 3. Resonance energy transfer. The excitation energy can be transferred by reso- nance energy transfer to a neighboring molecule if the energy level difference be- tween the two corresponds to the quantum of excitation energy. In this process, the energy transferred raises an electron in the receptor molecule to a higher energy state as the photoexcited e Ϫ in the original absorbing molecule returns to ground state. This so-called Förster resonance energy transfer is the mechanism whereby quanta of light falling anywhere within an array of pigment molecules can be transferred ultimately to specific photochemically reactive sites. 4. Energy transduction. The energy of excitation, in raising an electron to a higher energy orbital, dramatically changes the standard reduction potential, Ᏹ o Ј, of the pigment such that it becomes a much more effective electron donor. That is, the excited-state species, by virtue of having an electron at a higher energy level through light absorption, has become a more potent electron donor. Reaction of this excited-state electron donor with an electron acceptor situated in its vicinity leads to the transformation, or transduction, of light energy (photons) to chem- ical energy (reducing power, the potential for electron-transfer reactions). Trans- duction of light energy into chemical energy, the photochemical event, is the essence of photosynthesis. The Transduction of Light Energy into Chemical Energy Involves Oxidation–Reduction The diagram presented in Figure 21.8 illustrates the fundamental transduction of light energy into chemical energy (an oxidation–reduction reaction) that is the ba- sis of photosynthesis. Chlorophyll (Chl) resides in a membrane in close association with molecules competent in e Ϫ transfer, symbolized here as A and B. Chl absorbs a photon of light, becoming activated to Chl* in the process. Electron transfer from Chl* to A leads to oxidized Chl (Chlи ϩ , a cationic free radical) and reduced A (A Ϫ in the diagram). Subsequent oxidation of A Ϫ eventually culminates in re- duction of NADP ϩ to NADPH. The electron “hole” in oxidized Chl (Chlи ϩ ) is filled by transfer of an electron from B to Chlи ϩ , restoring Chl and creating B ϩ . B ϩ is re- stored to B by an e Ϫ donated by water. O 2 is the product of water oxidation. Note that the system is restored to its original state once NADPH is formed and H 2 O is oxidized. Proton translocations accompany these light-driven electron-transport NADP + NADPH I II III IV V A Chl B H 2 OO 2 A 1 2 Chl* B A – B A B A Chl B + A Chl B h␯ Chl + Chl + 1 2 1 2 1 2 FIGURE 21.8 Model for light absorption by chlorophyll and transduction of light energy into an oxidation– reduction reaction. I: Photoexcitation of Chl creates Chl*. II: Electron transfer from Chl* to A yields oxidized Chl (Chl ϩ ) and reduced A (A Ϫ ) III: An electron-transfer pathway from A Ϫ to NADP ϩ leads to NADPH formation and restoration of oxidized A (A). IV: Chl ϩ accepts an electron from B, restoring Chl and generating oxidized B (B ϩ ). V: B ϩ is reduced back to B by an electron origi- nating in H 2 O.Water oxidation is the source of O 2 formation. 21.3 What Kinds of Photosystems Are Used to Capture Light Energy? 637 reactions. Such H ϩ translocations establish a chemiosmotic gradient across the photosynthetic membrane that can drive ATP synthesis. Photosynthetic Units Consist of Many Chlorophyll Molecules but Only a Single Reaction Center In the early 1930s, Emerson and Arnold investigated the relationship between the amount of incident light energy, the amount of chlorophyll present, and the amount of oxygen evolved by illuminated algal cells. Emerson and Arnold were seeking to de- termine the quantum yield of photosynthesis: the number of electrons transferred per photon of light. Their studies gave an unexpected result: When algae were illu- minated with very brief light flashes that could excite every chlorophyll molecule at least once, only one molecule of O 2 was evolved per 2400 chlorophyll molecules. This result implied that not all chlorophyll molecules are photochemically reactive, and it led to the concept that photosynthesis occurs in functionally discrete units. Chlorophyll serves two roles in photosynthesis. It is involved in light harvesting and the transfer of light energy to photoreactive sites by exciton transfer, and it par- ticipates directly in the photochemical events whereby light energy becomes chem- ical energy. A photosynthetic unit (Figure 21.9) can be envisioned as an antenna of several hundred light-harvesting chlorophyll molecules (green) plus a special pair of photochemically reactive chlorophyll a molecules called the reaction center (orange). The purpose of the vast majority of chlorophyll in a photosynthetic unit is to harvest light incident within the unit and funnel it, via resonance energy trans- fer, to the reaction center chlorophyll dimers that are photochemically active. Most chlorophyll thus acts as a large light-collecting antenna, and it is at the reaction cen- ters that the photochemical event occurs. Oxidation of chlorophyll leaves a cationic free radical, Chlи ϩ , whose properties as an electron acceptor have important con- sequences for photosynthesis. Note that the Mg 2ϩ ion does not change in valence during these redox reactions. 21.3 What Kinds of Photosystems Are Used to Capture Light Energy? All photosynthetic cells contain some form of photosystem. Photosynthetic bacteria have only one photosystem; furthermore, they lack the ability to use light energy to split H 2 O and release O 2 . Cyanobacteria, green algae, and higher plants are oxygenic phototrophs because they can generate O 2 from water. Oxygenic phototrophs have two distinct photosystems: photosystem I (PSI) and photosystem II (PSII). Type I photo- systems use ferredoxins as terminal electron acceptors; type II photosystems use quinones as terminal electron acceptors. PSI is defined by reaction center chloro- phylls with maximal red light absorption at 700 nm; PSII uses reaction centers that exhibit maximal red light absorption at 680 nm. The reaction center Chl of PSI is re- ferred to as P700 because it absorbs light of 700-nm wavelength; the reaction center Chl of PSII is called P680 for analogous reasons. Both P700 and P680 are chlorophyll a dimers situated within specialized protein complexes. A distinct property of PSII is its role in light-driven O 2 evolution. Interestingly, the photosystems of photosynthetic bacteria are type II photosystems that resemble eukaryotic PSII more than PSI, even though these bacteria lack O 2 -evolving capacity. Chlorophyll Exists in Plant Membranes in Association with Proteins Detergent treatment of a suspension of thylakoids dissolves the membranes, releasing complexes containing both chlorophyll and protein. These chlorophyll–protein com- plexes represent integral components of the thylakoid membrane, and their organi- zation reflects their roles as either light-harvesting complexes (LHC), PSI complexes, or PSII complexes. All chlorophyll is apparently localized within these three macro- molecular assemblies. Reaction center Light-harvesting pigment (antenna molecules) h␯ ANIMATED FIGURE 21.9 Schematic diagram of a photosynthetic unit. See this figure animated at www.cengage.com/login. 638 Chapter 21 Photosynthesis PSI and PSII Participate in the Overall Process of Photosynthesis What are the roles of the two photosystems, and what is their relationship to each other? PSI provides reducing power in the form of NADPH. PSII splits water, pro- ducing O 2 , and feeds the electrons released into an electron-transport chain that couples PSII to PSI. Electron transfer between PSII and PSI pumps protons for chemiosmotic ATP synthesis. As summarized by Equation 21.2, photosynthesis in- volves the reduction of NADP ϩ , using electrons derived from water and activated by light, h␷. ATP is generated in the process. The standard reduction potential for the NADP ϩ /NADPH couple is Ϫ0.32 V. Thus, a strong reductant with an Ᏹ o Ј more neg- ative than Ϫ0.32 V is required to reduce NADP ϩ under standard conditions. By sim- ilar reasoning, a very strong oxidant will be required to oxidize water to oxygen be- cause Ᏹ o Ј( ᎏ 1 2 ᎏ O 2 /H 2 O) is ϩ0.82 V. Separation of the oxidizing and reducing aspects of Equation 21.2 is accomplished in nature by devoting PSI to NADP ϩ reduction and PSII to water oxidation. PSI and PSII are linked via an electron-transport chain so that the weak reductant generated by PSII can provide an electron to reduce the weak oxidant side of P700 (Figure 21.10). Thus, electrons flow from H 2 O to NADP ϩ , driven by light energy absorbed at the reaction centers. Oxygen is a by-product of the photolysis, literally “light-splitting,” of water. Accompanying electron flow is pro- duction of a proton gradient and ATP synthesis (see Section 21.6). This light- driven phosphorylation is termed photophosphorylation. The Pathway of Photosynthetic Electron Transfer Is Called the Z Scheme Photosystems I and II contain unique complements of electron carriers, and these carriers mediate the stepwise transfer of electrons from water to NADP ϩ . When the individual redox components of PSI and PSII are arranged as an e Ϫ transport chain according to their standard reduction potentials, the zigzag result resembles the let- ter Z laid sideways (Figure 21.11). The various electron carriers are indicated as fol- lows: “Mn complex” symbolizes the manganese-containing oxygen-evolving complex; D is its e Ϫ acceptor and the immediate e Ϫ donor to P680 ϩ ; Q A and Q B represent spe- cial plastoquinone molecules (see Figure 21.13) and PQ the plastoquinone pool; Fe-S stands for the Rieske iron–sulfur center, and cyt f, cytochrome f. PC is the abbreviation for plastocyanin, the immediate e Ϫ donor to P700 ϩ ; and F A , F B , and F X represent the membrane-associated ferredoxins downstream from A 0 (a specialized Chl a) and A 1 (a specialized PSI quinone). Fd is the soluble ferredoxin pool that serves as the e Ϫ donor to the flavoprotein (Fp), called ferredoxin–NADP ؉ reductase, which catalyzes reduction of NADP ϩ to NADPH. Cyt(b 6 ) N ,(b 6 ) P symbolizes the cyto- chrome b 6 moieties of the cytochrome b 6 f complex. PQ and the cytochrome b 6 f com- plex also serve to transfer e Ϫ from F A /F B back to P700 ϩ during cyclic photophos- phorylation (the pathway symbolized by the dashed arrow). Overall photosynthetic electron transfer is accomplished by three membrane- spanning supramolecular complexes composed of intrinsic and extrinsic poly- peptides (shown as shaded boxes bounded by solid black lines in Figure 22.11). These complexes are the PSII complex, the cytochrome b 6 f complex, and the PSI complex. The PSII complex is aptly described as a light-driven waterϺplastoquinone oxidoreductase; it is the enzyme system responsible for photolysis of water, and as such, it is also referred to as the oxygen-evolving complex, or OEC. PSII possesses a ° Ј ° Ј ° Ј ° Ј + PSII “blue” light < 680 nm PSI “red” light 700 nm P680 P700 Strong oxidant > +0.8 V Weak reductant ≅ 0 V Strong reductant < –0.6 V Weak oxidant ≅ 0.45 V 1 2 H 2 O NADP + ATP NADPHADP P i O 2 FIGURE 21.10 Roles of the two photosystems, PSI and PSII. Ferredoxin (Fd): A generic term for small pro- teins possessing iron-sulfur clusters that partici- pate in various electron-transfer reactions. 21.3 What Kinds of Photosystems Are Used to Capture Light Energy? 639 O 2 O 2 2 H + 4 H + Fp (FAD) +1.60 +1.20 +0.80 +0.40 Q A Q B 2 1 Protons released in lumen Protons taken up from stroma h␯ Protons released into lumen 0 –0.40 Pheo Chl a P680* Photosystem II o ' –0.80 –1.20 Mn complex (Cyt b 6 ) P P680 D PQ Fe-S Fd Cyt f h␯ A 0 A 1 F A F B F X P700* Photosystem I P700 PC (Cyt b 6 ) N H 2 O H 2 O (a) NADPH + NADP + H + PQ h␯ 2 1 PCPC Cyt b 6 Cyt b 6 Fd 4 + + Fd Photosystem I NADP + Photosystem II CF 1 CF 0 – ATP synthase Fp (FAD) Mn complex H + 2 H + 4 H + H + + PQ h␯ Fe-S Q B Fe Q A FeS A FeS B FeS X A 1 A 0 Pheo P680 Pheo P700 Stroma Lumen (b) NADPH ADP P i ATP Cyt f ACTIVE FIGURE 21.11 The Z scheme of photosynthesis. (a) The Z scheme is a diagrammatic rep- resentation of photosynthetic electron flow from H 2 O to NADP ϩ .The energy relationships can be derived from the Ᏹ o Ј scale beside the Z diagram.Energy input as light is indicated by two broad arrows,one photon appearing in P680 and the other in P700. P680* and P700* represent photoexcited states.The three supramolecular complexes (PSI, PSII, and the cytochrome b 6 f complex) are in shaded boxes. Proton translocations that establish the proton- motive force driving ATP synthesis are illustrated as well. (b) The functional relationships among PSII, the cyto- chrome bf complex,PSI, and the photosynthetic CF 1 CF 0 –ATP synthase within the thylakoid membrane.Test yourself on the concepts in this figure at www.cengage.com/login. 640 Chapter 21 Photosynthesis metal cluster containing 4 Mn 2ϩ atoms that coordinate two water molecules. As P680 undergoes four cycles of light-induced oxidation, four protons and four electrons are removed from the two water molecules and their O atoms are joined to form O 2 . A tyrosyl side chain of the PSII complex (see following discussion) mediates electron transfer between the Mn 2ϩ cluster and P680. The O 2 -evolving reaction requires Ca 2ϩ and Cl Ϫ ions in addition to the (Mn 2ϩ ) 4 cluster. Oxygen Evolution Requires the Accumulation of Four Oxidizing Equivalents in PSII When isolated chloroplasts that have been held in the dark are illuminated with very brief flashes of light, O 2 evolution reaches a peak on the third flash and every fourth flash thereafter (Figure 21.12a). The oscillation in O 2 evolution dampens over repeated flashes and converges to an average value. These data are interpreted to mean that the P680 reaction center complex cycles through five different oxida- tion states, numbered S 0 to S 4 . One electron and one proton are removed photo- chemically in each step. When S 4 is attained, an O 2 molecule is released (Figure 21.12b) as PSII returns to oxidation state S 0 and two new water molecules bind. (The reason the first pulse of O 2 release occurred on the third flash [Figure 21.12a] is that the PSII reaction centers in the isolated chloroplasts were already poised at S 1 reduction level.) Electrons Are Taken from H 2 O to Replace Electrons Lost from P680 The events intervening between H 2 O and P680 involve D, the name assigned to a specific protein tyrosine residue that mediates e Ϫ transfer from H 2 O via the Mn com- plex to P680 ϩ (see Figure 21.11). The oxidized form of D is a tyrosyl free radical species, Dи ϩ . To begin the cycle, an exciton of energy excites P680 to P680*, where- upon P680* transfers an electron to a nearby Chl a molecule, which is the direct elec- tron acceptor from P680 * . This Chl a then reduces a molecule of pheophytin, sym- bolized by “Pheo” in Figure 21.11. Pheophytin is like chlorophyll a, except 2 H ϩ replace the centrally coordinated Mg 2ϩ ion. This special pheophytin is the direct electron acceptor from P680*. Loss of an electron from P680* creates P680 ϩ , the electron acceptor for D. Electrons flow from Pheo via specialized molecules of plastoquinone, represented by “Q” in Figure 21.11, to a pool of plastoquinone (PQ) within the membrane. Because of its lipid nature, plastoquinone is mobile within the membrane and hence serves to shuttle electrons from the PSII supramolecular com- plex to the cytochrome b 6 f complex. Alternate oxidation–reduction of plasto- quinone to its hydroquinone form involves the uptake of protons (Figure 21.13). The asymmetry of the thylakoid membrane is designed to exploit this proton uptake and release so that protons (H ϩ ) accumulate within the lumen of thylakoid vesicles, establishing an electrochemical gradient. Note that plastoquinone is an analog of coenzyme Q, the mitochondrial electron carrier (see Chapter 20). Electrons from PSII Are Transferred to PSI via the Cytochrome b 6 f Complex The cytochrome b 6 f or plastoquinolϺplastocyanin oxidoreductase is a large (210 kD) multimeric protein possessing 26 transmembrane ␣-helices. This membrane protein complex is structurally and functionally homologous to the cytochrome bc 1 complex (Complex III) of mitochondria (see Chapter 20). It includes the two heme-containing electron transfer proteins for which it is named, as well as iron–sulfur clusters, which also participate in electron transport. The purpose of this complex is to mediate the trans- fer of electrons from PSII to PSI and to pump protons across the thylakoid membrane via a plastoquinone-mediated Q cycle, analogous to that found in mitochondrial e Ϫ transport (see Chapter 20). Cytochrome f (f from the Latin folium, meaning “foliage”) is a c-type cytochrome, with a reduction potential of ϩ0.365 V. Cytochrome b 6 in two forms (low- and high-potential) participates in the oxidation of plastoquinol e – e – e – e – O 2 (a) O 2 evolved/flash 4 8 12 16 20 24 Flash number (b) S 0 S 1 S 4 S 2 S 3 h␯ 2 H 2 O h ␯ h␯ h␯ ++++ H + H + H + H + FIGURE 21.12 Oxygen evolution requires the accumula- tion of four oxidizing equivalents in PSII. (a) O 2 evolution after brief light flashes. (b) The cycling of the PSII reac- tion center through five different oxidation states, S 0 to S 4 . One e Ϫ is removed photochemically at each light flash, moving the reaction center successively through S 1 ,S 2 ,S 3 , and S 4 .S 4 decays spontaneously to S 0 by oxidizing 2 H 2 O to O 2 . e – H 3 C H O H 3 C (CH 2 CH C CH 2 ) 9 H CH 3 H + +2 , 2 e – H + –2 , 2 H 3 C H OH H 3 C (CH 2 CH C CH 2 ) 9 H CH 3 OH O Plastoquinone A Plastohydroquinone A FIGURE 21.13 The structures of plastoquinone A and its reduced form, plastohydroquinone (or plastoquinol). Plastoquinone A has nine isoprene units and is the most abundant plastoquinone in plants and algae. 21.4 What Is the Molecular Architecture of Photosynthetic Reaction Centers? 641 and the Q cycle of the b 6 f complex. The cytochrome b 6 f complex can also serve in an alternative cyclic electron transfer pathway. Under certain conditions, electrons de- rived from P700* are not passed on to NADP ϩ but instead cycle down an alternative path, whereby reduced ferredoxin contributes its electron to PQ. This electron is then passed to the cytochrome b 6 f complex, and then back to P700 ϩ . This cyclic flow yields no O 2 evolution or NADP ϩ reduction but can lead to ATP synthesis via so-called cyclic photophosphorylation, discussed later. Plastocyanin Transfers Electrons from the Cytochrome b 6 f Complex to PSI Plastocyanin (PC in Figure 21.11) is an electron carrier capable of diffusion along the inside of the thylakoid and migration in and out of the membrane, aptly suited to its role in shuttling electrons between the cytochrome b 6 f complex and PSI. Plastocyanin is a low-molecular-weight (10.4 kD) protein containing a single copper atom. PC func- tions as a single-electron carrier (Ᏹ o Јϭϩ0.32 V) as its copper atom undergoes alter- nate oxidation–reduction between the cuprous (Cu ϩ ) and cupric (Cu 2ϩ ) states. PSI is a light-driven plastocyaninϺferredoxin oxidoreductase. When P700, the specialized chlorophyll a dimer of PSI, is excited by light and oxidized by transferring its e Ϫ to an adjacent chlorophyll a molecule that serves as its immediate e Ϫ acceptor, P700 ϩ is formed. (The standard reduction potential for the P700 ϩ /P700 couple is about ϩ0.45 V.) P700 ϩ readily gains an electron from plastocyanin. The immediate electron acceptor for P700* is a special molecule of chlorophyll. This unique Chl a (A 0 ) rapidly passes the electron to a specialized quinone (A 1 ), which in turn passes the e Ϫ to the first in a series of membrane-bound ferredoxins. This Fd series ends with a soluble form of ferredoxin, Fd s , which serves as the immediate electron donor to the flavoprotein (Fp) that catalyzes NADP ϩ reduction, namely, ferredoxinϺNADP ؉ reductase. 21.4 What Is the Molecular Architecture of Photosynthetic Reaction Centers? What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e Ϫ transfers to proton translocations so that ATP synthesis is possible? Part of the answer to this question lies in the membrane-associated nature of the photosystems. A major breakthrough occurred Structure of the cyanobacterial cytochrome b 6 f com- plex.The heme groups of cytochromes b 6 N, b 6 P, and f are shown in red; the iron-sulfur clusters are blue (pdb id ϭ 1BF5).The upper bundle of ϰ-helices defines the transmembrane domain. 642 Chapter 21 Photosynthesis in 1984, when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosyn- thetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in Chemistry. The R. viridis Photosynthetic Reaction Center Is an Integral Membrane Protein R. viridis is a photosynthetic prokaryote with a single photosystem that resembles PSII (even though it lacks an OEC and the capacity to oxidize water). The reaction center (145 kD) of the R. viridis photosystem is localized in the plasma membrane of these photosynthetic bacteria and is composed of four different polypeptides, designated L (273 amino acid residues), M (323 residues), H (258 residues), and cytochrome (333 amino acid residues) (Figure 21.14a). L and M each consist of five membrane- spanning ␣-helical segments; H has one such helix, the majority of the protein form- ing a globular domain in the cytoplasm (Figure 21.14b). The cytochrome subunit contains four heme groups; the N-terminal amino acid of this protein is cysteine. This cytochrome is anchored to the periplasmic face of the membrane via the hydropho- bic chains of two fatty acid groups that are esterified to a glyceryl moiety joined via a thioether bond to the Cys (Figure 21.14a). L and M each bear two bacteriochlorophyll molecules (the bacterial version of Chl) and one bacteriopheophytin. L also has a bound quinone molecule, Q A . Together, L and M coordinate an Fe atom. The photochemi- cally active species of the R. viridis reaction center, P870, is composed of two bacterio- chlorophylls, one contributed by L and the other by M. Photosynthetic Electron Transfer by the R. viridis Reaction Center Leads to ATP Synthesis The prosthetic groups of the R. viridis reaction center (P870, BChl, BPheo, and the bound quinones) are fixed in a spatial relationship to one another that favors photosynthetic e Ϫ transfer (Figure 21.14a,c). Photoexcitation of P870 (creation of P870*) leads to e Ϫ loss (P870 ϩ ) via electron transfer to the nearby bacteriochloro- Cytochrome with 4 heme groups hν P870 ML BChl BChl BPheo BPheo <1 psec 20 psec 230 psec Q A Q B Fe 100 ␮s H Note: The cytochrome subunit is membrane associated via a diacylglycerol moiety on its N-terminal Cys residue: H C H 3 NC O CH 2 SCHCH 2 CH 2 OO CCOO Membrane anchor + (a) (b) (c) FIGURE 21.14 The R.viridis reaction center (RC). (a) Diagram of the RC showing light activation and path of e Ϫ transfer. (b) Molecular graphic of the R.viridis RC. M and L are yellow and blue; H is orange; the cytochrome is green. (c) Deletion of the R. viridis RC protein chains reveals the spatial relationship between its heme, chlorophyll, and quinone prosthetic groups.The iron atom is represented by a sphere (pdb id ϭ 1PRC). . cycle, an exciton of energy excites P680 to P680 *, where- upon P680 * transfers an electron to a nearby Chl a molecule, which is the direct elec- tron acceptor from P680 * . This Chl a then reduces. from P680 The events intervening between H 2 O and P680 involve D, the name assigned to a specific protein tyrosine residue that mediates e Ϫ transfer from H 2 O via the Mn com- plex to P680 ϩ (see. Mg 2ϩ ion. This special pheophytin is the direct electron acceptor from P680 *. Loss of an electron from P680 * creates P680 ϩ , the electron acceptor for D. Electrons flow from Pheo via specialized