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In addition to the subunits shownElectron microscopy of sectioned chloroplasts shows ATP synthasecomplexes as knoblike projections on the outside (stromal) surface ofthylakoid membranes; these correspond to the ATP synthase complexes seen to project on the inside (matrix) surface of the inner mitochondrial membrane (see Fig. 18-15). Thus both the orientation of theATP synthase and the direction of proton pumping in chloroplasts areopposite to those in mitochondria. In both cases, the Fi portion of ATPsynthase is located on the more alkaline side of the membrane throughwhich protons flow down their concentration gradient; the direction ofproton flow relative to F1 is the same in both cases (Fig. 18-46).MitochondrionChloroplastBacterium (E.
coli)587Figure 18-46 Comparison of the topology of protonmovement and ATP synthase orientation in themembranes of mitochondria, chloroplasts, and thebacterium E. coli. In each case, the orientation ofthe proton gradient relative to the ATP synthase isthe same.The mechanism of chloroplast ATP synthase is also believed to beessentially identical to that of its mitochondrial analog; ADP and Pireadily condense to form ATP on the enzyme surface, but the release ofthis enzyme-bound ATP requires a proton-motive force (see Fig.
1823). As for the mitochondrial ATP synthase, the details of this mechanism remain to be determined.Cyclic Electron Flow Produces ATP but Not NADPH or O 2There is an alternative path of light-induced electron flow that allowschloroplasts to vary the ratio of NADPH and ATP formed during illuminations; this is called cyclic electron flow to differentiate it from thenormally unidirectional or noncyclic electron flow that proceedsfrom H2O to NADP + , as we have discussed thus far. Cyclic electronflow involves only photosystem I (Fig.
18-44). Electrons passed fromP700 to ferredoxin do not continue to NADP + , but move back throughthe cytochrome bf complex to plastocyanin. Plastocyanin donates electrons to P700, the illumination of which promotes electron transfer toferredoxin. Thus illumination of photosystem I can cause electrons tocycle continuously out of the reaction center of photosystem I and backinto it, each electron being propelled around the cycle by the energyyielded by absorption of one photon.
Cyclic electron flow is not accompanied by net formation of NADPH or the evolution of O2. However, itis accompanied by proton pumping and by the phosphorylation of ADPto ATP, referred to as cyclic photophosphorylation. The overallreaction equation for cyclic electron flow and photophosphorylation issimplyADP + PiATP + H 2 OCyclic electron flow and photophosphorylation are believed tooccur when the plant cell is already amply supplied with reducingpower in the form of NADPH but requires additional ATP for othermetabolic needs. By regulating the partitioning of electrons betweenNADP+ reduction and cyclic photophosphorylation, a plant adjusts theratio of NADPH and ATP produced in the light reactions to match theneeds for these products in the carbon fixation reactions and in otherenergy-requiring processes.Chapter 18 Oxidative Phosphorylation and Photophosphorylation589in which H2D symbolizes a hydrogen donor and D is its oxidized form.H2D thus may be water, hydrogen sulfide, lactate, or some other organic compound, depending upon the species.The Structure of a Bacterial PhotosystemReaction Center Has Been DeterminedThe three-dimensional structure of the photoreaction center of a photosynthetic bacterium, Rhodopseudomonas viridis, which is analogous insome ways to photosystem II in higher plants, is known from x-raycrystallographic studies (Fig.
18-47). This structure sheds light onhow phototransduction takes place in the bacterium, and presumablyin higher plants as well.The bacterial reaction center has four types of proteins: a c-typecytochrome, two subunits (L and M) associated with bacteriochlorophyll, and a fourth protein, the H subunit. A single reactioncenter contains four hemes (cytochromes), four bacteriochlorophyllsthat are similar to the chlorophylls of chloroplasts, two bacteriopheophytins, one inorganic Fe, and a quinone.
From a variety ofphysical studies, the extremely rapid sequence of events shown inFigure 18-47b has been deduced.A pair of closely spaced bacteriochlorophylls (the "special pair")constitutes the site of the initial photochemistry in the bacterial reaction center. The "electron hole" that develops in the chlorophyll is filledwith electrons from the c-type cytochrome, a role played by H2O inphotosystem II of higher plants.The bacteriochlorophylls, bacteriopheophytins, and quinone areheld rigidly in a fixed orientation relative to each other by the proteinsof the reaction center. The photochemical reactions among these components therefore take place in a virtually solid state, accounting forthe high efficiency and rapidity of the reactions; nothing is left tochance collision or dependent upon random diffusion.Figure 18-47 The purple sulfur bacterium Rhodopseudomonas viridis performs light-driven ATPsynthesis using machinery closely similar to photosystem II of higher plants, although the bacteriumhas no analog of photosystem I.
(a) Superimposedon the structure of the reaction center proteins (seeFig. 10-11) are the prosthetic groups that participate in the photochemical events. There are fourmolecules of bacteriochlorophyll, two of which (the"special pair," dark blue) constitute the site of thefirst photochemical changes after light absorption.The other two bacteriochlorophylls (orange)are called accessory pigments; their role in the photochemical events is not well understood.
Two bacterial pheophytins (light blue) lie beneath the bacteriochlorophylls, and beneath them, two bacterialquinones, QA and QB (green), separated by a nonheme iron atom (red). Shown at the top of the figure are four heme groups (red) associated with ctype cytochromes of the reaction center, (b) Thesequence of events that occurs upon excitation ofthe special pair of bacteriochlorophylls and the timescale of the electron transfers (in parentheses). Theexcited special pair passes an electron to bacteriopheophytin ® , from which the electron movesrapidly to the tightly bound quinone QA ® . Thisquinone much more slowly passes electrons to thediffusible quinone QB through the non-heme ironatom (3).
Meanwhile, the "electron hole" in the special pair is filled by an electron from one of thehemes of the four c-type cytochromes (4).a_ Hemes of c-typecytochromesLightBacteriochlorophylls(special pair)Bacteriochlorophylls(accessory pigments)BacteriopheophytinsQA(6/IS)(a)(b)590Part III Bioenergetics and MetabolismAlthough the reaction centers of plants have not yet been seen insuch detail, the similarities between the bacterial and eukaryotic reaction centers suggest that the principles deduced from studies of thegeometry and mechanism of the bacterial reaction center will alsoapply to the reaction centers of plants.Salt-Loving Bacteria Use Light Energy to Make ATPThe halophilic ("salt-loving") bacterium Halobacterium halobium conserves energy derived from absorbed sunlight by an interesting variation on the principle employed by true photosynthetic organisms.These unusual bacteria live only in brine ponds and salt lakes (GreatSalt Lake and the Dead Sea, for example), where the high salt concentration results from water loss by evaporation; indeed, they cannot livein NaCl concentrations lower than 3 M.
Halobacteria are aerobes andnormally use O2 to oxidize organic fuel molecules. However, the solubility of O2 is so low in brine ponds, in which the NaCl concentrationmay exceed 4 M, that these bacteria must sometimes call on anothersource of energy, namely sunlight. The plasma membrane of H. halobium contains patches of light-absorbing pigments, called purplepatches. These patches are made up of closely packed molecules of theprotein bacteriorhodopsin (see Fig. 10-10), which contains retinal(vitamin A aldehyde; see Fig. 9-18) as a prosthetic group.
When the cellsare illuminated, the bacteriorhodopsin molecules are excited by anabsorbed photon. As the excited molecules revert to their initial groundstate, an induced conformational change results in the release of protons outside the cell, forming an acid-outside pH gradient across theplasma membrane. Protons tend to diffuse back into the cell throughan ATP synthase complex in the membrane, very similar to that ofmitochondria and of chloroplasts, supplying the energy for ATP synthesis (Fig. 18-48).
Thus halobacteria can use light to supplement theATP synthesized by oxidative phosphorylation with the O2 that isavailable. However, halobacteria do not evolve O2, nor do they carryout photoreduction of NADP+; their phototransducing machinery istherefore much simpler than that of cyanobacteria or higher plants.Bacteriorhodopsin, with only 247 amino acid residues, is the simplest light-driven proton pump known.
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