Cytochrome b6f complex

Eventually, the electrons in PQBH2 pass through the cytochrome b6f complex (Fig. 19-49). The electron initially removed from P680 is replaced with an electron obtained from the oxidation of water, as described below. The binding site for plastoquinone is the point of action of many commercial herbicides that kill plants by blocking electron transfer through the cytochrome b6f complex and preventing photosynthetic ATP production. 

Cyanobacteria Use the Cytochrome b6f Complex and Cytochrome c6 in Both Oxidative Phosphorylation and Photophosphorylation... 

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

The light-driven splitting of H20 is catalyzed by a Mn-containing protein complex 02 is produced. The reduced plastoquinone carries electrons to the cytochrome b6f complex from here they pass to plastocyanin, and then to P700 to replace those lost during its photoexcitation. 

Electron flow through the cytochrome b6f complex drives protons across the plasma membrane, creating a proton-motive force that provides the energy for ATP synthesis by an ATP synthase. 

The simpler cytochrome bc] complexes of bacteria such as E. coli,102 Paracoccus dentrificans,116 and the photosynthetic Rhodobacter capsulatus117 all appear to function in a manner similar to that of the large mitochondrial complex. The bc] complex of Bacillus subtilis oxidizes reduced menaquinone (Fig. 15-24) rather than ubiquinol.118 In chloroplasts of green plants photochemically reduced plastoquinone is oxidized by a similar complex of cytochrome b, c-type cytochrome /, and a Rieske Fe-S protein.119 120a This cytochrome b6f complex delivers electrons to the copper protein plastocyanin (Fig. 23-18). 

Through the use of freeze-fracture and freezeetching techniques of electron microscopy, it is possible to see, embeddded in the thylakoid membranes, particles which may represent individual photosyn-thetic units (also called quantosomes).227 256-258 They are about 20 nm in diameter, and at least many of them presumably contain a reaction center surrounded by light-collecting chlorophyll-protein complexes. Others may represent the cytochrome b6f complex and... 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

If the reaction centers of photosystem I and photosystem II are segregated into separate regions of the thylakoid membrane, how can electrons move from photosystem I to photosystem II Evidently the plastoquinone that is reduced in photosystem II can diffuse rapidly in the membrane, just as ubiquinone does in the mitochondrial inner membrane. Plastoquinone thus carries electrons from photosystem II to the cytochrome b6f complex. Plastocyanin acts similarly as a mobile electron carrier from the cytochrome b f complex to the reaction center of photosystem I, just as cytochrome c carries electrons from the mitochondrial cytochrome bct complex to cytochrome oxidase and as a c-type cytochrome provides electrons to the reaction centers of purple bacteria (see fig. 15.13). 

In the Z scheme, photosystem II, the cytochrome b6f complex and photosystem I operate in series to move electrons from H20 to NADP+ and to create an electrochemical potential gradient for protons across the thylakoid membrane. In addition to this linear pathway, chloroplasts in some plant species may use a cyclic electron-transfer scheme that includes photosystem I and the cytochrome b6f... 

Electron flow through the cytochrome b6f complex results in proton translocation from the stroma to the thylakoid lumen. In addition, protons are released in the lumen when H20 is oxidized and are taken up from the stromal space when NADP+ is reduced. Protons move from the thylakoid lumen back to the stroma through an ATP-synthase, driving the formation of ATP. 

The electrons are now passed from PQH2 via the cytochrome bf complex (also called cytochrome b6f complex) to plastocyanin (PC). PC is a copper-containing protein that accepts electrons by the copper cycling between Cu2+ and Cu+ states ... 

Answer No NADPH is produced. Artificial electron acceptors can remove electrons from the photosynthetic system and stimulate 02 production. Ferricyanide competes with the cytochrome b6f complex for electrons and removes them from the system. Consequently, P700 (of photosystem I) does not receive any electrons that can be activated for NADP+ reduction. However, 02 is evolved because all components of photosystem II are oxidized (see Fig. 19-56). 

Answer Neither 02 nor NADPH is produced. At high [NADPH]/[NADP + ] ratios, electron transfer from reduced ferredoxin to NADP+ is inhibited and the electrons are diverted into the cytochrome b6f complex. These electrons return to P700 and ATP is synthesized by photophosphorylation. Because electrons are not lost from P700, none are needed from PSII. 

Britt, R. D., Sauer, K., Klein, M. P., Knaff, D. B., Kriauciunas, A., Yu, C. A., Yu, L., and Malkin, R., 1991, Electron spin echo envelope modulation spectroscopy supports the suggested coordination of two histidine ligands to the Rieske Fe-S centers of the cytochrome b6f complex of spinach and the cytochrome bcl complexes of Rhodospirillum rubrum, Rhodobacter sphaeroides, and bovine heart mitochondria. Biochemistry 30 1892nl901. 

Zhang H, Kurisu G, Smith JL, Cramer WA. A defined protein- 66. detergent-lipid complex for crystallization of integral membrane proteins The cytochrome b6f complex of oxygenic photosynthesis. Proc. Natl. Acad. Sci. U.S.A. 2003 100 5160-5163. 

G Mosser, C D rr, G Hauska and W Khibrandt (1994) A projection map of the cytochrome b6f complex from the spinach chloroplast membranes. International Congress on Electron Microscopy 1994, Les Editions de Physique 3 609-610... 

One at a time, QAH2 donates its 2 electrons to QB. Two additional stromal protons are used in the reduction of QB. Reduced QB (QBH2) donates its electrons to the cytochrome b6f complex. Finally, the cytochrome b6f complex donates its electrons to plastocyanin (PC), a mobile peripheral membrane protein. PC, a single-electron carrier, then transfers these electrons to P700 in PSI. 

However, in some circumstances (e.g., high light intensity) a ratio closer to 4 H+/2 e has been observed. With this ratio, approximately 1.3 ATP molecules are synthesized for each NADPH molecule. The reason for the ratio reduction is unknown. Some recent experimental evidence indicates that under high light intensity the cytochrome b6f complex fails to pump the additional 2 protons. 

FIG. 9 Dark-field scanning transmission electron microscopy image of an unstained cytochrome b6f complexed with the amphiphilic short poly anions A8-75 (a C8 modified poly(acrylic acid) of 8000). Dark bar tobacco mosaic virus added as an internal standard of diameter 18 nm. Dots complexes of diameter ca. 10-15 nm. Reprinted from Ref. 71. Copyright 1998 with permission from Elsevier.)... 

Tribet C., Mills D., Haider M., Popot J.-L. Scanning transmission electron microscopy study of the molecular mass of amphipol/cytochrome b6f complexes. Biochimie 1998 80 475-482. 

Plastoquinol interacts with the membrane-bound cytochrome b6f complex which contains cytochromes and iron-sulfur proteins. 

Plastoquinones QH is a molecule in the electron transfer system betweeen photosystem II (PSII) and photosystem I. The sequence of carriers is as follows Pheophytin -> QA -> QB -> QH -> Cytochrome b6f. Plastoquinone QH2 is the photo synthetic equivalent of coenzyme Q of the electron transport system. It moves freely in the thylakoid membrane, carry electrons between the photo system II complex and the cytochrome b6f complex (Figure 17.12). Electron transfer involving plastoquinones is shown here. Reduction of a plastoquinone creates a plastoquinol. 

Photosystems I and II, the cytochrome b6f complex, and ATP synthase (CFO-CFl) are all individual entities in the thylakoid membrane, but are not necessarily contiguous. Components that link the photosystems (plastoquinone in the lipid phase of the membrane and plastocyanin in the thylakoid lumen) are mobile. Figure 17.16 shows the arrangement of components of the two photosystems on the thylakoid membrane. Note that the interior membrane layers of the grana are rich in PSII, whereas the stroma lamellae are rich in PSI. 

Plastocyanin accepts electrons from cytochrome f of the cytochrome b6f complex and donates them to photosystem I. Plastocyanin contains a copper atom, which is reduced to Cu+ upon accepting an electron and Cu2+ upon losing the electron. 

The process depicted in Figure 17.12 of transferrring electrons from photosystem II (PSII) to photosystem I (PSI) and from water to PSII is called noncyclic electron flow. The generation of ATP by this process is called noncyclic photophosphorylation. An alternative pathway for the light reactions, called cyclic electron flow, utilizes the components of photosystem I, plastocyanin, and the cytochrome b6f complex. (Figure 17.17)... 

Cytochrome b6f is a is part of the electron transport chain that transfers electrons from photosystem II to photosystem I. Cytochrome b6f is a complex of proteins that includes cytochromes f, b6, and an iron sulfur protein. Cytochrome b6f accepts electrons from plastoquinone QH2 and passes them to plastocyanin (Figure 17.12). In addition to transferring electrons, the cytochrome b6f complex pumps protons into the thylakoid lumen, helping to build the proton gradient, which is used by the CFO-CFl complex to make ATP. 

Kurisu G, Zhang H, Smith J, and Cramer WA. Structure of the cytochrome b6f complex of oxygenic photosynthesis tuning the cavity. Science 2003 302 1009-1014. 

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