Reaction center electron transport

The thylakoid membranes contain the energy-transducing machinery light-harvesting proteins, reaction centers, electron-transport chains, and ATP synthase. They have nearly equal amounts of lipids and proteins. The lipid composition is highly distinctive about 40% of the total lipids are galactolipids and 4% are sulfolipids, whereas only 10% are... 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

The quantum yield of photosynthesis, the amount of product formed per equivalent of light input, has traditionally been expressed as the ratio of COg fixed or Og evolved per quantum absorbed. At each reaction center, one photon or quantum yields one electron. Interestingly, an overall stoichiometry of one translocated into the thylakoid vesicle for each photon has also been observed. Two photons per center would allow a pair of electrons to flow from HgO to NADP (Figure 22.12), resulting in the formation of 1 NADPH and Og. If one ATP were formed for every 3 H translocated during photosynthetic electron transport, 1 ATP would be synthesized. More appropriately, 4 hv per center (8 quanta total) would drive the evolution of 1 Og, the reduction of 2 NADP, and the phosphorylation of 2 ATP. 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

The participation of the phycobiliproteins in the absorption ofphotokinetically active light has been demonstrated above. Peaks of around 565 and 615 nm in the action spectra indicate the involvement of C-phycoerythrin andC-phycocanin. These pigments transfer energy to the reaction center of PS II and suggest the participation of the non-cyclic electron transport and coupled phosphorylation. 

Attaching the catalyst molecules to the electrode surface presents an obvious advantage for synthetic and sensor applications. Catalysis can then be viewed as a supported molecular catalysis. It is the object of the next section. A distinction is made between monolayer and multilayer coatings. In the former, only chemical catalysis may take place, whereas both types of catalysis are possible with multilayer coatings, thanks to their three-dimensional structure. Besides substrate transport in the bathing solution, the catalytic responses are then under the control of three main phenomena electron hopping conduction, substrate diffusion, and catalytic reaction. While several systems have been described in which electron transport and catalysis are carried out by the same redox centers, particularly interesting systems are those in which these two functions are completed by two different molecular systems. 

The cytochrome b(6)f complex mediates electron transfer between the PSI and PSII reaction centers by oxidizing hpophUic plastoquinol (PQH2) (see Figure 7.24) and reducing the enzymes plastocyanin or cytochrome Ce. The electronic connection also generates a transmembrane electrochemical proton gradient that can support adenosine triphosphate (ATP) synthesis instead of electron transport. 

The Fe-S Reaction Center (Type I Reaction Center) Photosynthesis in green sulfur bacteria involves the same three modules as in purple bacteria, but the process differs in several respects and involves additional enzymatic reactions (Fig. 19-47b). Excitation causes an electron to move from the reaction center to the cytochrome bei complex via a quinone carrier. Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP synthesis, just as in purple bacteria and in mitochondria. 

Ubiquinones function as electron transport agents within the inner mitochondrial membranes496 and also within the reaction centers of the photosynthetic membranes of bacteria (Eq. 23-32).484/488/494 The plasto-quinones also function in electron transport within... 

Many cytochromes c are soluble but others are bound to membranes or to other proteins. A well-studied tetraheme protein binds to the reaction centers of many purple and green bacteria and transfers electrons to those photosynthetic centers.118 120 Cytochrome c2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure.121 Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter 23).120,122 124 Some bacterial membranes as well as those of mitochondria contain a cytochrome bct complex whose structure is shown in Fig. 18-8.125,126... 

V). The centers resemble PSII of chloroplasts and have a high midpoint electrode potential E° of 0.46 V. The initial electron acceptor is the Mg2+-free bacteriopheophytin (see Fig. 23-20) whose midpoint potential is -0.7 V. Electrons flow from reduced bacteriopheophytin to menaquinone or ubiquinone or both via a cytochrome bct complex, similar to that of mitochondria, then back to the reaction center P870. This is primarily a cyclic process coupled to ATP synthesis. Needed reducing equivalents can be formed by ATP-driven reverse electron transport involving electrons removed from succinate. Similarly, the purple sulfur bacteria can use electrons from H2S. 

Figure 23-32 Simplified diagram of cyclic electron flow in purple bacteria. Two protons from the cytoplasm bind to QB2 in the reaction center to form QH2 (ubiquinol), which diffuses into the ubiquinone pool. From there it is dehydrogenated by the cytochrome kq complex with expulsion of two protons into the periplasm. A third and possibly a fourth proton may be pumped (green arrows) across the membrane, e.g., via the Q cycle (Fig. 18-9). The protons are returned to the cytoplasm through ATP synthase with formation of ATP. Some electrons may flow to the reaction centers from such reduced substrates as S2 and some electrons may be removed to generate NADPH using reverse electron transport.345...

The pathways involved in cyclic photophosphorylation in chloroplasts are not yet established. Electrons probably flow from the Fe-S centers Fdx, Fda, or Fdb back to cytochrome b563 or to the PQ pool as is indicated by the dashed line in Fig. 23-18. Cyclic flow around PSII is also possible. The photophosphorylation of inorganic phosphate to pyrophosphate (PP ) occurs in the chromatophores (vesicles derived from fragments of infolded photosynthetic membranes) from Rho-dospirillum rubrum. The PP formed in this way may be used in a variety of energy-requiring reactions in these bacteria.399 An example is formation of NADH by reverse electron transport. 

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