Reaction center, photosystem

Fig. 6. (A) Effect of composition of a binary soivent mixture on the apparent Chi a Aotal chlorophyli moiar ratio in extracts of spinach leaf tissue (0) and chloroplasts ( ). (B) relationship between the Chi a/Chl a molar ratio and the Chi a/P700 molar ratio for a number of P700-enriched subchloroplast particles by chloroform extraction (o) and by acetone extraction ( ). The solid line ciosest to the open circles is for Chi aVP700=1 and that nearest the filled circles for Chi aVP700= 2. See text for details. Figure source (A) Watanabe, Kobayashi, Maeda, Oba, Yoshida, Van de Meent and Amesz (1992) Function of the C13 -epimer chlorophylls in type I photosystem reaction centers. In N Murata (ed) Research In Photosynthesis, Vol III 4. Kluwer Acad PubI (B) Maeda, Watanabe, Kobayashi and Ikegami (1992) Presence of two chlorophyll a molecules at the core of photosystem I. Biochim Biophys Acta 1099 78.

T Watanabe, M Kobayashi, FI Maeda, T Oba, S Yoshida, EJ Van de Meent and J Amesz (1992) Function of the C13 -epimer chlorophylls in type I photosystem reaction centers. In N Murata (ed) Research in Photosynthesis, Vol III 3-10. KluwerAcad PubI... 

Specific numbers of photosystem reaction centers and electron transport rates for Synechococcus membranes, extracts and pellet (from extraction with 0.4% (w/w) SB 12)... 

Both PSI and PSII are necessary for photosynthesis, but the systems do not operate in the implied temporal sequence. There is also considerable pooling of electrons in intermediates between the two photosystems, and the indicated photoacts seldom occur in unison. The terms PSI and PSII have come to represent two distinct, but interacting reaction centers in photosynthetic membranes (36,37) the two centers are considered in combination with the proteins and electron-transfer processes specific to the separate centers. 

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,... 

Michel, H., Deisenhofer, J. Relevance of the photosynthetic reaction center from purple bacteria to the structure of photosystem II. BicKhemistry 27 1-7, 1988. 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e 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. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred 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 photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

Cluster Fx was also identified via its EPR spectral features in the RCI photosystem from green sulfur bacteria 31, 32) and the cluster binding motif was subsequently found in the gene sequence 34 ) of the (single) subunit of the homodimeric reaction center core (for a review, see 54, 55)). Whereas the same sequence motif is present in the RCI from heliobacteria (50), no EPR evidence for the presence of an iron-sulfur cluster related to Fx has been obtained. There are, however, indications from time-resolved optical spectroscopy for the involvement of an Fx-type center in electron transfer through the heliobacterial RC 56). 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

Lee I, Lee JW, Wamack RJ, Allison DP, Greenbaum E. Molecular electronics of a single photosystem I reaction center Studies with scanning tunneling microscopy and spectroscopy. Proc Natl Acad Sci USA 1995 97 1965-1969. 

Campbell D, Eriksson MJ, Oquist G, Gustafsson P, Clarke AK (1998) The cyanobacterium Synechococcus resists UV-B by exchanging photosystem II reaction-center D1 proteins. Proc... 

A recent study (Booth PJ, Crystall B, Giorgi LB, Barber J, Klug DR, Porter G (1990) Biochim. Biophys. Acta 1016 141) has shown that the free energy difference of the primary electron transfer is dominated by entropic contributions in photosystem II reaction centers as in bacterial reaction centers (Woodbury NWT, Parson WW (1984) Biochim. Biophys. Acta 767 345), so that the interpretation of the rate temperature dependenee should be revised. 

Photosystem 1 Reaction Center 4 bacteriochlorophylls, 2 bacteriopheophytins and a non-heme ferrous center [Fe(N-His) (0-Glu)] 131... 

Figure 1. Electron transfer in Photosystem I. The values plotted in the vertical direction are the mid-point potentials, and the numbers next to each arrow are the half-lives for electron transfer. P700 is the primary reaction center PsaA and PsaB are transmembrane proteins and PsaC is a peripheral cytoplasmic component. Courtesy of Professor Parag Chitnis, Iowa State University.

PHOTOSYNTHETIC REACTION CENTER PHOTOSYNTHETIC WATER OXIDATION PHOTOSYSTEM I pH profile for inactivation,... 

Fe proteins in nitrogenase, 38 252-254 photosystem I reaction center X proteins, 38 251-252... 

Rieke proteins, 47 337, 347-355 superoxide dismutases and, 45 129 Photosynthetic bacteria, 2[4Fe-4S] and [4Fe-4S] [3Fe-4S] ferredoxins, 38 255-257 Photosystem 1, 38 303-304 Pa/Fb proteins, 38 262-263 reaction center X proteins, single [4Fe-4S] ferredoxins cluster bridging two subunits, 38 251-252 Photosystem II, 46 328 interatomic separations, 33 228 mechanisms for water oxidation, 33 244-247... 

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