Plant photosystem II

In green plants, algae, and cyanobacteria, the primary photochemical events of photosynthesis occur in the protein-pigment complex called photosystem II (PSII). PSII consists of more than ten polypeptide chains and a number of co-factors important for electron transport.(i, 6) The co-factors are believed bound to two homologous polypeptides approximately 32 kD in size (D1 and D2). Photoexcitation of the PSD reaction center drives single electron transfer from the primary electron donor, P, (probably a dimer of chlorophyll a) to the primary electron acceptor, one of two pheophytin a molecules. The reduced pheophytin transfers the electron on to a primary plastoquinone 

The reaction center thus evolves the oxygen necessary for assimilating CO in the form of NADPH and pumps protons from the matrix to the inside of the thylakoid membrane. The proton gradient drives subsequent energy storage by adenosine triphosphate (ATP) synthesis and the ATP is later used in carbon-fixation reactions to make carbohydrates. 

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The reaction center of plant photosystem II, P680, passes electrons to plastoquinone, and the electrons lost from P680 are replaced by electrons from H20 (electron donors other than H20 are used in other organisms). 

K. H. Rhee, E.P. Morris, J. Barber, and W. Kuhibrandt. 1998. Three-dimensional structure of the plant photosystem II reaction centre at 8 A resolution Nature 396 283-286. (PubMed)... 

Photosynthetically active quinones include plastoquinone of green-plant photosystem II, ubiquinone and menaquinone in photosynthetic bacteria, and phylloquinone in photosystem I. Plastoquinone is present in green-plant photosystem II both as a tightly-bound and a loosely-bound electron carrier, designated Qa and Qb, respectively. Qa is photoreduced only to the semiquinone (PQ ) but Qb can accept two electrons, forming the plastohydroquinone (PQ-Hj) [see Chapters 5, 6 and 16 for further discussion]. Plastohydroquinone PQb H2 is the final reduction product of photosystem II and goes on to reduce the cytochrome bj complex as part of the electron transport and proton translocation processes [see Chapter 35 for detailed discussions]. 

Fig. 11. Electron-transfer schemes for the reaction centers of the green sulfur bacteria (A) and green filamentous bacteria (B). The reaction-center components of the green sulfur bacteria are compared to green-plant photosystem I and those of the green filamentous bacteria are compared to green-plant photosystem II or purple bacteria. The decay times and redox potentials are for Prosihecochloris aestuariiand Chloroflexus aurantiacus. See text lor discussion. Figure adapted from Amesz (1987)...

DB Knaff (1975) The effect of pH on the midpoint oxidation-reduction potentials of components associated with plant photosystem II. FEBS Lett 60 331-335... 

T-A Ono and Y Inoue (1988) Discrete extraction ofCa atom functional for02 evolution in higher plant photosystem II by a simple low pH treatment. FEES Lett 227 147-152... 

In addition, the recent demonstration of the 32kDa protein as an essential component of the higher plant photosystem II reaction center (6,7) has added a new dimension to this interest with still broader implications for crop improvement in agriculture. 

Rhee KH, Morris EP, Zheleva D, Hankamer B, Kiihlbrandt W and Barber J (1997) Two dimensional structure of plant Photosystem II at 8 A resolution. Nature 389 522-526 Rock CD and Zeevaart JAD (1991) The aba mutant of Arabidopsis thaliana is impaired in epoxy-carotenoid biosynthesis. Proc Natl Acad Sci USA 88 7496-7499 Rock CD, Bowlby NR, Hoffmann-Benning S and Zeevaart JAD (1992) The aba mutant of Arabidopsis thaliana (L) Heynh. has reduced chlorophyll fluorescence yields and reduced thylakoid stacking. Plant Physiol 100 1796-1801 Romer S, Humbeck K and Senger H (1990) Relationship between biosynthesis of carotenoids and increasing complexity of Photosystem I in mutant C-6D of Scenedesmus obliquus. Planta 182 216-222... 

The j3-carotene radical cation can be induced in RCs of plant Photosystem II (PS II) by illumination of ferricyanide-treated BB Y-type RC particles (Noguchi et al., 1994). The radical was observed by FTIR... 

Peter GF and Thornber P (1991) Biochemical composition and organisation of higher plant Photosystem II light harvesting proteins. J Biol Chem 266 16745-16754... 

Rhee K-H, Morris EP, Zhaleva D, Hankamer B, Kiihlbrandt W, and Barber J (1997) Two-dimensional structure of plant Photosystem II at 8 A resolution. Nature 389 522-526... 

Ono T-A, Inoue Y. Roles of Ca in O2 evolution in higher plant photosystem II effects of replacement of Ca site by other cations Arch Biochem Biophys 1989 275 440-448. 

Santini C, Tidu V, Tognon G et al. Three-dimensional structure of the higher-plant photosystem II reaction center and evidence for its dimeric organization in vivo. Eur J Biochem 1994 221(1) 307-315. 

Recent advances in the biochemistry of plant photosystem II (PSII) have led to the proposal that the complex composed of the D1 (32 kDa), D2 (34 kDa), and ochroine bssg(4 and 9 kDa) polypeptides constitutes the reaction center core (1). This complex, though similar in many respects to the reaction center complex of purple bacteria, has unique characteristics. Chief among these is its pigment stoichiometry. Unlike the bacterial reaction center complex, which binds six chlorin molecules, the Dl-D2-cytochrome bjjg complex has as many as 13 chlorin molecules (2). 

Recent EPR studies have shown that the amino acid tyrosine participates in a number of biological electron transfer reactions, including the oxidation of water to Oj in plant photosystem II, the reduction of Oj to water in cytochrome c oxidase, and the reduction of ribonucleotides to deoxyribonucleotides catalyzed by the enzyme ribonucleotide reductase. During the course of these electron transfer reactions, a tyrosine radical forms (4). The center of the EPR spectrum of the tyrosine radical in cytochrome c oxidase of the bacterium P. denitrificans occurs at 344.50 mT in a spectrometer operating at 9.6699 GHz (radiation belonging to the X band of the microwave region). Its -value is therefore... 

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