Photosynthetic electron transport in photosystem II

Photosystem II uses light energy to drive two chemical reactions: the oxidation of water and the reduction of plastoquinone. Five of redox components of PS II are known to be involved in transferring electrons from H2O to the plastoquinone pool: the water oxidizing manganese cluster (Mn)4, the amino acid tyrosine (Yz), the reaction center chlorophyll (P680), pheophytin, and two plastoquinone molecules, QA and Qb (Fig. 5). Tyrosine, P680, pheophytin (Pheo), QA, and QB are bound to two key polypeptides (Di and D2) that form the reaction center core of PS II and also provide ligands for the (Mn)4 cluster (Whitmarsh,

Fig. 4. Synthesis and structure of substituted N-phenylpyrazine-2-carboxamides (XIX).

1998). After primary charge separation between P680 (chlorophyll a) and pheophytin (Pheo), P680+/Pheo- is formed. Then electron is subsequently transferred from pheophytin to a plastoquinone molecule QA (permanently bound to PS II) acting as a one-electron acceptor.

PSII

Fig. 5. Scheme of the photosynthetic electron transport in photosystem II (PS II). (Taken from Photosystem II in http://www.bio.ic.ac.uk/research/barber/psIIimages/PSII.jpg with permission of Prof. Barber, Imperial College London).

From Qa- the electron is transferred to another plastoquinone molecule QB (acting as a two-electron acceptor); two photochemical turnovers of the reaction centre are necessary for the full reduction and protonation of QB. Because QB is loosely bound at the QB-site, reduced plastoquinone then unbinds from the reaction centre and diffuses in the hydrophobic core of the membrane and QB-binding site will be occupied by an oxidized plastoquinone molecule (Whitmarsh, 1998). Several commercial herbicides inhibit Photosynthetic elektron transport (PET) by binding at or near the QB-site, preventing access to plastoquinone (e.g. Oettmeier, 1992). Photosystem II is the only known protein complex that can oxidize water, which results in the release of O2 into the atmosphere. Oxidation of water is driven by the oxidized primary electron donor, P680+ which oxidizes a tyrosine on the D1 protein (Yz) and four Mn ions present in the water oxidizing complex undergo light-induced oxidation, too. Water oxidation requires two molecules of water and involves four sequential turnovers of the reaction centre whereby each photochemical reaction creates an oxidant that removes one electron. The net reaction results in the release of one O2 molecule, the deposition of four protons into the inner water phase, and the transfer of four electrons to the QB-site (producing two reduced plastoquinone molecules) (Whitmarsh & Govindjee, 1999). PET in chloroplasts can be estimated by electrochemical measurements of oxygen concentration using Clark electrode (PET through the whole photosynthetic apparatus is registered) or by spectrophotometric methods enabling the monitoring of PET through individual parts of photosynthetic apparatus. The site of action of PET inhibitors can be more closely specified by the use of chlorophyll fluorescence (e.g. Joshi & Mohanty, 2004) or by electron paramagnetic resonance (EPR) (e.g. Dolezal et al., 2001a).

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