Oxidation reactions, Photosystem protonation

The oxidation of water to dioxygen occurs as the consequence of Photosystem 11-dependent generation of a very strong oxidant. Protons liberated by the water-oxidation reaction then contribute to the thylakoid transmembrane electrochemical gradient that drives ATP synthesis. Brudvig et aL describe how flash-induced proton-release measurements have resolved key steps that provide insights on how the 02-evolving center of PSll mediates this four-electron oxidation of water. 

COX is an extensive membrane-bound ensemble in which cytochrome a3 and Cub cooperate to form an oxygen reduction site and reduced cytochrome c containing iron(II) is oxidized through proximity to Cua- As a consequence of this reaction four protons are pumped across the membrane to set up a potential that helps to power ATP synthase. In plants, and some bacteria, the latter enzyme is serviced by another complex, membrane-bound protein ensemble, photosystem II. 

The electrons undergo the equivalent of a partial oxidation process ia a dark reaction to a positive potential of +0.4 V, and Photosystem I then raises the potential of the electrons to as high as —0.7 V. Under normal photosynthesis conditions, these electrons reduce tryphosphopyridine-nucleotide (TPN) to TPNH, which reduces carbon dioxide to organic plant material. In the biophotolysis of water, these electrons are diverted from carbon dioxide to a microbial hydrogenase for reduction of protons to hydrogen ... 

Figure 5-19. Schematic representation of reactions occurring at the photosystems and certain electron transfer components, emphasizing the vectorial or unidirectional flows developed in the thylakoids of a chloroplast. Outwardly directed election movements occur in the two photosystems (PS I and PS II), where the election donors are on the inner side of the membrane and the election acceptors are on the outer side. Light-harvesting complexes (LHC) act as antennae for these photosystems. The plastoquinone pool (PQ) and the Cyt b(f complex occur in the membrane, whereas plastocyanin (PC) occurs on the lumen side and ferredoxin-NADP+ oxidoreductase (FNR), which catalyzes electron flow from ferredoxin (FD) to NADP+, occurs on the stromal side of the thylakoids. Protons (H+) are produced in the lumen by the oxidation of water and also are transported into the lumen accompanying electron (e ) movement along the electron transfer chain.

The dark green complex [Co PhB(Bu lm)3 NHBu ] can further react with a radical proton abstractor (a phenol radical) to form the corresponding cobalt(Ill)-imido complex in a redox reaction that is reported to model a cracial step in the oxidation of water to elemental oxygen in the catalytic system of photosystem II [406] (see Figure 3.131). A similar iron(III) complex exists as well [415]. 

We can now estimate the overall stoichiometry for the light reactions. The absorption of 4 photons hy photosystem II generates 1 molecule of O2 and releases 4 protons into the thylakoid lumen. The 2 molecules of plastoquinol are oxidized hy the Q cycle of the cytochrome bf complex to release 8 protons into the lumen. Finally, the electrons from 4 molecules of reduced plastocyanin are driven to ferredoxin by the absorption of 4 additional photons. The 4 molecules of reduced ferredoxin generate 2 molecules of NADPH. Thus, the overall reaction is ... 

Figure 1. The major transmembrane photosynthetic reaction centers (RC) (top) and respiratory complexes (bottom) are composed of light (zigzag) activated chains (dark gray) of redox centers (open polygons) that create a transmembrane electric field and move protons (double arrows) to create a transmembrane proton gradient, fulfilling the requirements of Mitchell s chemiosmotic hypothesis. Diffusing substrates include ubiquinone (hexagon) and other sources of oxidants and reductants. PSI and PSII, photosystems I and II, respectively.

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