Oxygen-evolving complex electron transfer

Because the four protons produced in this reaction are released into the thylakoid lumen, the oxygen-evolving complex acts as a proton pump, driven by electron transfer. The sum of Equations 19-12 through 19-15 is... 

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The oxygen-evolving complex, responsible for the transfer of electrons from... 

Fig. 4.7. The Z scheme of the higher plant photosynthetic chain visualized as an arrangement of multiprotein complexes. The two RC of PSII and PSI are arranged in parallel across the membrane and are intereonnected by the b /f complex. The electron transfer pathway within this complex follows a modified Q cycle scheme analogous to that proposed for bacterial photosynthesis. The oxygen-evolving complex is proposed to face the inner thylakoid lumen and to release protons in this compartment. The association of cytochrome 6-559 with PSII-RC and the cyclic role of ferredoxin are also depicted. Proton-binding and proton-releasing sites are illustrated (from Ref. 93).

S2, S3, and S4. S is the most reduced state and S4 is the most oxidized state of the complex. It is now believed that the Mn cluster, a group of four manganese atoms bound to MSP near the PSII reaction center, is mostly responsible for these transitions. As the oxygen-evolving complex abstracts electrons and protons from HzO, it cycles through the five oxidation states. The electrons are transferred one at a time to a tyrosine residue on the Dj polypeptide and then to the P680 reaction center. The protons released in the process remain in the thylakoid lumen, where they contribute to the pH gradient that drives ATP synthesis. 

The oxidation of two molecules of H2O to form O2 requires the removal of four electrons, but absorption of each photon by PSIl results in the transfer of just one electron. A simple experiment, described in Figure 8-39, resolved whether the formation of O2 depends on a single PSIl or multiple ones acting in concert. The results indicated that a single PSIl must lose an electron and then oxidize the oxygen-evolving complex four times in a row for an O2 molecule to be formed. 

Oxygen produced in photosynthesis comes from water. The oxygen-evolving complex is part of the series of electron-transfer reactions from water to NADPH. Carbon dioxide is involved in the dark reactions, which are different reactions that take place in another part of the chloroplast... 

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