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  1. 1. Plant Photosynthetic Reaction Centers
  2. 2. In plants, algae and cyanobacteria: Oxygenic and anoxygenic photosynthesis 2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2 carbon dioxide + water + light energy → carbohydrate + oxygen Today, the average rate of energy capture by photosynthesis globally is ~130 terawatts which is about six times larger than the current power consumption of human civilization.
  3. 3. However, there are some types of bacteria that carry out anoxygenic photosynthesis, which consumes carbon dioxide but does not release oxygen. CO2 + (AsO3 3–) + photons → (AsO4 3–) + CO carbon dioxide + arsenite + light energy → arsenate + carbon monoxide (used to build other compounds in subsequent reactions) Examples of organisms belonging to anoxygenic photosynthesis are the filamentous green bacteria, the green sulfur bacteria, the purple bacteria and the heliobacteria. The basic difference between oxygenic and bacterial photosynthesis is that bacteria have only one type of reaction center, whereas oxygenic species have two (Photosystem I and II). These photosynthetic bacteria can use light energy to extract electrons from molecules other than water such as sulfur compounds or simple organic molecules.
  4. 4. Green plants and algae have two different types of reaction centers that are part of larger supercomplexes known as photosystem I (P700) and photosystem II (P680). Transforming light energy into charge separation
  5. 5. The purple bacterial reaction center shares a striking similarity to Photosystem II of plants and cyanobacteria. Photosystem II
  6. 6. The overall reaction catalyzed by photosystem II is:
  7. 7. The colors of photosynthesis The light-absorbing molecules include green chlorophylls and orange carotenoids.
  8. 8. Photosystem II: the reaction center
  9. 9. Photosystem II: harvesting light Photosystems have large antennas of light-absorbing molecules that harvest light and transfer their energy inwards to the reaction center.
  10. 10. The oxygen-evolving center strips an electron from water and passes it to a tyrosine amino acid, which then delivers it to the chlorophyll, making it ready to absorb another photon. It contains cluster of manganese ions (magenta), calcium (cyan), chloride ion and oxygen atoms (red). It grips two water molecules and removes four electrons, forming oxygen gas and four hydrogen ions. It is surrounded by histidines, aspartates and glutamates. Photosystem II: oxygen-evolving center
  11. 11. Photosystem II: Synechococcus elongatus structure The lumenal side reaction center proteins (D1 & D2): A through E. The peripheral antenna proteins (CP43 & CP47): six membrane spanning α-helices. The PsbO protein: the water splitting enzyme of photosynthesis. PsbV (cytochrome c-550): plays role in the oxygen-evolving complex of photosystem II.
  12. 12. Because of the oxidation of water and the release of dioxygen, the Photosystem II reaction center does not operate in a cycle. Bacterial RC vs Photosystem II The released protons from water oxidation are deposited on one side of the thylakoid membrane to help build the electrochemical gradient. In the bacterial reaction centre, the electron is obtained from a reduced compound haem group in a cytochrome subunit or from a water-soluble cytochrome-c protein. In addition, Photosystem II differs from the bacterial reaction centre in that it has many additional subunits that bind additional chlorophylls to increase efficiency.
  13. 13. Cytochrome b6f complex It catalyzing the transfer of electrons from plastoquinol to plastocyanin. Crystal structure from C. reinhardtii Each monomer consists of four large subunits: cytochrome f (32 kDa), a cytochrome b6 (25 kDa), a 19 kDa Rieske iron-sulfur protein (19 kDa), subunit IV (17 kDa); four small subunits (3-4 kDa): PetG, PetL, PetM, and PetN. The total molecular weight is ~217 kDa.
  14. 14. Reaction mechanism of cytochrome b6f complex The cytochrome b6f complex is responsible for ‘non-cyclic’ and ‘cyclic’ electron transfer between two mobile redox carriers, plastoquinone (QH2) and plastocyanin H2O Photosystem II  (1) QH2  Cyt b6f  Pc  Photosystem I  NADPH (2) QH2  Cyt b6f  Pc  Photosystem I  Q
  15. 15. Plastocyanin Plastocyanin (Cu2+Pc) is reduced by cytochrome f according to the following reaction: Cu2+Pc + e- → Cu+Pc After dissociation, Cu+Pc diffuses through the lumen until recognition/binding occurs with P700+ which oxidizes Cu+Pc according to: Cu+Pc → Cu2+Pc + e-