Lesson plan
Oxygen-evolving complex
The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the active site of a water-oxidizing enzyme involved in the photo-oxidation of water during the light reactions of photosynthesis. OEC is surrounded by 4 core proteins of photosystem II at the membrane-lumen interface. The mechanism for splitting water involves absorption of three photons before the fourth provides sufficient energy for water oxidation. Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states, denoted S 0 to S 4 , with S 0 the most reduced and S 4 the most oxidized. Energy from the photons captured by photosystem II moves the system from state S 0 to S 1 to S 2 to S 3 and finally to S 4 . S 4 reacts with water producing free oxygen: : 2H2O -> O2 + 4H+ + 4e- This conversion resets the catalyst to the S 0 state. The active site of the OEC consists of a cluster of manganese and calcium with the formula Mn 4 Ca 1 O x Cl 1–2 (HCO 3 ) y . This cluster is bound to D 1 and CP 43 subunits and stabilized by peripheral membrane proteins. Many characteristics of it have been examined by flash photolysis experiments, electron paramagnetic resonance (EPR), and X-ray spectroscopy. The mechanism of the complex is proposed to involve an Mn-oxide which couples by O-O bond formation to a calcium oxide/hydroxide.
Article summary
The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the active site of a water-oxidizing enzyme involved in the photo-oxidation of water during the light reactions of photosynthesis. OEC is surrounded by 4 core proteins of photosystem II at the membrane-lumen interface. The mechanism for splitting water involves absorption of three photons before the fourth provides sufficient energy for water oxidation. Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states, denoted S 0 to S 4 , with S 0 the most reduced and S 4 the most oxidized. Energy from the photons captured by photosystem II moves the system from state S 0 to S 1 to S 2 to S 3 and finally to S 4 . S 4 reacts with water producing free oxygen: : 2H2O -> O2 + 4H+ + 4e- This conversion resets the catalyst to the S 0 state. The active site of the OEC consists of a cluster of manganese and calcium with the formula Mn 4 Ca 1 O x Cl 1–2 (HCO 3 ) y . This cluster is bound to D 1 and CP 43 subunits and stabilized by peripheral membrane proteins. Many characteristics of it have been examined by flash photolysis experiments, electron paramagnetic resonance (EPR), and X-ray spectroscopy. The mechanism of the complex is proposed to involve an Mn-oxide which couples by O-O bond formation to a calcium oxide/hydroxide.
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Discussion questions
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