P680 is reduced by a redox-active tyrosine residue, specifically TyrZ, which in turn obtains electrons from the Mn4Ca2+O4 cluster of the oxygen-evolving complex (OEC).
Detailed Explanation
P680 is the primary electron donor in Photosystem II (PSII), a protein complex vital for oxygenic photosynthesis in plants, algae, and cyanobacteria. Upon absorbing light energy, P680 becomes photoexcited, leading to the separation of charge and the formation of P680•+ (its oxidized form). To continue the photosynthetic process, P680•+ must be reduced to regain its electron.
Here's a breakdown of the process:
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Oxidation of P680: Light energy excites P680, causing it to release an electron. This electron is passed along the electron transport chain. P680 now becomes P680•+, a strong oxidant.
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Role of TyrZ: A specific tyrosine residue, denoted as TyrZ (or Tyr161 in PsbA), acts as an intermediary electron carrier. TyrZ is located close to P680•+ within the PSII complex.
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Electron Transfer from TyrZ to P680•+: TyrZ donates an electron to P680•+, reducing it back to P680. This process neutralizes the positive charge on P680•+ and allows it to participate in another cycle of photoexcitation and electron donation.
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Replenishing TyrZ: The oxidized TyrZ (TyrZ•) then extracts electrons from the oxygen-evolving complex (OEC), also known as the water-splitting complex.
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Oxygen-Evolving Complex (OEC): The OEC is a cluster of four manganese ions, two calcium ions, and oxygen atoms (Mn4Ca2+O4). This cluster is responsible for oxidizing water molecules (H2O) to produce oxygen (O2), protons (H+), and electrons. The electrons from water oxidation are used to reduce TyrZ• back to TyrZ.
In summary, the reduction of P680•+ is an indirect process, relying on a chain of electron transfers that ultimately source electrons from water molecules, facilitated by the OEC and mediated by TyrZ.
