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D4. Photosystem II

The net reaction carried out by PS2 is the oxidation of water and reduction of plastoquinone.

\[\mathrm{PQ  +  H_2O  \rightarrow  PQH_2 +  O_2\, (g)}\]

Note that water is not converted to 2H2 +  O2  , as in the electrolysis of water.  Rather the Hs are removed from water as protons in the lumen of the cholorplast, since the part of PSII which oxides water is near the lumenal end of the transmembrane complex.   Protons required to protonated the reduced (anionic) form of plastaquinone to form PQH2, an activity of PSII found closer to the stroma, derive from the stroma.  which then can be used to protonated the "anionic" form of reduced PQ to form PQH2.

A quick look at standard reduction potentials shows that the passing of electrons from water (dioxygen  SRP = +0.816 V) to plastoquinone (approx  SRP of  0.11 ) is not thermodynamically favored.  The process is driven thermodynamically by the energy of the absorbed photons.

An early crystal structure of PSII from a photosynthetic cyanobacterium showed that it consists of 17 polypeptide subunits with metal and pigment cofactors and over 45,000 atoms.  (Zouni, Nature, 409, 739, 2001).  Of particular interest is the P680 chlorophyll reaction center, which consists of four monomeric chlorphylls adjacent to a cationic Tyr-D side chain which destabilizes the chlorophyll molecules.  When H2O gets oxidized to form dioxygen, 4 electrons must be remove by photoactivated P680.  In PSII, this process occurs in 4, single electron steps, with the electrons first being transferred to a Mn4 cluster cofactor (of composition Mn4Ca1Cl1-2(HCO3)y.   This inorganic Mn cluster is often called OEC (oxygen evolving complex) or WOC (water oxidizing complex).  The electrons passed through the Mn complex are delivered to P680 by a photoactive Tyr free radical (Tyr Z).  The actual structure of the OEC could not be resolved in this structure, but other structural and spectroscopic data support the structure below (Chem. Rev., 2001, 101, 21-35), which also shows a possible mechanism for electron and proton transfer from water to form dioxygen.  5 discrete intermediates of the OEC, S0-S4, are suggested from the experimental data (Kok cycle).  These were postulated  from experiments in which spinach chloroplast were illuminated with short light pulses.  A pattern of dioxygen release was noted that repeated after 4 flashes.  Ultimately, light absorption by P680 forms excited state P680* which donates an electron to pheophytin (which passes them to quinones) to form P680+, which receives electrons from the OEC, specifically the TyrZ radical.   As we will see in a bit, this structure and mechanism has been called into question by new crystallographic structures.

Figure:   OEC - Mechanism of Water Oxidation

Investigators have made non-peptide mimetics of superoxide dismutase to facilitate therapeutic removal of excess superoxide formed in brain and heart tissue.    These may arise  after an oxidative burst from reperfusion of these tissues after heart or brain attacks.  Likewise, scientist are trying to build synthetic PSII-OEC complexes which could be used to form dioxygen or hydrogen gas for fuels. 

In summary, for PSII in plants

  1. a pair of chlorophylls (P680) in the D subunits absorb light (maximum absorbance around 680 nm) and reach an excited state
  2. electron transfer from P680 to a nearby chlorophyll with a lower energy level for the excited state electron occurs.  This chlorophyll has 2 H+ ions in the chlorophyll instead of Mg2+occurs.  The P680 now becomes cationic, P680+.
  3. This "anionic" chlorophyll transfers an electron to oxidized plastoquinone. 
  4. The P680+, a strong oxidizing agent, removes one electron from H2O-OEC complex. 

Steps 1-4 repeat three more times, each requiring another photon and each cycle producing another electron which passes on through the system.  Remember that when O2 acts as an oxiding agent, it gains four electrons.  The first produces superoxide, the next peroxide, and two more produce oxide which when protonated is water.  Hence two waters and four cycles are required to remove the four electrons required to produce dioxygen.

A similar mechanism is found in PSI, except plastacyanin, not dioxygen is oxidized, with electrons moved to ferrodoxin.  This is likewise a difficult process since the reduction potential for  oxidized plastocyanin (the form that can act as a reducing agent) is +0.37  while for ferrodoxin it is -0.75.  This transfer of electrons is an uphill thermodynamic battle since the more positive the standard reduction potential, the better the oxidizing agent and the more likely the agent becomes reduced.  What drives this uphill flow of electrons.  Of course, it is the energy input from the photon.

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