Structural Organization of the PS-I Rxn Center

1)Components

2)Stoichiometry

3)Physical Organization

4)Final electron acceptors

 

 

In order to oxidize water, P680+ must generate an oxidizing potential > than +0.8 V.  Its midpoint potential is estimated to be ~1.2 V, a very high oxidizing potential.  This must arise from the particular binding environment of P680 in the PSII complex.  It is thought that the D1/D2 heterodimer polypeptides of PSII ligate the chlorophyll a dimer and play a key role in this process.  In the excited singlet state, P680* transfers an e^- to a molecule of pheophytin a (Ph) generating a radical pair P680⁺Ph^-.  The time constant for this reaction is 3- 21 ps.  The Ph acceptor appears to be bound to the D1 subunit.  The P680⁺Ph^- radical pair is in equilibrium with the excited state ¹P680*Ph and normally decays by e^- transfer from Ph^- to a molecule of plastoquinone (PQ) called Q_A with a half time of ~300 ps.  The PQ is a very hydrophobic molecule and apparently moves in the thylakoid membrane between PSII and the cytochrome b₆/cytochrome f complex.  A schematic representation of the major structural and functional components of PSII are shown in Figs. 2.7 and 2.8 of Plant Biochemistry `97 by Dey and Harborne on reserve and Figs. 3.18 & 3.22 of the Heldt `97 text:

The normal reductant for P680⁺ is a tyrosine residue on the D1 protein, Tyr161, referred to as Y_z.  The water-oxidizing active site is a cluster of 4 Mn^+3/+4 atoms bound to the luminal side of the PSII complex apparently ligated to the D1 and D2 proteins.  The water-splitting Mn cluster re-reduces Y_z⁺.  A folding model of the D1 protein in the thylakoid is given in Fig. 2.10 of the Dey and Harborne text.  Note the 5 membrane spanning a-helices, the Y_z donor to P680, the His 198 ligand to P680, His 215 & 272 non-heme Fe ligands and the D, H, E, D, and A amino acid residues implicated in Mn ligation.

 

 

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All materials © 1998, 1999, 2000, Dr. David Hildebrand or Dr. Bob Houtz, unless otherwise noted.
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This page was last modified January 17, 2000.