Cofactors of Photosystem II

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Chlorophyll a. Many Chlorophyll a surround P680, the reaction center. Chlorophyll a passes energy from light to P680 which when excited loses an electron to Pheo. ⁽¹⁾

Pheophytin a. Pheophytin a (Pheo) is the primary electron acceptor in PSII (2). Excited P680 passes a single unpaired electron to Pheo to form P680+ and Pheo-,a radical pair. Pheo passes the electron it received onto a plastoquinone, PQa. ⁽¹⁾

Fe. An electron is passed from plastoquinone (PQa) to Fe to PQb ⁽¹⁾.

Tyrosine. Trysine residue, D1-Tyr161, participates in the transfer of an electron from the Mn cluster to reduce the P680 center ⁽¹⁾.

Mn. Mn acts as a charge storage device for 4 electrons which come from water. After the electrons are removed in single electron steps O2 is released. Mn is always ready for the reduction of tyrosine. ⁽¹⁾

Mn and Tryosine. The oxidixed Tyr receives electrons from the Mn cluster ⁽¹⁾.

Reaction Center. These cofactors are the ones proposed to be a part of the reaction center. In addition to these, several Beta-carotenes as well as Qa and Qb are involved. ⁽¹⁾

Other Cofactors of PSII

Cd

Protoporphyrin ix containing fe [ Heme ]

2-[(3-hydroxy-2-methyl-5-phosphonooxymethyl -pyridin-4- ylmethyl)-amino]-2-methyl-succinic acid [ N-pyridoxyl-2-methylaspartic acid-5-monophosphate ] (PLA)

A Closer View ⁽³⁾

Zoom to Chlorophyll a, Nitrogen in Blue, Mg in Green

Zoom to Pheophytin a, Nitrogen in Blue

Zoom to Tyrosine

Zoom to Protoporphyrin ix containing Fe [ Heme ]

Zoom to PLA

1. http://www.bio.ic.ac.uk/research/barber/psIIimages/PSII.html

2. http://www.life.uiuc.edu/govindjee/Part2/23_Klimov.pdf

3. http://www.biochem.ucl.ac.uk/bsm/pdbsum/1fe1/main.html

Britt Peterson and Megan Dunlavey