Photosystem II

Model of the photosystem II core from Jonathan Nugent

The structure is a model (1) based on use of the bacterial reaction center as a template. The authors have added residues in "missing" loops, and refined the structure by a combination of computational approaches.

In the initial view, the protein is oriented with the acceptor side (stroma facing domain) up, and the donor-side (lumen facing domain) down. The D1 subunit is colored cyan, and the D2 subunit yellow. The chromophores are shown as dark green spacefilling models.

The chromophores

Chlorophylls (Chl) of the special pair (red), and the ancillary pair (cyan), pheophytins (Ph) (magenta), plastoquinones (Q_A, yellow; Q_B, orange), Fe (dark green). The white molecule is a bicarbonate which is thought to be involved in H⁺ processing at the Q_B-site. The Mn-cluster involved in oxygen evolution is shown in green.

The protein

The protein added as a gray wireframe model.
The protein as a spacefilling model to show polarity of surface. Hydrophobic residues are colored white, neutral polar green, acidic red, basic blue. Prosthetic groups are magenta wireframe models, except Mn cluster (cyan).
The protein as a ribbon model with D1 colored cyan and D2 colored yellow.

The donor side

Stop the rotation before going on

Zoom in on the Mn-cluster, and show residues shown by site-directed mutation to be involved in the donor side mechanism (Y161, D170, F186, E189, H190, N191; colored by type - see above left. Labels are at C_b-atoms).

The acceptor side

Zoom in on the acceptor side. The residues shown are those within 7.2 Å of the plastoquinone. Residues in color are changed in herbicide resistant mutants of green plants or Chlamydomonas, putative ligands, or residues which change the kinetics of electron transfer at the Q_B-site on mutation (H215, V219, A251, H252, F255, G256, R257, S264, H272, L275, - color-coded by amino acid type). The plastoquinone Q_B is colored orange, Fe green.

(See panel at top left for amino acid color coding)

Ruffle, S.V., Donnelly, D., Blundell, T.L., and Nugent, J.H.A. (1993) A three-dimensional model of the photosystem II reaction center in Pisum sativum (Pea). Photosytnhesis Research (in press).