Probing of Apoprotein Function in the Photosystem II Reaction Centre by Proteolytic Modification
The photochemically active pigments of photosystem II (PSII) are housed in an apoprotein environment provided by the Dl and D2 polypeptides which also contain binding sites for the acceptor quinones (QA and QB). The organisation of the polypeptides and chromophores has been inferred (1–3) from various similarities and homologies between PSII reaction centres and the photosynthetic reaction centres of purple bacteria, such as Rhodopseudomonas viridis, which have been structurally resolved in considerable detail (e.g. 4). By analogy with the L and M polypeptides of bacterial reaction centres, Dl and D2 each contain 5 transmembrane helical spans. There is strong sequence homology between all four polypeptides in Helix 4, which contains ligands for the (bacterio) chlorophyll photochemical electron donor (“special pair”) and a non-haem ferrous ion. QA and QB appear to be hydrogen bonded by residues in Helix 4 and the connection between Helices 4 and 5 of Dl/L and D2/M respectively. The two quinone binding sites serve slightly different functions. The QA site is permanently filled by quinone which undergoes photochemical reduction to semiquinone by pheophytin, and reoxidation by QB. The QB quinone, on the other hand, is released as a quinol following its two-electron reduction by a double turnover of QA. The emtpy QB site is then refilled by an oxidised quinone molecule. Other molecules are able to displace the quinone from the QB site and thereby block electron flow. These include herbicides such as dichlorophenyl dimethylurea (DCMU) which inhibits PSII, and certain triazine herbicides, which are effective on both PSII and bacterial photosystems.
KeywordsPSII Reaction Centre Photosynthetic Reaction Centre Triazine Herbicide Bacterial Reaction Centre Quinone Binding Site
Unable to display preview. Download preview PDF.
- 1.Trebst, A. (1986) Z. Naturforsch. 41c, 240–245Google Scholar
- 13.Renger, G., Hagemann, R. and Vermaas, W.F.J. (1984) Z. Naturforsch. 39c, 362–367Google Scholar
- 15.Vermaas, W.F.J. (1985) Ph.D. Thesis, Agricultural Univ. WageningenGoogle Scholar
- 16.Telfer, A., Marder, J.B. and Barber, J. (1989) In: Photosynthesis: Molecular Biology and Bioenergetics (Eds. Singhal, G.S. et al.), Narosa Pubi. House, New Delhi, pp 175–188Google Scholar