PARENT SESSION

Symposium S6A Type II reaction centres: Excited state dynamics and donor side
Thursday September 2nd, 2004 10:20 AM-12:20 PM Room 511D
Chair: Bill Rutherford
Co-Chair: Marilyn Gunner

The carotenoid electron transfer pathway in photosystem II. Alison Telfer^*,1, ¹ Department Biological Sciences, London, UK

ABSTRACT- The extreme oxidising potential of the P680^.+/P680 redox couple (required to catalyse water splitting) results in unwanted oxidation reactions which damage the PSII reaction centre and the D1 protein in particular. This leads to turnover of the D1 protein and, in excess light when damage exceeds the rate of repair, to the phenomenon of photoinhibition. The core of Photosystem II binds a number of -carotene molecules, two of which are known to be specifically associated with the reaction centre (RC). Unlike the carotenoids in other photosynthetic reaction centres and light harvesting systems, these RC specific -carotenes do not quench chlorophyll triplet states or act as very efficient light harvesting pigments. They are, however, important structurally and do quench singlet oxygen offering some protection against oxidative damage. If electron donation from water is impaired electron transport around the PSII reaction center occurs via high potential cytochrome b559 and the -carotenes. A monomeric chlorophyll molecule, known as Chl_Z, can also be oxidised. The very high potential of the oxidised primary donor means that some carotenoid oxidation is inevitable. The carotene cation radical is very unstable and also unable to quench singlet oxygen, thus its repair by cytochrome b599_HP is essential if it is to be an efficient anti-oxidant. The most recent X-ray structure of PSII has identified a number of carotenoids in the PSII core including two which may well be the reaction centre specific -carotenes. The electron transfer pathways around PSII will be discussed in relation to this new structural information.

KEY WORDS: beta-carotene, triplet state, cytochrome b559, singlet oxygen