Investigations on the size and specificity of the plastoquinone pool of photosystem 2 membrane fragments

The quinone acceptor pool capacity of photosystem 2 (PS2) membrane fragments is diminished compared to that of intact thylakoids. The extent of quinone pool reconstitution was highly specific for the type of quinone molecules used. The far most efficient component was the natural plastoquinone-9 (PQ-9) that exhibited significant reconstitution effects at very low ratios of PQ-9 per PS2. In marked contrast, the widely used exogenous acceptor phenyl-p-benzoquinone (Ph-p-BQ) did not increase the pool capacity even at a molar Ph-p-BQ:PS2 ratio of 150:1. The samples reconstituted with PQ-9 provided a most suitable material for all experiments performed in the absence of exogenous artificial accepters in order to avoid interference by disturbing artefacts. The inherent potential of PQ-9 reconstituted PS2 membrane fragments was clearly demonstrated by monitoring the characteristic oscillation pattern of flash-induced oxygen evolution. These measurements permit for the first time unambiguous determination of the population of superreduced redox states of the water oxidising complex (WOC) in PS2 membrane fragments. A selective population of up to 90 % can be achieved for redox state S-2 by suitable treatment of PQ-9 reconstituted samples with NH2OH.