EFFECTS OF PH AND CHEMICALS AFFECTING POLYPEPTIDE CONFORMATION ON INHIBITION OF REDUCING AND OXIDIZING SIDES OF PHOTOSYSTEM-2 BY DCMU IN PEA-CHLOROPLASTS

Inhibition of photosystem 2 (PS2) reducing side in pea chloroplasts by 10(-8) M DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea], resulting fr om its binding at the secondary quinone acceptor site is manifested by suppression of millisecond time range delayed fluorescence (DF). In c oncentrations higher than 10(-4) M DCMU binds to another site in PS2 a nd inhibits the oxidizing side, thus leading to suppression of DF with t1/2 congruent-to 2 s. Based upon DF measurements, DCMU-affinity of t he second site was increased upon chaotropic action of oleic acid, whi le addition of polyethylene glycol that stabilizes polypeptides lead t o preferential decrease in affinity of the main binding site. The affi nities of the two sites differred also in their dependence upon pH of the medium. Thus both binding sites can have high or low afinities to DCMU depending on the state of their microenvironment. A lower affinit y to DCMU of the site responsible for inhibition of the oxidizing side is probably related to its more rigid conformation.