TURNOVER CONTROL OF PHOTOSYSTEM .2. USE OF REDOX-ACTIVE HERBICIDES TO FORM THE S3 STATE

The O2-evolving center of photosystem II, which contains an active-site tetramanganese-oxo cluster, catalyzes the four-electron oxidation of two water molecules to dioxygen, with the concomitant production of four H+ and four electrons. During catalytic turnover, the manganese-oxo cluster steps through five intermediate oxidation states, which are known as the S(i) states (i = 0-4). While methods have been found to manipulate the system into S1 and S2 in high yields, efficient production of the S3 state in good yield at high concentration has not yet been achieved. Previous methods have suffered from the requirement of low protein concentration so that actinic flashes are saturating; the use of temperature to control S-state advancement under continuous illumination, which can lead to S-state scrambling; or the use of herbicides that bind to the Q(B) site and restrict the system to one turnover. We describe here a method for the high-yield production of the S3 state in highly-concentrated samples of photosystem II, through the use of electron-accepting herbicides which bind to the Q(B) site, Redox-active herbicides can be used, in principle, to limit S-state cycling to any desired number of turnovers, given the appropriate herbicide. This work has fundamental methodological implications not only for the study of photosystem II but also for other multistate redox protein systems.