STEADY-STATE KINETIC ANALYSES OF PHOTOSYSTEM-II ACTIVITY CATALYZED BY LIPOPHILIC ELECTRON-ACCEPTORS

Steady-state kinetic analyses of the Photosystem II activity elicited by quinones and quinonediimines in KCN/Hg-inhibited chloroplasts reveal that: 1. 1. Quinones generate a single reaction in which one quinone competes with another for photoreduction by Photosystem II. 2. 2. Quinonediimines also compete with one another for photoreduction, but the electron transport activity elicited by quinonediimines is the composite of two separate reactions, one of which is sensitive to ionic strength and atebrinbinding. 3. 3. The relationship between a quinone and a quinonediimine with respect to photoreduction by Photosystem II is non-competitive rather competitive. These findings are interpreted to indicate that quinones and quinonediimines accept electrons from two separate sites near the reducing site of Photosystem II. © 1979.