Interaction of linolenic acid with bound quinone molecules in Photosystem II. Time-resolved optical and electron spin resonance studies

Time-resolved spectroscopic techniques, including optical flash photolysis and electron spin resonance spectroscopy, have been utilized to monitor electron-transport activity in Photosystem II subchloroplast particles. These studies have indicated that in the presence of 100 μM linolenic acid (1) a high initial fluorescence yield (F_i) is observed upon steady-state illumination of the dark-adapted sample; (2) flash-induced absorption transients (t > 10 μs) in the region of 820 nm, attributed to P-680⁺, are first slowed, then abolished; and (3) electron spin resonance Signal II_s and Signal II_f (Z⁺) are not detectable. Upon reversal of linolenic acid inhibition by washing with bovine serum albumin, optical and electron spin resonance transients originating from the photooxidation of P-680 are restored. Similarly, the variable component of fluorescence is recovered with an accompanying restoration of Signal II_s and Signal II_f. The data indicate that linolenic acid affects two inhibition sites in Photosystem II: one located between pheophytin and Q_A on the reducing side, and the other between electron donor Z and P-680 on the oxidizing side. Since both sites are associated with bound quinone molecules, we suggest that linolenic acid interacts at the level of quinone binding proteins in Photosystem II.