On the mechanism of linolenic acid inhibition in Photosystem II.

Recent studies in our laboratory have reexamined the interaction of the unsaturated fatty acid, linolenic acid, with Photosystem II and have documented two principal regions of inhibition: one associated with the donor complex (Signal 2f or D1) to the reaction center, and the other located on the reducing side between pheophytin and Qa (Golbeck, J.H. and Warden, J.T. (1984) Biochim. Biophys. Acta 767, 263-271). A further characterization of fatty acid inhibition of secondary electron transport in Photosystem II at room and cryogenic temperatures is presented in this paper. These studies demonstrate that linolenic acid, and related fatty acid analogs, eliminate the transient absorption increase at 320 nm, attributed to Qa-; abolish the production, either chemically or photochemically, of the ESR signal (Q-Fe) associated with the bound quinone acceptor, Qa-; and prevent the photooxidation of Signal 2(1t)(D1) at cryogenic temperature. Linolenic-acid-treated samples are characterized by a high initial fluorescence yield (Fi) equivalent to the maximum level of fluorescence (Fmax); however, the spin-polarized triplet, associated with the reaction-center electron donor, P-680, is observed only in inhibited samples that have been prereduced with sodium dithionite. These results suggest the presence of an additional acceptor intermediate between pheophytin and Qa. The donor-assisted photoaccumulation of pheophytin anion in Photosystem II particles, as monitored by the decline of fluorescence yield, is inhibited by linolenic acid. Redox titrations of the fluorescence yield in control and inhibited preparations demonstrate that the midpoint potential for the primary acceptor for Photosystem II is insensitive to the fatty acid (Em approximately -583 mV) and thus indicate that primary photochemistry is functional during linolenic-acid inhibition. These data are consistent with the hypothesis that unsaturated fatty acids inhibit secondary electron transport in Photosystem II via displacement of endogenous quinone from quinone-binding peptides.