The site and mechanism of duroquinol oxidation by the cytochrome b6- f complex in Synechococcus sp.

Abstract

Duroquinol (tetramethyl- p-hydroquinone) served as an excellent electron donor to Photosystem I and promoted methyl viologen photoreduction in 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea-poisoned cells of the thermophilic cyanobacterium Synechococcus sp. at a rate comparable to or, more likely, higher than that of the photoreduction with water as electron donor. The restored activity was inhibited by 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB). The inhibition is competitive and partially reversed by increasing duroquinol concentration. Spectrophotometric experiments with flashes demonstrated that duroquinol accelerates the reduction of P-700, cytochrome c-533 and cytochrome f in the DBMIB-poisoned cells similarly and in a way just opposite to the inhibitory action of DBMIB: the electron donor specifically increased the fraction of the three electron carriers that was reduced with the 2 ms half-time. The results indicate that duroquinol does not serve as a reductant of the plastoquinone pool, nor of P-700 and cytochrome c-533, but directly donates its electrons to the cytochrome b 6- f complex by binding to the plastoquinone-binding site and that DBMIB inhibits the quinol oxidation by binding competitively to the same site.