Photoreduction of cytochrome b559 in a Photosystem-II subchloroplast particle

Abstract

A Photosystem-II reaction-center particle derived from spinach chloroplasts by Triton treatment contains only one kind of cytochrome, namely, cytochrome b 559, in the amount of slightly more than 2 per 100 total chlorophyll molecules. Cytochrome b 559 is present in the oxidized form, has a standard redox potential of 58 mV, and undergoes photoreduction. Photoreduction of cytochrome b 559 and its re-oxidation in the dark can take place in the Photosystem-II particle without the addition of any exogenous electron donors such as diphenylcarbazide. The same reaction can even be better observed under anaerobic condition when oxidative photobleaching of bulk chlorophyll is prevented. Whereas the photoreduction of 2,6-dichlorophenolindophenol catalyzed by the Photosystem-II particle and supported by diphenylcarbazide is completely inhibited by 2 · 10 −5 M 3-(3,4-dichlorophenyl)-1,1-dimethylura (DCMU) or carbonylcyanide- m-chlorophenylhydrazone, the photoreduction of cytochrome b 559 is inhibited only 40 and 20%, respectively. The quantum efficiency of cytochrome b 559 photoreducation is near unity. An Arrhenius plot yields an activation energy of 7.5 kcal/mole. The cytochrome reaction ceases when the reaction mixture becomes frozen. No photooxidation of cytochrome b 559 at 77°K has been observed even when it was pre-reduced chemically. Observations made thus far support the notion that cytochrome b 559 is reduced by a reducing component formed in the electron-transport chain and re-oxidized by an oxidizing component formed on the oxidizing side of Photosystem-II. Thus, the light-induced redox reactions of cytochrome b 559 form a small cycle around Photosystem-II. The nature for the lack of inhibition of cytochrome b 559 photoreduction by DCMU is not yet clear.