The sites of electron donation of Photosystem I to methyl viologen

Abstract

Effectiveness of methyl viologen as an electron acceptor was studied by oxygen polarography and millisecond time range flash photolysis spectroscopy at room temperature on three types of chloroplast preparation deficient in NADP + photoreduction activity. HgCl 2-treated chloroplasts which had completely lost Fe-S center B and also NADP + photoreduction activity were still partially active in methyl viologen photoreduction. Compared with untreated chloroplasts, V max of the oxygen uptake in this preparation was almost halved and its apparent K m for methyl viologen was about 10 times greater. Photosystem I particles extracted with digitonin from the treated chloroplasts showed, in the absence of methyl viologen, a flash-induced absorption transient at 430 nm whose magnitude and decaying time were very similar to those of the particles extracted from untreated chloroplasts. However, the former required a concentration of methyl viologen for stabilization of P-700 + more than 10 times higher than control particles. The shape of the difference spectrum of the faster decaying component in the presence of methyl viologen was similar to P-430. Our conclusions are: (1) this spectral component represents the redox of Fe-S center A, and chloroplasts can transfer electrons from center X to center A even when their center B is destroyed; (2) center B is the main site of electron donation to methyl viologen, and center A can donate electrons to methyl viologen although with a lower affinity. Chloroplasts anaerobically photoinactivated under strongly reducing conditions in which electron transport between A 0 and center X was impaired showed very low oxygen uptake activity which was almost insensitive to methyl viologen. Dependence of oxygen uptake on methyl viologen concentration by aerobically photoinactivated chloroplasts in which three Fe-S centers were partially destroyed somewhat resembled that of HgCl 2-treated chloroplasts.