Contrasting effects of plastocyanin on the photoreduction and photooxidation of cytochrome f in chloroplasts

Flash photolysis-electron spin resonance studies of photosystem I. A fast reduction of component of P-700+

A 300 mus decay component of ESR Signal I (P-700+) in chloroplasts is observed following a 10 mus actinic xenon flash. This transient is inhibited by treatments which block electron transfer from Photosystem II to Photosystem I (e.g. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), KCN and HgCl2). The fast transient reduction of P-700+ can be restored in the case of DCMU or DBMIB inhibition by addition of an electron donor couple (2,6-dichlorophenol indophenol (Cl2Ind)/ascorbate) which supplies electrons to cytochrome f. However, this donor couple is inefficient in restoring electron transport in chloroplasts which have been inhibited with the plastocyanin inactivators, KCN and HgCl2. Oxidation-reduction measurements reveal that the fast P-700+ reduction component reflects electron transfer from a component with Em = 375 +/- 10 mV (pH = 7.5). These data suggest the assignment of the 300-mus decay kinetics to electron transfer from cytochrome f (Fe2+) to P-700+, thus confirming the recent observations of Haehnel et al. (Z. Naturforsch. 26b, 1171-1174 (1971)).

The kinetics and specificity of electron transfer from cytochromes and copper proteins to P700

The rates of electron transfer to P700 from plastocyanin and cytochrome f have been compared with those from three other c-type cytochromes and azurin, a copper protein resembling plastocyanin. Three different disruptive techniques were used to expose P700; digitonin, Triton X-100 and sonication. The following rate constants were measured at 25 degrees C, pH 7.0, with digitonin-treated chloroplasts: plastocyanin, 8 x 10(7)M(-1) x s(-1); red-algal cytochrome c-553, 1.9 x 10(7)M(-1) x s (-1); Pseudomonas cytochrome c-551, 8 x 10(6)M(-1) x s (-1); azurin, less than or = 3 x 10(5)M(-1) x s (-1); cytochrome f, less than or = 2 x 10(4)M(-1) x s (-1); mammalian cytochrome c, less than or = 2 x 10(4)M(-1) x s (-1). For electron transfer from plastocyanin, the effects of ionic strength, pH and temperature were also studied, and saturation effects found in earlier work were avoided by a full consideration of the various secondary reactions and inclusion of superoxide dismutase. The relative rates are discussed in relation to photosynthetic electron transport.

EPR signals of oxidized plastocyanin in intact algae

(1) An electron paramagnetic resonance (EPR) signal was observed at g = 2.05 in the low temperature spectra of intact cells of green, red and blue-green algae and of spinach chloroplasts. The g-value and the shape of the signal were similar to that of purified, soluble plastocyanin. (2) The amount of the copper protein, determined from the EPR signal height, was estimated to be nearly the same in all the studied organisms on the basis of the concentration of chlorophyll. Furthermore, it was found that the amount of the copper protein, determined from the EPR signal height in spinach chloroplasts corresponds with that of plastocyanin as determined chemically by Katoh, S., Suga, I., Shiratori, I. and Takamiya, A. (1961) Arch. Biochem. Biophys. 94, 136-141. (3) Experiments with far-red and red illumination show that the site of the copper protein in vivo is in the electron transport pathway between Photosystems 1 and 2. Plastocyanin is not oxidized by illumination at 77 degrees K, indicating that no electron transfer occurs between the primary electron donor of Photosystem 1, P700, and plastocyanin at that temperature. Furthermore, the experiments suggest that in the intact cells of the studied algae, plastocyanin is not only reduced by Photosystem 2 but also by cyclic electron transport around Photosystem 1.