Electron uptake and delivery sites on plastocyanin in its reactions with the photosynthetic electron transport system

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Abstract
Plastocyanin [from French bean, pea and lettuce] is stoichiometrically and specifically labeled upon reduction by Cr(II)aq ions, yielding a substitution-inert Cr(III) adduct at the protein surface. The effect of the modification on the activity of plastocyanin in electron transfer between photosystems II and I was investigated. The photoreduction and photooxidation by chloroplasts or by photosystem I reaction centers, respectively, of native and Cr(III)-labeled plastocyanin were compared. Whereas the photoreduction rates of native and Cr-labeled plastocyanin were indistinguishable, the rates of photooxidation of the modified protein were markedly attenuated relative to those of the native one. This difference in reactivity clearly reflects the perturbation of the electron transfer pathway to P700. These findings, in conjunction with the structure of plastocyanin and the locus of Cr(III) binding on its surface, imply that there are most probably 2 physiologically significant, electron transfer sites on plastocyanin. The site involved in the electron transfer to P700 is most likely in the region of tyrosine-83 and the negatively charged patch proximal to it. By elimination, the 2nd site is centered at the hydrophobic region of histidine-87.