Electron Donation to Photosystem I

Abstract

Electron donation to photosystem I was studied in highly resolved particles from spinach. Divalent cations increased the efficiency of electron donation from spinach plastocyanin to P700+ through a decrease in the apparent Km for plastocyanin. Cytochrome f was not an efficient electron donor for P700+ in the presence or absence of divalent cations. Cytochrome f photooxidation could be observed in the presence of both plastocyanin and divalent cations.The efficiencies of electron donors from eukaryotic and prokaryotic algae to P700+ were also examined. Divalent cations enhanced the effectiveness of electron donors from eukaryotic organisms, while inhibiting electron donors from prokaryotic organisms. The prokaryotic electron donors were also much more efficient donors than were the electron donors from eukaryotic organisms. A correlation between the Km for the electron donor and its isoelectric point suggests that the net charge on the donor protein is a major determinant of the efficiency for electron donation. The data presented raise interesting questions with respect to the evolution of electron donation to photosystem I and the possibility of an additional electron carrier between plastocyanin and P700+.

A comparative laser‐flash absorption spectroscopy study of Anabaena PCC 7119 plastocyanin and cytochrome c6 photooxidation by photosystem I particles

“…Such a large difference in the effective salt concentration cannot be explained simply as arising from an ionic strength effect (which should only provide a factor of four), but rather suggests a more specific role for Mg2+. In the presence of multivalent cations, the electron exchange appears to proceed via encounters of catiodprotein complexes, which dissociate at high ionic strength [29, 301. The above results are in good agreement with those obtained with basic donor proteins under steady-state conditions [6]. Actually, increasing ionic strengths in general, and divalent cations in particular, inhibit electron transfer to P700' from electron donors having isoelectric points greater than 5.5, as is the case for Anabaena cytochrome c6 and plastocyanin which eave isoelectric points close to 9 (unpublished results).…”

“…The interaction between plastocyanin and PSI has been extensively studied in a wide variety of organisms, in spite of the difficulties and variability in the procedures used to obtain the PSI particles, as well as the differences in reactivity of such particles towards their respective copper proteins [5-91. Although the mechanism of electron transfer from plastocyanin to oxidized R O O (P700') is still quite controversial [2], the electrostatic nature of the process, the specific role played by divalent cations and the effect of pH have been clearly shown [4,[6][7][8][9][10][11]. These results have mostly been interpreted in terms of factors controlling the interaction of plastocyanin with the reaction center, which involves intermediate complex formation within a bimolecular kinetic model [6, 71.…”

“…Although the mechanism of electron transfer from plastocyanin to oxidized R O O (P700') is still quite controversial [2], the electrostatic nature of the process, the specific role played by divalent cations and the effect of pH have been clearly shown [4,[6][7][8][9][10][11]. These results have mostly been interpreted in terms of factors controlling the interaction of plastocyanin with the reaction center, which involves intermediate complex formation within a bimolecular kinetic model [6, 71.…”

Photosynthesis in Tall Fescue

“…Also, externally added PC did not increase either V6-802 or I-16-2 PSI electron flow. This is consistent with the speculation that PC is located in the lumen space of thylakoids (8,15,29).…”

“…Davis et al (8) also presented evidence that Mg2' increased the affinity of PSI particles toward eukaryotic PC and lowered the affinity for prokaryotic PC. The prokaryotic PC lacks the negatively charged domain that is present in eukaryotic PC (8). Davis correlated the Km for the electron donor PC and its isoelectric point, stressing that the net charge on PC is a major determinant in the effectiveness of electron donation and binding to PSI.…”