Cyanobacterial Photosystem I lacks specificity in its interaction with cytochrome c sub(6) electron donors

In cyanobacteria, plastocyanin and cytochrome c sub(6), the alternate donor proteins to Photosystem I, can be acidic, neutral or basic; the role of electrostatics in their interaction with photosystem I varies accordingly. In order to elucidate whether these changes in the electron donors' properties correlate with complementary changes in the docking site of the corresponding photosystem, we have investigated the kinetics of reactions between three cytochrome c sub(6) with isoelectric points of 5.6, 7.0 and 9.0, with Photosystem I particles from the same three genera of cyanobacteria which provided the cytochromes. The model systems compared here thus sample the full range of charge properties observed in cytochromes c sub(6): acidic, basic and neutral. The rate constants and dependence on ionic strength for photosystem I reduction were distinctive for each cytochrome c sub(6), but independent of Photosystem I. We conclude that the specific structural features of each cytochrome c sub(6) dictate their different kinetic behaviours, whereas the three photosystems are relatively indiscriminate in docking with the electron donors.