Effect of Trypsin Treatment of Photosystem I Particles on the Electron Donation to P700

Proteolysis of photosystem I particles had no effect on P700 oxidation but did inhibit the rate of P700(+) reduction. The V(max) values were decreased for both dichlorophenol and plastocyanin, but the K(m) values were unaffected indicating that trypsin treatment altered electron transfer rather than the binding of the donor to the photosystem I complex. The salt dependence of P700(+) reduction was unaffected. The effects of P700(+) reduction were the same for the preparations of different workers (Shiozawa, Alberte, Thornber 1974 Arch Biochem Biophys 165: 388; and Bengis, Nelson 1975 J Biol Chem 250: 2783).In both cases, the 70-kilodalton, chlorophyll-containing polypeptide was digested confirming its role in transferring electrons from plastocyanin to P700. The fact that the preparation of Shiozawa et al. lacks subunit (III) but still used plastocyanin as the electron donor rules out a role for this subunit as "the plastocyanin binding protein." Subunit III was also digested in the Bengis and Nelson preparation.