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Effect of Trypsin Treatment of Photosystem I Particles on the Electron Donation to P700 ¹

Department of Biochemistry, The Ohio State University, Columbus, Ohio 43210

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.

³ Author to whom all correspondence should be addressed.

¹ Supported in part by National Science Foundation Grant PCM 78-06969.