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

Laser-flash absorption spectroscopy has been used to investigate the kinetics of electron transfer from reduced cytochrome c6 and plastocyanin, isolated from Anabaena PCC 7119, to oxidized P700 in photosystem-I particles isolated from the same cyanobacterium and from spinach. For all metalloproteins and photosystems, the observed rate constant has a non-linear protein-concentration dependence, thus suggesting complex formation preceding electron transfer. Plastocyanin and cytochrome c6 have similar association constants for complex formation with spinach photosystem I, but the copper protein exhibits a higher intracomplex-electron-transfer rate constant (twofold). With Anabaena photosystem I, the two redox proteins are more effective with respect to both complex formation (5-10-fold) and electron transfer (1.5-4-fold) than with the spinach photosystem. In all cases, the observed rate constants for electron-transfer monotonically decrease with increasing NaCl or MgCl2 concentration. This is interpreted in terms of the involvement of attractive electrostatic interactions, which result in the initial collision complex having the most productive orientation for the electron transfer process, without a requirement for further reorientation. The magnitude of the response to MgCl2 suggests the occurrence of specific ion effects as well. In the absence of added salts, the reduction rate of oxidized P700 increases with pH from approximately 6 to 8, but decreases slightly at pH 8.5.