A COMPARATIVE LASER-FLASH ABSORPTION-SPECTROSCOPY STUDY OF ANABAENA PCC-7119 PLASTOCYANIN AND CYTOCHROME C-6 PHOTOOXIDATION BY PHOTOSYSTEM-I PARTICLES

Laser-flash absorption spectroscopy has been used to investigate the k inetics of electron transfer from reduced cytochrome c6 and plastocyan in, 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 compl ex 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 MgCl 2 concentration. This is interpreted in terms of the involvement of at tractive electrostatic interactions, which result in the initial colli sion complex having the most productive orientation for the electron t ransfer 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 rat e of oxidized P700 increases with pH from approximately 6 to 8, but de creases slightly at pH 8.5.