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The dependence of the P(700)(+)/P(700) midpoint potential on kinetics of reduction of P(700)(+) in vivo has been examined in a series of site-directed mutants of Chlamydomonas reinhardtii in which the histidyl axial ligand to the Mg(2+) of the P(700) chlorophyll a has been changed to several different amino acids. In wild-type photosystem I, the potential of P(700)(+)/P(700) is 447 mV and the in vivo half-time of P(700)(+) reduction by its natural donor, plastocyanin, is 4 micros. Substitution of the axial histidine ligand with cysteine increases the potential of P(700)(+)/P(700) to 583 mV and changes the rate of P(700)(+) reduction to 0.8 micros. Mutants with a range of potentials between 447 and 583 mV show a strong correlation of the P(700)(+)/P(700) potential to the rate of reduction of P(700)(+) by plastocyanin. There is also an increase in the rate of photosystem I-mediated electron transfer from the artificial electron donor DCPIP to methyl viologen in thylakoid membranes. The results indicate that the overall rate constant of P(700)(+) reduction is determined by the rate of electron transfer between the copper and P(700)(+) and confirmed that in vivo there is a preformed complex between plastocyanin and photosystem I. Using approximations of the Marcus electron transfer theory, it is possible to estimate that the distance between the copper of plastocyanin and P(700)(+) is approximately 15 A. On the basis of this distance, the plastocyanin docking site should lie in a 10 A hollow formed by the lumenal exposed loops between transmembrane helices i and j of PsaA and PsaB.

biochemistry

Electron Transfer from Plastocyanin to the Photosystem I Reaction Center in Mutants with Increased Potential of the Primary Donor in <i>Chlamydomonas</i> <i>reinhardtii</i>

Electron Transfer from Plastocyanin to the Photosystem I Reaction Center in Mutants with Increased Potential of the Primary Donor in Chlamydomonas reinhardtii