The dependence of the P700+/P700 midpoint potential on kinetics of reduction of P700+ in vivo has been examined in a series of site-directed mutants of Chlamydomonas reinhardtii in which the histidyl axial ligand to the Mg2+ of the P700 chlorophyll a has been changed to several different amino acids. In wild-type photosystem I, the potential of P700+/P700 is 447 mV and the in vivo half-time of P700+ reduction by its natural donor, plastocyanin, is 4 μs. Substitution of the axial histidine ligand with cysteine increases the potential of P700+/P700 to 583 mV and changes the rate of P700+ reduction to 0.8 μs. Mutants with a range of potentials between 447 and 583 mV show a strong correlation of the P700+/P700 potential to the rate of reduction of P700+ 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 P700+ reduction is determined by the rate of electron transfer between the copper and P700+ 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 P700+ is ∼15 Å. On the basis of this distance, the plastocyanin docking site should lie in a 10 Å hollow formed by the lumenal exposed loops between transmembrane helices i and j of PsaA and PsaB.
ASJC Scopus subject areas