Electron transfer from plastocyanin to the photosystem I reaction center in mutants with increased potential of the primary donor in Chlamydomonas reinhardtii

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.