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 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 Mg2+ 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,us. 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 mus. 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 similar to15 Angstrom. On the basis of this distance, the plastocyanin docking site should lie in a 10 Angstrom hollow formed by the lumenal exposed loops between transmembrane helices i and j of PsaA and PsaB.