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 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.
Ramesh
V M
VM
Department of Plant Biology and Center for the Study of Early Events in Photosynthesis, P.O. Box 871601, Arizona State University, Tempe 85287-1601, USA.
Guergova-Kuras
Mariana
M
Joliot
Pierre
P
Webber
Andrew N
AN
D000818:Animals; D016825:Chlamydomonas reinhardtii / Q000737:chemistry* / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D002734:Chlorophyll / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004579:Electron Transport / Q000235:genetics; D045342:Light-Harvesting Protein Complexes; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D045331:Photosystem I Protein Complex; D010970:Plastocyanin / Q000737:chemistry* / Q000235:genetics; D013053:Spectrophotometry; D020524:Thylakoids / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D002734:Chlorophyll; C001785:chlorophyll P 700; D010970:Plastocyanin