Spectroscopic characterization of PS I core complexes from thermophilic Synechococcus sp. Identical reoxidation kinetics of A1- before and after removal of the iron-sulfur-clusters FA and FB.
Monomeric and trimeric PS I complexes missing the three stromal subunits E,C and D (termed PS I core complexes) were prepared from the thermophilic cyanobacterium Synechococcus sp. by incubation with urea. The subunits E,C and D are sequentially removed. In the monomeric PS I the subunit C is removed with a half life of approx. 5 min. This is about eight times faster than in the trimeric PS I complex. In parallel with the removal of the FA/B containing subunit C the reduction kinetics of P700+ changed from a half life of about 25 ms to about 750 microseconds. The partner of P700+ in the 750 microseconds charge recombination was identified to be FX by the difference spectrum of this phase. There are some minor differences in the spectra of trimeric and monomeric PS I core complexes. At 77K the forward electron transfer from A1- to FX is blocked in the major fraction of the PS I core complexes and P700+ A1- recombines with a half life of about 220 microseconds. In the remaining fraction P700+FX- is formed and decays with a half life of approx. 10 ms at 77 K. The kinetics of the forward electron transfer from A1- to the iron-sulfur-clusters was measured in the native PS I and the corresponding core complexes. The reoxidation kinetics of A1- are identical in both cases (t1/2 = 180 ns). We conclude that FX is an obligatory intermediate in the normal forward electron transfer.
Lüneberg
J
J
Max-Volmer-Institut für Biophysikalische und Physikalische Chemie, Technische Universität Berlin, Germany.
Fromme
P
P
Jekow
P
P
Schlodder
E
E
D000458:Cyanobacteria / Q000737:chemistry*; D004579:Electron Transport; D004591:Electrophoresis, Polyacrylamide Gel; D006207:Half-Life; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D007700:Kinetics; D046911:Macromolecular Substances; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D013053:Spectrophotometry; D013816:Thermodynamics; D014508:Urea / Q000494:pharmacology
D007506:Iron-Sulfur Proteins; D046911:Macromolecular Substances; D045322:Photosynthetic Reaction Center Complex Proteins; D014508:Urea