CHARACTERIZATION OF A REDOX-ACTIVE CROSS-LINKED COMPLEX BETWEEN CYANOBACTERIAL PHOTOSYSTEM-I AND SOLUBLE FERREDOXIN

A covalent stoichiometric complex between photosystem I (PSI) and ferr edoxin from the cyanobacterium Synechocystis sp, PCC 6803 was generate d by chemical cross-linking, The photoreduction of ferredoxin, studied by laser flash absorption spectroscopy between 460 and 600 nm, is a f ast process in 60% of the covalent complexes, which exhibit spectral a nd kinetic properties very similar to those observed with the free par tners. Two major phases with t(1/2) <1 mu s and approximate to 10-14 m u s are observed at two different pH values (5.8 and 8.0), The remaini ng complexes do not undergo fast ferredoxin reduction and 20-25% of th e complexes are still able to reduce free ferredoxin or flavodoxin eff iciently, thus indicating that ferredoxin is not bound properly in thi s proportion of covalent complexes, The docking site of ferredoxin on PSI was determined by electron microscopy in combination with image an alysis, Ferredoxin binds to the cytoplasmic side of PSI, with its mass center 77 Angstrom distant from the center of the trimer acid in clos e contact with a ridge formed by the subunits PsaC, PsaD and PsaE, Thi s docking site corresponds to a close proximity between the [2Fe-2S] c enter of ferredoxin and the terminal [4Fe-4S] acceptor F-II of PSI and is very similar in position to the docking site of flavodoxin, an alt ernative electron acceptor of PSI.