Electrogenic reduction of the primary electron donor P700 by plastocyanin in photosystem I complexes

An electrometric technique was used to investigate electron transfer betwee n spinach plastocyanin (Pc) and photooxidized primary electron donor P700 i n photosystem I (PS I) complexes from the cyanobacterium Synechocystis sp, PCC 6803. In the presence of Pc, the fast unresolvable kinetic phase of mem brane potential generation related to electron transfer between P700 and th e terminal iron-sulfur acceptor F-B was followed by additional electrogenic phases in the microsecond and millisecond time scales, which contribute ap proximately 20% to the overall electrogenicity. These phases are attributed to the vectorial electron transfer from Pc to the protein-embedded chlorop hyll dimer P700(+) within the PsaA/PsaB heterodimer, The observed rate cons tant of the millisecond kinetic phase exhibited a saturation profile at inc reasing Pc concentration, suggesting the formation of a transient complex b etween Pc and PS I with the dissociation constant K-d of about 80 muM. A sm all but detectable fast electrogenic phase was observed at high Pc concentr ation. The rate constant of this phase was independent of Pc concentration, indicating that it is related to a first-order process. (C) 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemic al Societies.