Determination of Reaction Site Charge on Plastocyanin from the Cyanobacterium Anabaena variabilis Based on Electrostatic Interactions with Small Inorganic Complexes.

In order to investigate whether plastocyanin (PC) from the cyanobacterium Anabaena variabilis, like PC from green algae and higher plants, can exhibit dual path electron transfer (ET) we have determined the reaction site charge of A. variabilis PC. Kinetic measurements of the reactions between PC and [Co(phen)3]2+, [Co(phen)3]3+ and the neutral, sulfonated analogue [Co(phen-SO3)3] as a function of pH (6.3-8.8) and ionic strength (0.05-0.20 M) are reported. These data are supplemented by the midpoint potentials for A. variabilis in the same pH and ionic strength ranges. Three different approaches to reaction site charge determination are used. These rest on the ionic strength dependence of work terms and reaction free energy, on a comparison of the rate constants for the oxidation of PC by [Co(phen)3]3+ and [Co(phen-SO3)3], and on the pH profile of the ET rate constants. Consistent values for the reaction site charge are obtained, with only a weak pH dependence. The state of protonation of His-59 (pK(a) = 7.3), situated at the conceivable remote reaction site, thus does not Affect the reactivity of PC to any significant extent. These results are consistent with the absence of dual path reactivity for A. variabilis PC and a reaction pattern dominated by ET at the adjacent site close to the copper atom, for small positively charged reaction partners.