Conformational changes of plastocyanin induced by redox state or metal substitution: Effect on electron transfer to photosystem 1

Abstract

We are using flash-photolysis studies to elucidate the mechanism of electron transfer (ET) from plastocyanin (Pc) to photosystem I (PSI) in spinach. Two different models can describe the kinetics of the reaction: the CC-model where a rate-limiting Conformational Change occurs in the Pc-PSI complex before the ET or the RE-model where the ET is assumed to be Reversible as a consequence of a higher PSI affinity for reduced Pc compared to oxidized Pc. We will report on kinetic studies with different metal-substituted forms of Pc that can be considered to be redox-inert structural analogs of Cu+-Pc or Cu2+-Pc. Addition of apo- and Ag+-Pc (model of Cu+-Pc) results in a reversible inhibition of the ET from Cu+-Pc to PSI, with affinities for the inhibitors that are comparable to that for Cu+-Pc. However, Zn2+-Pc (model of Cu2+-Pc) is a much weaker inhibitor of the reaction. This indicates that the nature of the metal ion can influence the structure of Pc, which in turn can lead to different affinities to PSI. We will also report on high-resolution X-ray diffraction studies of both the oxidized and reduced states of Cu-Pc. There are small, but significant differences in the structures, in particular in regions that make contact with PSI, which could lead to different affinities to PSI for the two redox states of Pc.