The comparative structural and functional analyses of Pc and Cyt c6 isolated from different organisms, ranging from cyanobacteria to green algae and plants, reveal that the two proteins possess two functional equivalent areas at the surface level -one hydrophobic (site 1), the other electrostatically charged (site 2) - for the interaction with both Cyt f and PSI. The electrostatic patch should be responsible for driving the formation of the transient complex with the corresponding membrane partner, and the hydrophobic patch should serve to establish the electron transfer pathway. In spite of the intrinsic structural differences between Pc and Cyt c6, their two functional areas have evolved in a parallel way, thus giving rise to similarities such that each donor protein can replace the other with similar kinetic efficiency.
Lastly, the redox interaction between the soluble proteins, Pc and Cyt c6, and their membrane-embedded partners, Cyt b6f and PSI, has been optimized during evolution to improve the specificity and efficiency. In fact, the reaction mechanism would have evolved from a simple collision, type I, to a more sophisticated model involving not only formation of a transient complex, type II, but even a further rearrangement of the two partners to facilitate the final electron transfer step, type III.
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