A comparative structural and functional analysis of cyanobacterial plastocyanin and cytochrome c (6) as alternative electron donors to Photosystem I.

Plastocyanin and cytochrome c (6) are two soluble metalloproteins that act as alternative electron carriers between the membrane-embedded complexes cytochromes b (6) f and Photosystem I. Despite plastocyanin and cytochrome c (6) differing in the nature of their redox center (one is a copper protein, the other is a heme protein) and folding pattern (one is a beta-barrel, the other consists of alpha-helices), they are exchangeable in green algae and cyanobacteria. In fact, the two proteins share a number of structural similarities that allow them to interact with the same membrane complexes in a similar way. The kinetic and thermodynamic analysis of Photosystem I reduction by plastocyanin and cytochrome c (6) reveals that the same factors govern the reaction mechanism within the same organism, but differ from one another. In cyanobacteria, in particular, the electrostatic and hydrophobic interactions between Photosystem I and its electron donors have been analyzed using the wild-type protein species and site-directed mutants. A number of residues similarly conserved in the two proteins have been shown to be critical for the electron transfer reaction. Cytochrome c (6) does contain two functional areas that are equivalent to those previously described in plastocyanin: one is a hydrophobic patch for electron transfer (site 1), and the other is an electrically charged area for complex formation (site 2). Each cyanobacterial protein contains just one arginyl residue, similarly located between sites 1 and 2, that is essential for the redox interaction with Photosystem I.