In the photosynthetic redox chain, cytochrome f  donates an electron to plastocyanin (or in some cases to cytochrome c ₆), which carries it to photosystem 1. We have studied the transient complex formed between both proteins with NMR from plants and from cyanobacteria.
In these complexes, the hydrophobic patch of plastocyanin binds near the haem of the cytochrome, resulting in a short electron transfer distance between the iron and the copper. In the plant complex, plastocyanin uses also extensive electrostatic interactions, with negative charges on plastocyanin and positive charges on the cytochrome f . In the cyanobacterial complex from Nostoc , a charge-reversal is observed, resulting in a similar orientation of the proteins within the comlex. In the complex from the cyanobacterium Phormidium laminosum , plastocyanin approaches in a 'head-on' fashion, resulting in an almost purely hydrophobic complex.
Apparently, in a homologous system, with proteins of very similar three-dimensional structure, different mechanisms for transient complex formation have evolved, all resulting in fast electron transfer.

Complex of cytochrome f  (salmon) and plastocyanin (cyan) from Nostoc. Relevant negative residues in red, positive in blue. Haem in green.
Diaz-Moreno et al. (2005) JBC  280, 18908-18915

The complex of plant cytochrome f  (ribbon) with the haem in green and plastocyanin (traces of 10 best structures) with the copper as blue sphere.




Ubbink et al. (1998)Structure  6, 323-335
Lange et al. (2005) BBA Bioenergetics  1707, 179-188