Crystal structure of plantacyanin, a basic blue cupredoxin from spinach

The crystal structure of the basic blue protein (plantacyanin) from spinach (SBP) has been solved to a resolution of 2.05 Angstrom by molecular replac ement using the homologous protein from cucumber (CBP) as a model. Although the sequence identity of 58% between both proteins is only moderate, the t hree-dimensional structures turned out to be highly similar and the buried residues, which form the hydrophobic core of the protein, are almost comple tely conserved. However, the redox potentials of both proteins differ by 40 mV? and a comparison of the two structures leads to a single lysine replac ing a proline in the cucumber sequence, which causes a shift of the peptide chain and thus a subtle distortion of the copper ligand geometry in respec t to CBP. The crystal contained three monomers of SEP in the asymmetric uni t which show considerable variations in outer loop regions owing to crystal packing, but not in the regions presumed to be essential for redox partner recognition and redox potential fine tuning of the copper centers. Still, bond length variations at the copper site are at the same scale between the monomers of SEP as they are in respect to CBP, indicating that in the oxid ized state the protein does not impose a high conformational strain on the copper.