PLASTOCYANIN, AN ELECTRON-TRANSFER PROTEIN

Plastocyanin (Pc) is a copper (Cu)-containing blue protein, that funct ions as a mobile electron carrier between cytochrome (cyt) f and Photo system 1 (PS1) in oxygenic organisms. The atomic structure is known an d can be described as a beta-barrel with hydrophobic residues in the i nterior of the protein. To increase the understanding about structure- function relationships, site-directed mutagenesis of Pc has proven to be very useful. Mainly two spectroscopic techniques, optical and EPR s pectroscopy, have been used to investigate how the copper-site is affe cted by different mutations. The redox properties of the mutants have been investigated and factors that affect the reduction potential are discussed. Absorption and EPR spectra and reduction potentials for the surface mutants are similar to those of the corresponding wild-type. However, mutants around the Cu ion affected the mentioned properties. Comparisons are made with other cupredoxins. Five site-directed mutant s of spinach Pc, Pc(Leu12His), Pc(Leu15His), Pc(Thr79His), Pc(Lys81His ) and Pc(Tyr83His), have been modified by covalent attachment of a pho toactive ruthenium (Ru)-complex at the surface-exposed histidine resid ues. The rates of the internal electron-transfer reactions exhibit an exponential dependence on the metal-to-metal separation with a decay f actor of 1.1 Angstrom(-1). A reorganization energy for the Cu-to-Ru el ectron-transfer reaction of 1.2 eV was determined. Interprotein electr on-transfer reactions involving genetically modified Pc are described. Ionic-strength and pH dependencies indicated that electrostatic inter actions are involved in the complex formation between Pc and PS 1, whi ch was confirmed by mutations in the acidic patches of Pc. A very spec ific interaction was further verified by replacements of hydrophobic r esidues. Position 10, 12, 36, 87 and 90 were found to be very importan t for the formation of an active complex. A comparison between availab le structures of Pc and cyt cs, both effective donors to PS 1, is made . The physiological electron donor to Pc, cyt f, is briefly described.