Preparation and characterization of a chemically modified plastocyanin.

The reaction of plastocyanin with tetranitromethane results in the nitration of only one of the three tyrosyl residues present in the protein. The modification does not affect the blue copper chromophore as both the characteristic visible spectrum of the chromophore and the redox potential of the protein are unchanged. Photochemical assays show that the modified plastocyanin is fully active in the reduction of photooxidized P700 and in the photooxidation of cytochrome f. The pK of the nitro-tyrosyl residue is about 7.3 indicating that the modified residue may be located in a negatively charged environment. Examination of the recently published X-ray structure of poplar plastocyanin suggests that Tyr-80 would be a likely candidate for the site of modification.