Crystal structure determinations of oxidized and reduced plastocyanin fromthe cyanobacterium Synechococcus sp PCC 7942

The crystal structures of oxidized and reduced plastocyanins from Synechoco ccus sp. PCC 7942 have been determined at 1.9 and 1.8 Angstrom resolution r espectively, at pH 5.0. The protein consists of only 91 amino acid residues , the smallest number known for a plastocyanin, and apparently lacks the mo stly conserved acidic patch that is believed to be important for recognitio n with electron-transfer partners. The protein has two acidic residues, Glu 42 and Glu85, around Tyr83, which is thought to be a possible conduit for e lectrons, but these are neutralized by Arg88 and Lys58. Residue Arg88 inter acts with Tyr83 through a pi-pi interaction in which the guanidinium group of the former completely overlaps the aromatic ring of the tyrosine. Reduct ion of the protein at pH 5.0 causes a lengthening of one Cu-N(His) bond by 0.36 Angstrom, despite the small rms deviation of 0.08 Angstrom calculated for the backbone atoms. Moreover, significant conformational changes of Arg 88 and Lys58, along with the movement of a water molecule adjacent to the O H group of Tyr83, were observed on reduction; the guanidinium group of Arg8 8 rotates by more than 11 degrees, and the water molecule moves by 0.42 Ang strom. The changes around the copper site and the alterations around Tyr83 may be linked to the reduction of the copper.