Abstact

The crystal structures of oxidized and reduced plastocyanins from Synechococcus sp. PCC 7942 have been determined at 1.9 and 1.8 A resolution, respectively, at pH 5.0. The protein consists of only 91 amino acid residues, the smallest number known for a plastocyanin, and apparently lacks the mostly conserved acidic patch that is believed to be important for recognition with electron-transfer partners. The protein has two acidic residues, Glu42 and Glu85, around Tyr83, which is thought to be a possible conduit for electrons, but these are neutralized by Arg88 and Lys58. Residue Arg88 interacts with Tyr83 through a pi-pi interaction in which the guanidinium group of the former completely overlaps the aromatic ring of the tyrosine. Reduction of the protein at pH 5.0 causes a lengthening of one Cu-N(His) bond by 0.36 A, despite the small rms deviation of 0.08 A calculated for the backbone atoms. Moreover, significant conformational changes of Arg88 and Lys58, along with the movement of a water molecule adjacent to the OH group of Tyr83, were observed on reduction; the guanidinium group of Arg88 rotates by more than 11 degrees, and the water molecule moves by 0.42 A. The changes around the copper site and the alterations around Tyr83 may be linked to the reduction of the copper.