Crystal Structure Determinations of Oxidized and Reduced Plastocyanin from the Cyanobacterium Synechococcus sp. PCC 7942,

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Abstract
The crystal structures of oxidized and reduced plastocyanins from Synechococcus sp. PCC 7942 have been determined at 1.9 and 1.8 Å 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 π−π 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 Å, despite the small rms deviation of 0.08 Å 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°, and the water molecule moves by 0.42 Å. The changes around the copper site and the alterations around Tyr83 may be linked to the reduction of the copper.