Plastocyanin
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Plastocyanin is a single strand protein of the thylakoid membrane, (molecular weight 10,500), that plays an integral part in photosynthesis. This protein has a copper center, consisting of one copper atom bonded to two nitrogen atoms and two sulfur atoms. This tetrahedral geometry is unusual for copper compounds in nature, but seems to minimize energies of oxidation and reduction for the copper atom at high pH.
Plastocyanin is involved in electron transport. At photosystem I it reacts with the P700+ complex losing one electron in the process. Plastocyanin replaces the electron by accepting one from cytochrome f of the cytochrome b6f complex.