plastocyanin

Plastocyanin is a small copper-containing protein that functions as an electron carrier in the photosynthetic electron transport chain of plants, algae, and cyanobacteria. It is a soluble protein located in the thylakoid lumen and shuttles electrons between cytochrome f of the cytochrome b6f complex and photosystem I.

Plastocyanin contains a single copper ion in a type 1 (blue) copper center. The Cu is coordinated by a cysteine thiolate and two histidine ligands, with a weak axial ligand, typically a methionine. This geometry gives plastocyanin its characteristic blue color and facilitates rapid, reversible electron transfer between Cu(II) and Cu(I). The redox potential of plastocyanin is typically around +0.3 to +0.4 V versus the standard hydrogen electrode, depending on pH and environment.

In plants, plastocyanin is encoded in the nucleus, translated as a precursor with a chloroplast transit peptide, and targeted to the thylakoid lumen where it becomes mature and active. In cyanobacteria and some algae, plastocyanin is encoded in the respective genomes. Functionally, plastocyanin accepts an electron from cytochrome f and donates it to photosystem I, acting as a mobile one-electron carrier that supports linear electron flow and ATP synthesis during photosynthesis.

Under copper limitation or genetic variation, some organisms can substitute plastocyanin with cytochrome c6 as an alternative, copper-independent electron carrier, highlighting the link between metal availability and the photosynthetic electron transport chain.