plastokyanin

Plastocyanin, occasionally spelled plastokyanin, is a small copper-containing protein that plays a key role in photosynthetic electron transport. It is found in the thylakoid lumen of chloroplasts in green plants and algae and in cyanobacteria, serving as a mobile electron carrier between the cytochrome b6f complex and photosystem I. In plants, plastocyanin is one of several copper-containing electron carriers that participate in linear electron flow from water splitting to NADP+ reduction.

Chemically, plastocyanin is a type 1 blue copper protein of about 8–10 kDa. Its active site contains a single copper ion coordinated tetrahedrally by a cysteine thiolate, two histidine nitrogens, and a weakly coordinating methionine. This arrangement gives plastocyanin its characteristic blue color and enables rapid, reversible redox chemistry: Cu2+ ⇄ Cu+.

Functionally, plastocyanin accepts an electron from cytochrome f and passes it to the P700 chlorophyll pair in photosystem I, facilitating the reduction of ferredoxin and ultimately NADP+ formation. The protein is soluble in the lumen and diffuses to encounter its partners, with a typical redox potential around +0.3 to +0.5 V, depending on species and conditions. In organisms that lack plastocyanin, cytochrome c6 can fulfill a similar role.

Genetic and evolutionary notes: plastocyanin is part of the cupredoxin family. The PETE gene encodes plastocyanin in many plants, while cyanobacteria and some algae frequently use the petE designation. Plastocyanin provides insight into copper handling in biological electron transfer and remains a widely studied model for blue copper proteins.