Plastocyanin (PC) is a small (approx 10 kDa), type 1 (blue) copper protein that functions as an electron donor to Photosystem I (PSI) from cytochrome (cyt) f in both chloroplast systems and in some strains of cyanobacteria.
Defined by 4 residues: HIS:A-39CYS:A-82HIS:A-85MET:A-90