Contains two b-type cytochromes, two c-type cytochromes (cn and f), and a [2Fe-2S] Rieske cluster. The enzyme plays a key role in photosynthesis, transferring electrons from photosystem II (EC 18.104.22.168) to photosystem I (EC 22.214.171.124). Cytochrome c-552 can act as acceptor instead of plastocyanin, but more slowly. In chloroplasts, protons are translocated through the thylakoid membrane from the stroma to the lumen. The mechanism occurs through the Q cycle as in EC 126.96.36.199, quinol---cytochrome-c reductase (complex III) and involves electron bifurcation.
EC 188.8.131.52 created 1984 as EC 184.108.40.206, transferred 2011 to EC 220.127.116.11, transferred 2018 to EC 18.104.22.168