Orange carotenoid protein

Orange carotenoid-binding protein
crystal structure of orange carotenoid protein
Identifiers
Organism	Arthrospira maxima
Symbol	Ocp
Alt. symbols	slr1963
PDB	1M98
UniProt	P83689

Orange carotenoid protein (OCP) is a water-soluble protein which plays a role in photoprotection in diverse cyanobacteria. It is the only photoactive protein known to use a carotenoid as the chromophore. The protein consists of two domains, with a single molecule non-covalently bound between the two domains. It is a very efficient quencher of excitation energy absorbed by the primary light-harvesting antenna complexes of cyanobacteria, the phycobilisomes. The quenching is induced by blue-green light. It is also capable of preventing oxidative damage by directly scavenging singlet oxygen (¹O₂).

OCP was first described in 1981 by Holt and Krogmann who isolated it from the unicellular cyanobacterium Arthrospira maxima, although its function would remain obscure until 2006. The crystal structure of the OCP was reported in 2003 and the protein was shown to be an effective quencher of singlet oxygen. In 2000, it was demonstrated that cyanobacteria could perform photoprotective independent of lipid phase transitions, differential transmembrane pH, and inhibitors. The action spectrum for this quenching process suggested the involvement of carotenoids, and the specific involvement of the OCP was later demonstrated by Kirilovsky and coworkers in 2006. In 2008, OCP was shown to require photoactivation by strong blue-green light for its photoprotective quenching function.

For a long time, cyanobacteria were considered incapable of performing (NPQ) as a photoprotective mechanism, relying instead on a mechanism of energy redistribution between the two photosynthetic reaction centers, PSII and PSI, known as "state transitions".