Diverse activities of carotenoid cleavage oxygenases

Apocarotenoids are isoprenoid compounds that contain shortened carbon backbones compared to the naturally occurring carotenoids from which they are derived by oxidative cleavage. Cleavage of the carotenoid backbone can occur through nonspecific nonenzymatic (e.g., radical formation) or enzymatic oxidation (e.g., peroxidases). However, in biological systems where apocarotenoids exhibit specific biological activities, the oxidative cleavage of the carotenoid backbone is catalyzed by a class of enzymes known as carotenoid cleavage enzymes. Unlike unspecific carotenoid cleavage, these enzymes catalyze cleavage of specific double bonds of the carotenoid backbone. Enzymes with preferences for different carotenoid substrates and activities for cleaving different sites within a specific carotenoid have been identified and are discussed in detail in this chapter. Because the mechanism by which these enzymes catalyze oxidative cleavage, either via a monooxygenase or dioxygenase mechanism, is currently controversial, we use the term carotenoid cleavage oxygenases (CCOs).