Lutein, a dihydroxy derivative of α-carotene (β,ε-carotene), is the most abundant carotenoid in photosynthetic plant tissues where it plays important roles in light-harvesting complex-II structure and function. The synthesis of lutein from lycopene requires at least four distinct enzymatic reactions: β- and ε-ring cyclizations and hydroxylation of each ring at the C-3 position. Three carotenoid hydroxylases have already been identified in Arabidopsis, two nonheme diiron β-ring monooxygenases (the B1 and B2 loci) that primarily catalyze hydroxylation of the β-ring of β,β-carotenoids and one heme-containing monooxygenase (CYP97C1, the LUT1 locus) that catalyzes hydroxylation of the ε-ring of β,ε-carotenoids. In this study, we demonstrate that Arabidopsis CYP97A3 (the LUT5 locus) encodes a fourth carotenoid hydroxylase with major in vivo activity toward the β-ring of α-carotene (β,ε-carotene) and minor activity on the β-rings of β-carotene (β,β-carotene). A cyp97a3-null allele, lut5-1, causes an accumulation of α-carotene at a level equivalent to β-carotene in wild type, which is stably incorporated into photosystems, and a 35% reduction in β-carotene-derived xanthophylls. That lut5-1 still produces 80% of wild-type lutein levels, indicating at least one of the other carotene hydroxylases, can partially compensate for the loss of CYP97A3 activity. From these data, we propose a model for the preferred pathway for lutein synthesis in plants: ring cyclizations to form α-carotene, β-ring hydroxylation of α-carotene by CYP97A3 to produce zeinoxanthin, followed by ε-ring hydroxylation of zeinoxanthin by CYP97C1 to produce lutein.
Joonyul Kim; DellaPenna, D. Defining the primary route for lutein synthesis in plants: The role of Arabidopsis carotenoid ß-ring hydroxylase CYP97A3. Proceedings of the National Academy of Sciences (2006) 103 (9) 3474-3479. [DOI: 10.1073/pnas.0511207103]