Xanthophyll biosynthesis: molecular and functional characterization of carotenoid hydroxylases from pepper fruits (Capsicum annuum L.)

To dissect the mechanism by which carotenoid hydroxylases catalyze xanthophyll formation, we have cloned two pepper cDNAs encoding beta-cryptoxanthin and zeaxanthin biosynthetic enzymes. Using an in vitro system, we find that both enzymes are ferredoxin dependent and that their activity is strongly inhibited by iron chelators such as o-phenanthroline or 8-hydroxyquinoline. This suggests the transfer of a reducing equivalent from NADPH to the hydroxylase via ferredoxin and the involvement of an iron activated oxygen insertion process. Based on sequence analysis, the putative histidine clusters involved in the iron coordination were identified and their roles evaluated. Following site-directed mutagenesis of the identified histidine residues hydroxylase activity was totally inactivated. Collectively, our data indicate that carotenoid hydroxylases belong to a new class of diiron proteins structurally related to membrane fatty acid desaturases. Mechanistically, both types of enzymes exploit iron activated oxygen to break the C-H bond with concomitant formation of double bond or oxygen insertion. We propose that the same mechanism operates for beta-carotene ketolase and probably for other carotenoid oxygenases as well.