Promiscuous activity of beta-carotene hydroxylase CrtZ on epoxycarotenoids leads to the formation of rare carotenoids with 6-hydroxy-3-keto-epsilon-ends

Carotenoids with rare 6-hydroxy-3-keto-epsilon-end groups, such as piprixanthin, vitixanthin, or cochloxanthin, found in manakin birds or plants, are rare carotenoids with high antioxidant activity. The same chemical structure is found in abscisic acid or blumenol, apocarotenoids found in plants or fungi. In this study, we serendipitously discovered that the promiscuous activity of the beta-carotene hydroxylase CrtZ, a diiron-containing membrane protein, can catalyze the formation of 6-hydroxy-3-keto-epsilon-end by using epoxycarotenoids antheraxanthin or violaxanthin as substrate. We suggest that the reaction mechanism is similar to that of a rhodoxanthin biosynthetic enzyme. Our results provide a further understanding of the reaction mechanism of diiron-containing beta-carotene hydroxylases, as well as insight into the biosynthesis of natural compounds with 6-hydroxy-3-keto-epsilon-end carotenoid derivatives.