Nonnatural biosynthetic pathway for 2-hydroxylated xanthophylls with C 50 -carotenoid backbone.

Carotenoids are structurally diverse pigments with various important biological functions. There has been a large interest in the search for novel carotenoid structures, since only a slight structural changes can result in a drastic difference in their biological functions. Carotenoid-modifying enzymes show remarkable substrate promiscuity, allowing rapid access to a vast set of novel carotenoids by combinatorial biosynthesis. We previously constructed a nonnatural carotenoid biosynthetic pathway in Escherichia coli that can produce C carotenoids having a longer chain than their natural C counterparts. In this study, a carotenoid 2,2'-hydroxylase (crtG) from Brevundimonas sp. SD212 was coexpressed together with our laboratory-engineered C-zeaxanthin and C-astaxanthin biosynthetic pathways. We identified six novel nonnatural C-xanthophylls, namely, C-nostoxanthin, C-caloxanthin, C-adonixanthin, C-4-ketonostoxanthin, C-2-hydroxyastaxanthin, and C-2,2'-dihydroxyastaxanthin.