Rhodoxanthin synthase from honeysuckle; a membrane diiron enzyme catalyzes the multistep conversation of β-carotene to rhodoxanthin

Rhodoxanthin is a vibrant red carotenoid found across the plant kingdom and in certain birds and fish. It is a member of the atypical retro class of carotenoids, which contain an additional double bond and a concerted shift of the conjugated double bonds relative to the more widely occurring carotenoid pigments, and whose biosynthetic origins have long remained elusive. Here, we identify LHRS (Lonicera hydroxylase rhodoxanthin synthase), a variant β-carotene hydroxylase (BCH)–type integral membrane diiron enzyme that mediates the conversion of β-carotene into rhodoxanthin. We identify residues that are critical to rhodoxanthin formation by LHRS. Substitution of only three residues converts a typical BCH into a multifunctional enzyme that mediates a multistep pathway from β-carotene to rhodoxanthin via a series of distinct oxidation steps in which the product of each step becomes the substrate for the next catalytic cycle. We propose a biosynthetic pathway from β-carotene to rhodoxanthin.