Figure 1.

Biosynthesis of cyclic carotenoids in plants. The symmetrical linear lycopene (ψ,ψ-carotene; ψ indicates a linear or acyclic end group) is converted to the bicyclic β-carotene (β, β-carotene) by the LCYb of Arabidopsis. LCYe of Arabidopsis adds only one ɛ-ring to lycopene to form the monocyclic δ-carotene (ɛ,ψ-carotene). Together, the two cyclases produce α-carotene (β,ɛ-carotene) and β-carotene. Both α- and β-carotene serve as precursors for production of carotenoids typically found in the photosynthetic apparatus of green plants, including zeaxanthin, violaxanthin, neoxanthin, and lutein. For those few plants that contain substantial amounts of carotenoids with two ɛ-rings (e.g., lactucaxanthin), it may be surmised that the endogenous LCYe converts lycopene to ɛ-carotene.