Fig. 1.

Scheme of carotenoids biosynthesis. Carotenoids synthesis can be divided into four stages: precursor supply, skeleton desaturation, terminal cyclization, and product tailoring. The conjugated double bands (c.d.b.) formed in desaturation stage are boxed with different color. The numbering of C₄₀ carotenoids is exemplified by using β-carotene. The symmetric oxidations of β-ring generate zeaxanthin, canthaxanthin, and astaxanthin. The cleavage positions are presented on β-carotene, which leads to the symmetric products of retinoids and the asymmetric product of ionones. GPS GGPP synthase, PSY phytoene synthase, PDS phytoene desaturase, ZDS ζ-carotene desaturase, LYC lycopene cyclase