Carotenoids are tetraterpenoids which are synthezised from the common precursor isopentenyl diphosphate. The first dedicated step is the formation of phytoene via phytoene synthase. Subsequent desaturation reactions lead to the production of lycopene. Cyclization of lycopene by the action of either #beta# - or #epsilon# -cyclase (alone or in combination) results in the formation of #beta#- or #alpha#-carotene, respectively. Addition of different oxygen-containing side groups gives rise to the various xanthophylls. The three #beta#-carotene derived xanthophylls zeaxanthin, antheraxanthin and violaxanthin can be interconverted and are constituents of the xanthophyll cycle.
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