Tangerine Dreams

Abstract
Carotenoids are long (mainly 40-carbon) isoprenoid chains that may be linear or cyclized at one or both ends of the molecule. The first committed step in carotenoid biosynthesis in plants and cyanobacteria is the production of the colorless compound phytoene via the condensation of two molecules of geranylgeranyl diphosphate by phytoene synthase. In photosynthetic organisms, phytoene desaturase (PDS) and ζ-carotene desaturase (ZDS) catalyze successive dehydrogenation reactions of phytoene to yield the colored molecules ζ-carotene and lycopene, respectively. In nonphotosynthetic (anoxygenic) bacteria, the desaturation of phytoene to lycopene is catalyzed by a single enzyme, bacterial phytoene desaturase (CrtI). The sequence of CrtI is unrelated to that of PDS or ZDS, suggesting that some of the steps in carotenoid biosynthesis arose independently in photosynthetic organisms (higher plants and cyanobacteria) and nonphotosynthetic bacteria (Hirschberg et al., 1997).