phytoène

Phytoene is a colorless carotenoid hydrocarbon with the chemical formula C40H64. It is an early intermediate in carotenoid biosynthesis, produced by plants, algae, and many bacteria that synthesize carotenoids.

Biochemically, phytoene is formed by the head-to-head condensation of two geranylgeranyl pyrophosphate (GGPP) units, a reaction catalyzed by phytoene synthase (CrtB). It then undergoes desaturation to introduce additional double bonds. In bacteria, phytoene desaturase (CrtI) converts phytoene to lycopene through several intermediates. In plants, the desaturation steps are mediated by phytoene desaturase (PDS) and zeta-carotene desaturase (ZDS), with plastid terminal oxidase aiding the process, yielding lycopene that is subsequently cyclized to form β-carotene and other carotenoids.

Because of its limited conjugation, phytoene is colorless; its role is primarily as a biosynthetic precursor to the colored carotenoids that protect photosynthetic membranes and participate in light harvesting.

Occurrence and research: Phytoene is widespread among organisms that synthesize carotenoids. In biotechnology, its synthase gene is used to study and engineer carotenoid pathways, and phytoene levels serve as an indicator of pathway flux. Some investigations explore its biological activity, but its physiological function is mainly that of an intermediate rather than a primary functional pigment.