Phytoene synthase (, prephytoene-diphosphate synthase, 15-cis-phytoene synthase, PSase, geranylgeranyl-diphosphate geranylgeranyltransferase) is a transferase enzyme involved in the biosynthesis of carotenoids. It catalyzes the conversion of geranylgeranyl pyrophosphate to phytoene. This enzyme catalyses the following chemical reaction

1. 2 geranylgeranyl diphosphate 15-cis-phytoene + 2 diphosphate (overall reaction)

2. diphosphate + prephytoene diphosphate

3. 15-cis-phytoene + diphosphate

This enzyme requires Mn2+ for activity. It belongs to squalene/phytoene synthase family of proteins.

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