In enzymology, a copalyl diphosphate synthase (EC 5.5.1.12) is an enzyme that catalyzes the chemical reaction

geranylgeranyl diphosphate ${\displaystyle \rightleftharpoons }$ (+)-copalyl diphosphate

Hence, this enzyme has one substrate, geranylgeranyl diphosphate, and one product, (+)-copalyl diphosphate.

This enzyme belongs to the family of isomerases, specifically the class of intramolecular lyases. The systematic name of this enzyme class is (+)-copalyl-diphosphate lyase (decyclizing). This enzyme participates in diterpenoid biosynthesis.

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