THCA synthase, a flavoprotein, uses a flavin adenine dinucleotide (FAD) cofactor to catalyze the oxidative cyclization of the monoterpene moiety of cannabigerolic acid (CBGA).
The overall chemical reaction is: CPGA + O2 \rightleftharpoons THCA + H2O2 A hydride is transferred from CBGA to reduce FAD, concerted by deprotonation of a hydroxyl group by a tyrosine residue.