Requires Mg2+?and Mn2+?for activity. This is a key enzyme in the biosynthesis of the antimalarial endoperoxide artemisinin [3]. Catalyses the formation of both olefinic [e.g. amorpha-4,11-diene, amorpha-4,7(11)-diene, ¦Ã-humulene and ¦Â-sesquiphellandrene] and oxygenated (e.g. amorpha-4-en-7-ol) sesquiterpenes, with amorpha-4,11-diene being the major product. When geranyl diphosphate is used as a substrate, no monoterpenes are produced [2].