Involved in the biosynthesis of germacrene-derived sesquiterpene lactones. Catalyzes three consecutive oxidations of germacrene A to produce germacrene A acid. Could also catalyze the three-step oxidation of non-natural substrate amorphadiene to artemisinic acid. Can use beta-elemene as substrate. (+)-(R)-germacrene A + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = germacra-1(10),4,11(13)-trien-12-oate + 4 H(+) + 4 H2O + 3 oxidized [NADPH--hemoprotein reductase] Inhibited by cytochrome C, miconazole, aminobenzotriazole, metyrapone and clotrimazole. Optimum pH is 8.0. Secondary metabolite biosynthesis; terpenoid biosynthesis. Belongs to the cytochrome P450 family.