Involved in the biosynthesis of germacrene D. Can use farnesyl diphosphate as substrate, but not geranyl diphosphate. Produces mainly (+)-germacrene D along with germacrene B and a number of minor by-products. (2E,6E)-farnesyl diphosphate = (+)-germacrene D + diphosphate Magnesium, manganese, cobalt or nickel, but not zinc or copper. Binds 3 magnesium ions per subunit. Optimum pH is 7.5-8.5. Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsa subfamily.