Catalyzes the formation of the (-)-3-isothujone precursor sabinene from geranyl diphosphate. The enzyme also produces significant amounts of gamma-terpinene, terpinolene and limonene. (2E)-geranyl diphosphate = (1R,5R)-sabinene + diphosphate Binds 3 Mg(2+) ions per subunit. Terpene metabolism; sabinene hydrate biosynthesis. Monomer. 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.