Monoterpene synthase involved in the biosynthesis of volatile compounds widely used in aromatherapy and folk medicine, and present in culinary herbs (PubMed:24943828). Mediates the conversion of (2E)-geranyl diphosphate (GPP) into alpha-pinene and, as minor compounds, into alpha-phellandrene, limonene and alpha-terpinolene (PubMed:24943828). (2E)-geranyl diphosphate = alpha-pinene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed at low levels in leaves. 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.