Involved in monoterpene (C10) olefins biosynthesis. The major product is (+)-alpha-pinene. (2E)-geranyl diphosphate = (1R,5R)-alpha-pinene + diphosphate Binds 3 Mg(2+) ions per subunit. Optimum pH is 7.0. Optimum temperature is 30 degrees Celsius. Terpene metabolism; (-)-alpha-pinene biosynthesis; (-)-alpha-pinene from geranyl diphosphate: step 1/1. Trichome. 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. Tpsb subfamily.