Terpene cyclases, Class 1 (C1) of the class 1 family of isoprenoid biosynthesis enzymes, which share the 'isoprenoid synthase fold' and convert linear, all-trans, isoprenoids, geranyl (C10)-, farnesyl (C15)-, or geranylgeranyl (C20)-diphosphate into numerous cyclic forms of monoterpenes, diterpenes, and sesquiterpenes. Also included in this CD are the cis-trans terpene cyclases such as trichodiene synthase. The class I terpene cyclization reactions proceed via electrophilic alkylations in which a new carbon-carbon single bond is generated through interaction between a highly reactive electron-deficient allylic carbocation and an electron-rich carbon-carbon double bond. The catalytic site consists of a large central cavity formed by mostly antiparallel alpha helices with two aspartate-rich regions located on opposite walls. These residues mediate binding of prenyl phosphates via bridging Mg2+ ions, inducing proposed conformational changes that close the active site to solvent, stabilizing reactive carbocation intermediates. Mechanistically and structurally distinct, class II terpene cyclases and cis-IPPS are not included in this CD. Taxonomic distribution includes bacteria, fungi and plants.