This CD includes a diverse group of monomeric plant terpene cyclases (Tspa-Tspf) that convert the acyclic isoprenoid diphosphates, geranyl diphosphate (GPP), farnesyl diphosphate (FPP), or geranylgeranyl diphosphate (GGPP) into cyclic monoterpenes, diterpenes, or sesquiterpenes, respectively; a few form acyclic species. Terpnoid cyclases are soluble enzymes localized to the cytosol (sesquiterpene synthases) or plastids (mono- and diterpene synthases). All monoterpene and diterpene synthases have restrict substrate specificity, however, some sesquiterpene synthases can accept both FPP and GPP. 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 diphosphates, via bridging Mg2+ ions (K+ preferred by gymnosperm cyclases), inducing conformational changes such that an N-terminal region forms a cap over the catalytic core. Loss of diphosphate from the enzyme-bound substrate (GPP, FPP, or GGPP) results in an allylic carbocation that electrophilically attacks a double bond further down the terpene chain to effect the first ring closure. Unlike monoterpene, sesquiterene, and macrocyclic diterpenes synthases, which undergo substrate ionization by diphosphate ester scission, Tpsc-like diterpene synthases catalyze cyclization reactions by an initial protonation step producing a copalyl diphosphate intermediate. These enzymes lack the aspartate-rich sequences mentioned above. Most diterpene synthases have an N-terminal, internal element (approx 210 aa) whose function is unknown.
Jiyao W et al.(2023). "The conserved domain database in 2023.",