Structure of trichodiene synthase from Fusarium sporotrichioides provides mechanistic inferences on the terpene cyclization cascade.

The x-ray crystal structure of recombinant trichodiene synthase from Fusarium sporotrichioides has been determined to 2.5-A resolution, both unliganded and complexed with inorganic pyrophosphate. This reaction product coordinates to three Mg(2+) ions near the mouth of the active site cleft. A comparison of the liganded and unliganded structures reveals a ligand-induced conformational change that closes the mouth of the active site cleft. Binding of the substrate farnesyl diphosphate similarly may trigger this conformational change, which would facilitate catalysis by protecting reactive carbocationic intermediates in the cyclization cascade. Trichodiene synthase also shares significant structural similarity with other sesquiterpene synthases despite a lack of significant sequence identity. This similarity indicates divergence from a common ancestor early in the evolution of terpene biosynthesis.