Crystal Structures of Aristolochene Synthase‐Substrate Complexes

Abstract

The universal sesquiterpene precursor farnesyl diphosphate, a flexible linear 15-carbon isoprenoid, is cyclized in a reaction catalyzed by aristolochene synthase to form the bicyclic hydrocarbon aristolochene. The 2.1 Å resolution X-ray crystal structure of tetrameric aristolochene synthase from Aspergillus terreus complexed with substrate farnesyl diphosphate (FPP) reveals the binding of intact FPP to monomers A–C and coproduct PPi and one Mg2+ ion binding to monomer D. The 1.89 Å resolution structure of aristolochene synthase complexed with substrate analogue 2-fluorofarnesyl diphosphate (2F-FPP) reveals 2F-FPP binding to all subunits of the tetramer and one Mg2+ ion binding to monomer D. In both structures, all the monomers adopt “open” active site conformation. However, some structural changes in monomers C and D indicate partial transition to the closed state. Tetramerization of the catalytically active dimer may hinder the full transition from the “open” to the “closed” active site conformation that is required for catalysis. Binding interactions of FPP and 2F-FPP trapped in non-productive conformations and incomplete Mg2+ clusters provide structural insight into the primary enzyme-substrate molecular recognition events by sesquiterpene cyclases. NIH Grant GM 56838.