X‐ray Crystal Structure of Aristolochene Synthase from Aspergillus terreus and Evolution of Templates for the Cyclization of Farnesyl Diphosphate

Abstract

Aristolochene synthase from Aspergillus terreus catalyzes the cyclization of the universal sesquiterpene precursor, farnesyl diphosphate, to form the bicyclic hydrocarbon aristolochene. The 2.2 Å resolution X-ray crystal structure of aristolochene synthase reveals a tetrameric quaternary structure in which each subunit adopts the α-helical class I terpene synthase fold with the active site in the “open”, solvent-exposed conformation. Intriguingly, the 2.15 Å resolution crystal structure of the complex with Mg2+3-pyrophosphate reveals ligand binding only to tetramer subunit D, which is stabilized in the “closed” conformation required for catalysis. In both conformations, but especially so in the closed conformation, the active site contour is highly complementary in shape to that of aristolochene, and a catalytic function is proposed for the pyrophosphate anion based on its orientation with regard to the presumed binding mode of aristolochene. A similar active site contour is conserved in aristolochene synthase from P. roqueforti despite the substantial divergent evolution of these two enzymes, while strikingly different active site contours are found in the sesquiterpene cyclases 5-epi aristolochene synthase and trichodiene synthase. Thus, the terpenoid cyclase active site plays a critical role as a template to bind the flexible polyisoprenoid substrate in the proper conformation for catalysis. NIH Grants GM 56838 (D.W.C.) and GM 30301 (D.E.C.)