Alteration of product formation by directed mutagenesis and truncation of the multiple-product sesquiterpene synthases δ-selinene synthase and γ-humulene synthase

Two recombinant sesquiterpene synthases from grand fir, δ-selinene synthase and γ-humulene synthase, each produce more than 30 sesquiterpene olefins from the acyclic precursor farnesyl diphosphate. These enzymes contain a pair of DDxxD motifs, on opposite lips of the presumptive active site, which are thought to be involved in substrate binding and could promote multiple orientations of the substrate alkyl chain from which multiple families of cyclic olefins could derive. Mutagenesis of the first aspartate of either DDxxD motif resulted in depressed kcat, with lesser effect on Km, for δ-selinene synthase and afforded a much simpler product spectrum composed largely of monocyclic olefins. Identical alterations in γ-humulene synthase produced similar kinetic effects with a simplified product spectrum of mostly acyclic and monocyclic olefins. Although impaired in product diversity, none of the mutant synthases lost entirely the capacity to generate complex structures. These results confirm the catalytic significance of the DDxxD motifs and imply that they also influence permitted modes of cyclization. Deletion of an N-terminal arginine pair in δ-selinene synthase (an element potentially involved in substrate isomerization) altered kinetics without substantially altering product outcome. Finally, mutation of an active-site tyrosine residue thought to play a role in proton exchange had little influence; however, substitution of a nearby active site aspartate dramatically altered kinetics and product outcome.