Most terpene synthases contain plasticity residues responsible for diversified terpene products and functional evolution,which provide a potential for improving catalytic efficiency.Artemisinin,a sesquiterpene lactone from Artemisia annua L.,is widely used for malaria treatment.We have characterized a new sesquiterpene synthase from A.annua-bisabolol synthase(AaBOS),which has high sequence identity to amorpha-4,11-diene synthase(AaADS),a key enzyme in artemisinin biosynthesis.Domain-swapping and structure-based mutagenesis led to the identification of several plasticity residues,whose alteration changed the product profile of AaBOS to-humulene as the major product.To elucidate the underlying mechanisms,we solved the crystal structures of AaBOS.Among the plasticity residues,position 399,located in the substrate binding pocket,is crucial for both enzymes.In AaBOS,substitution of Leu with Thr(AaBOSL339T-humulene production; whereas AaADST399L produces amorpha-4,7(11)-diene and strikingly AaADST399S resulted in a substantial increase of the activity of amorpha-4,11-diene production,likely a result of accelerated product release.Recently,we identified other plasticity residues in AaADS,and a synergistic effect was observed in improving catalytic efficiency when these mutations were combined.Our work demonstrates that substitution of plasticity residues holds a potential for improving catalytic efficiency of the enzyme.