ORIGINS OF ISOPRENOID DIVERSITY: A STUDY OF STRUCTURE-FUNCTION RELATIONSHIPS IN SESQUITERPENE SYNTHASES

• Main Author: Greenhagen, Bryan T.
• Format: Others
• Published: UKnowledge 2003
• Subjects: Terpene Synthase|Terpene Cyclase|Isoprenoid Biosynthesis|Phytoalexin biosynthesis|Cytochrome P450
• Online Access: http://uknowledge.uky.edu/gradschool_diss/440; http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1443&context=gradschool_diss

Plant sesquiterpene synthases catalyze the conversion of the linear substrate farnesyl diphosphate, FPP, into a remarkable array of secondary metabolites. These secondary metabolites in turn mediate a number of important interactions between plants and their environment, such as plant-plant, plant-insect and plant-pathogen interactions. Given the relative biological importance of sesquiterpenes and their use in numerous practical applications, the current thesis was directed towards developing a better understanding of the mechanisms employed by sesquiterpene synthases in the biosynthesis of such a diverse class of compounds. Substrate preference for sesquiterpene synthases initially isolated from Nicotiana tabacum (TEAS), Hyoscyamus muticus (HPS) and Artemisia annuna (ADS) were optimized with regards to a divalent metal ion requirement. Surprisingly, careful titration with manganese stimulated bona fide synthase activity with the native 15-carbon substrate farnesyl diphopshate (FPP) as well as with the 10-carbon substrate geranyl diphosphate (GPP). Reaction product analysis suggested that the GPP could be used to investigate early steps in the catalytic cascade of these enzymes. To investigate how structural features of the sesquiterpene synthases translate into enzymatic traits, a series of substrate and active site residue contacts maps were developed and used in a comparative approach to identify residues that might direct product specificity. The role and contribution of several of these residues to catalysis and product specificity were subsequently tested by the creation of site-directed mutants. One series of mutants was demonstrated to change the reaction product to a novel sesquiterpene, 4-epi-eremophilene, and while another series successfully transmutated TEAS into a HPS-like enzyme. This is the first report of a rational redesign of product specificity for any terpene synthase. The contact map provides a basis for the prediction of specific configurations of amino acids that might be necessary for as yet uncharacterized sesquiterpene synthases from natural sources. This prediction was tested by the subsequent isolation and validation that valencene synthase, a synthase from citrus, did indeed have the amino acid configuration as predicted. Lastly, an in vitro system was developed for analyzing the interaction between sesquiterpene synthases and the corresponding terpene hydroxylase. Development of this in vitro system is presented as a new important tool in further defining those biochemical features giving rise to the biological diversity of sesquiterpenes.