Multiproduct terpene synthases : catalytic promiscuity and cyclization of substrate geometric isomers
Terpenes constitute the largest and the most diverse class of natural products. The wealth of terpenes can be attributed to highly promiscuous enzymes called terpene synthases. Apart from single product enzymes, there also exist multiproduct terpene synthases generating a bouquet of acyclic and cyclic products. This thesis is aimed at using substrate analogs to study catalytic promiscuity and mechanistic pathways of multiproduct terpene synthases. The mechanistic details of reaction cascade in TPS4 and TPS5 from Zea mays were evaluated by using isotope sensitive branching. Labeled substrates with deuterium atoms completely surrounding the key cationic intermediates were used as metabolic probes. Apart from the confirmation of mechanistic cascade, kinetic isotope effects on terminating deprotonations led to an enhanced formation of alcohols over olefinic products. Deuterium labeled (2Z)-substrate geometric isomers, mimicking the nerolidyl diphosphate intermediate with TPS4 and TPS5 generated the same product profile but with increased cyclic products. Major increase in enzymatic turnover was also observed with (2Z)-substrates emphasize the rate limiting effect of the initial isomerization step. In contrast, MtTPS5 from Medicago truncatula showed a new product profile with majority of products formed via a C1-C11 ring closure to the humulyl cation over the natural cadalane skeleton. This demonstrates the possibility of using substrate geometry as tool to generate novel products. The structural characteristics of multiproduct terpene synthases remain unresolved due to absence of defined crystal structures. With some initial signs of success as co-crystallization candidates, 3-bromo analogs of substrates were found to be potent competitive inhibitors of MtTPS5 and other terpene synthases. Consequently with this work, catalytic promiscuity of multiproduct terpene synthases can be employed to design better biocatalysts with improved turnover and generate novel products.