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Mechanistic studies on (s)-norcoclaurine synthase and dimethylallyltryptophan synthase Luk, Louis Yu Pan

Abstract

In alkaloid biosynthesis, there are a limited number of enzymes that can catalyze an aromatic electrophilic substitution. One example is norcoclaurine synthase, which catalyzes an asymmetric Pictet-Spengler condensation of dopamine and 4-hydroxyphenylacetaldehyde to give (S)-norcoclaurine. This is the first committed step in the biosynthesis of the benzylisoquinoline alkaloids that include morphine and codeine. In this work, the gene encoding for the Thalictrum flavum norcoclaurine synthase is highly overexpressed in Escherichia coli and the His-tagged recombinant enzyme is purified for the first time. A continuous assay based on circular dichroism spectroscopy is developed and used to monitor the kinetics of the enzymatic reaction. Dopamine analogues bearing a methoxy or hydrogen substituent in place of the C-1 phenolic group were readily accepted by the enzyme whereas those bearing the same substituents at C-2 were not. This supports a mechanism involving a two-step cyclization of the putative iminium ion intermediate that does not proceed via a spirocyclic intermediate. The reaction of [3,5,6-²H₃]-dopamine was found to be slowed by a kinetic isotope effect of 1.7 ± 0.2 on the value of kcat/KM. This is interpreted as showing that the deprotonation step causing re-aromatization is partially rate determining in the overall reaction. Dimethylallyltryptophan synthase is an aromatic prenyltransferase that catalyzes an electrophilic aromatic substitution between dimethylallyl diphosphate (DMAPP) and L-tryptophan. The synthase catalyzes the first committed step in the ergot alkaloid biosynthesis. The enzymatic reaction could follow either an SN1 reaction involving a discrete dimethylallyl cation intermediate or an SN2 mechanism in which the indole ring directly displaces diphosphate in a single step. In this work, positional isotope exchange experiments are presented in support of an SN1 pathway. When [1-¹⁸O]-DMAPP is subjected to the synthase reaction, 15% of the ¹⁸O-label is found to have scrambled from a bridging to a non-bridging position on the α-phosphorus. Kinetic isotope effect studies show that steps involved in the formation of the arenium ion intermediate are rate-determining, and therefore the scrambling occurs during the lifetime of the dimethylallyl cation/diphosphate ion pair. Similarly, when the unreactive substrate analogue, 6-fluorotryptophan, was employed, complete scrambling of the ¹⁸O-label in DMAPP was observed.

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