J. Am. Chem. Soc., published online 21 July 2011, doi:10.1021/ja2034969

Dimethylallyltryptophan (DMAT), generated by DMAT synthase, is an alkaloid with a prenyl group attached to the C-4 position of a tryptophan indole ring. The attachment of the prenyl group to the less reactive C-4 indole position instead of the more nucleophilic C-3 position has been suggested to occur through either of two mechanisms, one involving a Cope rearrangement to transfer the alkyl group from C-3 to C-4 during the reaction. However, direct evidence for this proposal was lacking. Luk et al. now identify a relevant intermediate in their analysis of an active site mutant of DMAT synthase. Lys174 had previously been suggested to catalyze the final step of the second proposed mechanism, in which the prenyl group is connected directly to the C-4 position from the beginning. Kinetic analysis of a Lys174Ala mutant did show a decrease in kcat consistent with this role. However, characterization of the reaction products surprisingly indicated the prenyl group was attached at C-3, and its connectivity was perfectly poised for a Cope rearrangement to yield the normal product. Finally, analysis of an existing crystal structure of the enzyme in complex with a nonhydrolyzable analog provided no rationale for how the enzyme might drive reactivity at the unfavored C-4 position. Together, these data call for further investigation into this interesting enzyme.