Enzymatic Fluoroethylation by a Fluoroethyl Selenium Analogue of S-Adenosylmethionine

Fluorine is a unique element with important roles in medicinal chemistry, agrochemistry, and materials chemistry. The fluoroethyl group is an important fluoroalkyl functional unit that is widely used in clinical drugs, 19F probes and 18F PET diagnostic drugs. Chemo- and regioselective fluoroethylation is difficult in chemical synthesis. To date, no enzymatic reaction for selective fluoroethylation has been reported. Based on the widespread natural methyl donor S-adenosine-l-methionine (SAM), we designed and synthesized a fluoroethyl SAM analogue (FEt-SAM). A stability study revealed that FEt-SAM was very labile under physiological conditions and gave the fluorine-elimination product vinyl-SAM. We circumvented this problem by replacing the S in FEt-SAM with Se to give fluoroethyl Se-adenosyl-l-selenomethionine (FEt-SeAM). By using halide methyltransferase (HMT) and its mutant for the in situ production of FEt-SeAM, we created cascade reactions of the HMT mutant with methyltransferases and fluoroethylated several O-, N-, S-, and C-nucleophiles. For methyltransferases that did not recognize FEt-SeAM well, such as DnrK and NovO, simple mutagenesis of the conserved hydrophobic residues (Leu and Ile) in the SAM binding pocket to smaller amino acids significantly increased the activities. Therefore, we have provided a useful tool for the late-stage fluoroethylation of natural products and drugs. This method could also be used to enzymatically prepare probes for 19F NMR and 18F PET tests.