A Fluorescence-Based Supramolecular Tandem Assay for Monitoring Lysine Methyltransferase Activity in Homogeneous Solution

The demand for practical and convenient enzyme assays for histone lysine methyltransferases (HKMTs) emerges along with the rapid development of this young class of enzymes. A supramolecular reporter pair composed of p‐sulfonatocalix[4]arene (CX4) and the fluorescent dye lucigenin (LCG) has been used to monitor enzymatic trimethylation of lysine residues in peptide substrates. The assay affords a switch‐ON fluorescence response and operates in a continuous, real‐time, and label‐free fashion. The underlying working principle relies on the higher affinity of the macrocycle towards the trimethylated product of the enzymatic reaction as compared to the substrate, which allows the assay to be carried out in the product‐selective mode. The final product incorporates a trimethylammonium moiety, a known high‐affinity binding motif for CX4. Two substrates corresponding to the H3 N‐terminal tail, namely, S2 (RTKQTARKSTGGKAP) and S6 (QTARKSTGGS), were selected as model compounds for methylation with the Neurospora crassa Dim‐5 enzyme and investigated by the newly developed supramolecular tandem HKMTs assay. Only the longer substrate S2 underwent methylation in solution. The potential of the assay for inhibitor screening was demonstrated by means of inhibition studies with 1,10‐phenanthroline to afford an inhibition constant of (70±20) μM. The methylation of peptides by histone lysine methyltransferases, the enzymes responsible for epigenetic regulation, can be continuously monitored in homogeneous solution by using a label‐free supramolecular tandem assay strategy. The high affinity of the trimethylated enzymatic product towards p‐sulfonatocalix[4]arene, used in combination with the displacement of lucigenin as a dye, allows for a time‐resolved switch‐ON fluorescence response as a fingerprint of the catalytic activity (see figure).