LAUSR

The capsaicin receptor, TRPV1 ion channel, is a polymodal nociceptor and attractive drug target. Recent advances in cryo-electron microscopy revealed the vanilloid-binding pocket in high resolution, which allowed atomic interactions mediating capsaicin binding to be proposed. In the current model, capsaicin adopts a “tail-up, head-down” pose; key interactions include two hydrogen bonds formed between the amide of capsaicin and Thr551 and between the hydroxyl on the vanillyl group and Glu571 (of mouse TRPV1). Guided by this model, the vanilloid-insensitive TRPV2 channel has been successfully transformed to be vanilloid-sensitive with minimal alterations. In this study, we provide further evidence from kinetic and equilibrium analyses in support of the capsaicin binding model. Eliminating the key hydrogen bonds by point mutations right-shifted the concentration-response relationship, which was achieved by accelerating the current OFF rate and decelerating the ON rate. These observations are consistent with the expectation that mutations would weaken capsaicin binding. Surprisingly, though resiniferatoxin (RTX) has been proposed to bind in a similar fashion as capsaicin, kinetic and equilibrium analyses revealed that removing these crucial hydrogen bonds did not alter RTX activation kinetics as they did capsaicin, suggesting that RTX may behave differently.