Structural decoding of reversible covalent linkage of odorants in human olfactory receptor OR6A2

Olfactory receptors (ORs) are a diverse superfamily of G protein-coupled receptors responsible for odor detection that are also implicated in non-olfactory physiological functions. OR6A2, a class II OR, selectively senses medium-chain aldehydes and belongs to a cluster of ORs genetically associated with the "soapy" perception of cilantro. It also modulates macrophage-mediated inflammatory responses. Structural studies of ORs have long been challenging. Using a back-mutation strategy, we engineered a functional OR6A2 variant (bmOR6A2) from a consensus OR6 (consOR6). Structures of bmOR6A2 in complex with aldehydes reveal a novel ligand-recognition mechanism involving a reversible Schiff base linkage with residue K^4.60, validated by mass spectrometry. By integrating structures of consOR6, molecular dynamics simulations, and functional assays, we identified a conserved D^45.51Y^6.55Y^7.41 triad critical for activation in class II ORs. These findings establish a practical strategy for decoding odorant recognition, offering new insights into olfaction signaling and applications in fragrance and therapeutic development.