Optimal Structure of Odorant-Receptor Interaction for Effective Olfactory Discrimination

An important task of olfactory sensing is the discrimination of different odors. An odor captures the chemical state of the environment in a mixture of smell molecules, called odorants. Olfactory sensing is realized by the selective binding of odorants to a set of olfactory receptors (ORs), which in turn activates the corresponding olfactory sensory neurons, constructing the brain's first representation of the odor. Despite the high-dimensional nature of olfactory sensing, recent measurements with human ORs suggest that the odorant-receptor interaction is sparse; only a small fraction of all available pairs interact, while the interaction strengths vary. Why did the OR system evolve this way? More generally, what are the optimal interaction structures for effective olfactory discrimination? We investigate these questions by combining studies of model systems and analyses of experimental data. In particular, we discuss how the graph-theoretic properties of the interaction should reflect the environmental odor statistics. We also point out that the human OR system adopts such optimal design.