OR51E2 binds to acetate, propionate (C 2 H 5 COO -), and lactate (2-hydroxylpropanate) . It is not found only in olfactory neurons: it is also expressed in kidneys , helping to regulate blood glucose concentration; carotid arteries , sensing blood oxygen content; and sperm cells , aiding in their movement. In this work, we focus on propionate, which is bound to OR51E2 in the single-particle cryogenic electron microscopy (cryo-EM) structure elucidated by Billesbølle et al. . Propionate is a short-chain fatty acid "with a pungent, disagreeable, rancid odor" ; it is, however, an important component of some cheeses, especially Emmental .