Molecular biology of mammalian bitter taste receptors. A review.

Bitter taste prevents mammals from ingesting food potentially contaminated with bitter-tasting toxins, which are frequent and structurally diverse. It is mediated by a family of heptahelical G protein-coupled receptors, called taste 2 receptors or TAS2Rs or T2Rs. The number of TAS2R genes differs greatly across mammalian species, suggesting that some species depend less than others on the protective effect of TAS2Rs. TAS2Rs are glycosylated to become substrates for chaperones required for them to complete biosynthesis and cell surface expression. TAS2Rs recognize several to many bitter compounds, a property that accounts for our ability to detect the numerous bitter substances. Although TAS2Rs oligomerize, heteromers apparently do not increase the number of detectable bitter substances. A small molecule antagonist that inhibits bitter taste in vivo blocks several TAS2Rs in vitro, thereby sharing the property with bitter agonists to interact with several receptors. TAS2Rs possess only one binding pocket, in which they accommodate their ligands by contacting different but overlapping sets of amino acids in the upper parts of transmembrane segments 2-7. Pronounced genetic variability has been observed for human TAS2R genes, including single nucleotide polymorphisms, copy number variations and indels, which determine functionally different receptor variants and influence subjects' sensitivity for the bitterness of specific compounds. However, whether differences in taste sensitivity influence preferences in nutrition remains elusive and is only indirectly supported.