Humans can perceive thousands of compounds as bitter-tasting despite having around 25 bitter taste receptors, encoded by the Taste 2 Receptor (TAS2R or T2R) gene family (Adler et al. 2000, Chandrashekar et al. 2000, Matsunami et al. 2000), which signal via the Gi family G-protein Gustducin (Caicedo et al. 2003). Some receptors recognize only a few agonists while others have moderate or broad agonist ranges (Meyerhof et al. 2009). Although there is no clear correlation between bitterness and toxicity (Glendinning 1994), it is generally believed that this sense prevents mammals from ingesting potentially harmful food constituents.
Bitter compounds are numerous, esitimates for natural bitter compounds are in the tens of thousands. They are structurally diverse, including hydroxy fatty acids, fatty acids, peptides, amino acids, amines, amides, azacycloalkanes, N-heterocyclic compounds, ureas, thioureas, carbamides, esters, lactones, carbonyl compounds, phenols, crown ethers, terpenoids, secoiridoids, alkaloids, glycosides, flavonoids, steroids, halogenated or acetylated sugars, and metal ions (DuBois et al. 2008). The representative set of compounds in this reaction are taken from Table 1 in Meyerhof et al. (2009).