Receptor Discovery

There are currently five recognized primary senses of taste: umami, which is the savory taste of glutamate, sweet, salt, bitter and sour. Scientists generally believe that each taste sensation is recognized by a distinct taste receptor or family of taste receptors on the surface of taste bud cells in the mouth or on the tongue. A taste receptor functions either by physically binding to a flavor ingredient in a process analogous to the way a key fits into a lock or by acting as a channel to allow ions to flow directly into a taste cell. The brain recognizes tastes by determining which of the numerous receptors in the mouth have been contacted by a given flavor ingredient. Savory, sweet and bitter taste compounds bind to taste receptors, called G protein-coupled receptors (GPCRs), specific to each taste on the surface of taste bud cells. In contrast, the taste of salt and the sour taste are thought to be recognized by taste ion channels that allow the passage of particular ions into the taste bud cells. The following summarizes the proteins involved in savory, sweet and salt taste.

Savory Receptor. Glutamate is a natural component of foods including tomatoes, mushrooms, parmesan cheese, and meats. It is often added to foods in the form of MSG to provide a savory flavor. The human savory receptor is composed of two proteins called hT1R1 and hT1R3. The T1R proteins are members of the GPCR family and are expressed on the surface of certain taste bud cells.

Sweet Receptor. The human sweet receptor is composed of two proteins called hT1R2 and hT1R3. The hT1R3 protein is shared in common with the savory receptor. Like the savory receptor, the sweet receptor is also a member of the GPCR family and is expressed on the surface of certain taste bud cells.

Bitter Receptor. A novel family of GPCRs termed T2Rs that mediates the response to bitter compounds has been identified. Analysis of the human genome revealed that the hT2R family is composed of about 25 receptors, each one recognizing a different class of bitter compounds. Senomyx recently reported the identification of two novel ligand-receptor pairs: hT2R61 which is activated by 6-nitrosaccharin, a bitter derivative of saccharin; and hT2R44 which is activated by denatonium, one of the most bitter tasting compounds known to man, and 6-nitrosaccharin. These discoveries, and the identification of additional T2R receptor-ligand pairs, are part of our ongoing program to identify mediators of bitter taste.

Salt Receptor. In contrast to the GPCRs that mediate savory, sweet, and bitter tastes, sodium ions and to a lesser extent potassium ions, are thought to produce a salt taste via ion channels present on the surface of taste bud cells. Ion channels are receptors that span the cell membrane and allow a flow of ions into or out of cells. Published work suggests that the Epithelial Sodium Channel, or ENaC, is a mediator of salt taste.