The Sense of Taste--Why Food Tastes the Way it Does
The Five Basic Tastes: sweet, salty, bitter, sour, umami
Receptors in the Plasma Membrane
Most water-soluble signal molecules bind to specific sites on receptor proteins in the plasma membrane
There are three main types of membrane receptors:
-G-protein-linked receptors
-Receptor tyrosine kinases
-Ion channel receptors
Signaling Pathway
ReceptionTransductionResponse
-reception – ligand (signal molecule) binds to receptor
-signal transduction – 2nd messenger relays signal to intracellular pathways
-sensory neuron activation – electrical impulse sent to brain
-perception – brain interprets signal (response)
Tastes Receptors: Receptors linked to ion channels
-salty →
sodium ion channels
-sour →
hydrogen ion channels
G-protein coupled receptors (GPCRs) linked to gustducin (G-protein)
-sweet →
T1R2/T1R3 receptors
-bitter →
TAS2R receptor family
-umami →
T1R1/T1R3, mgluR1/mGluR4 receptor familes (MSG, IMP, GMP)
G-protein receptors
A G-protein-linked receptor is a plasma membrane receptor that works with the help of a G protein
The G-protein acts as an on/off switch: If GDP is bound to the G protein, the G protein is inactive
Ion channel receptors
An ion channel receptor acts as a gate when the receptor changes shape
When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na + or Ca2+, through
a channel in the receptor
Distribution of Taste Receptors
Taste buds (left) contain 50–150 taste receptor cells (TRCs) depending on the species. TRCs project microvilli to the
apical surface of the taste bud, to form taste pore. Taste buds are grouped on surfaces of taste papillae.
Bitter TAS2R38 Gene
-located on chromosome 7
-codes for a G-protein coupled taste receptor
- can be bound by bitter tasting compounds
-gene has several different alleles that display Mendelian Inheritance
-T taster allele (PAV)
-t nontaster allele (AVI)
-differences are due to single base changes
TAS2R38 Single Nucleotide Polymorphisms (SNPs)
Signaling Pathway
1. arrival of 1st messenger
2. ligand binds receptor
3. receptor activation
4. signal transduction
5. neuron activation
6. perception