Sour taste is perceived by which receptors?

Correct Answer: B. TRPP3

Sour taste is mediated by TRPP3 (transient receptor potential channel, polycystin subfamily, member 3), also known as PKD2L1. This ion channel is selectively expressed on a subset of taste receptor cells on the tongue and soft palate. When acidic stimuli (H⁺ ions) contact the taste bud, they directly activate TRPP3 channels, causing depolarization and triggering action potentials that signal sourness to the brain via the chorda tympani (facial nerve) and glossopharyngeal nerve. The mechanism is fundamentally different from sweet, umami, and bitter tastes, which rely on G-protein-coupled taste receptors (T1R family). TRPP3 is an ionotropic receptor responding directly to protons, making it the primary molecular sensor for acidic taste. This distinction is critical in sensory physiology: while T1R1, T1R2, and T1R3 form heterodimeric complexes for sweet and umami perception, sour taste operates through a separate ion-channel mechanism. Indian physiology curricula (Guyton & Hall, Ganong) emphasize this functional segregation of taste modalities at the molecular level.

Why the other options are wrong

A. T1R1 — T1R1 is a taste receptor that forms a heterodimer with T1R3 to detect umami taste (glutamate and nucleotides like MSG). It is a G-protein-coupled receptor, not an ion channel. This is a common trap for students who confuse the T1R family members without understanding their specific ligand selectivity and the fundamental difference between metabotropic (T1R) and ionotropic (TRPP3) taste mechanisms. C. T1R3 — T1R3 is part of two heterodimeric complexes: T1R1+T1R3 (umami) and T1R2+T1R3 (sweet taste). It binds sugars and amino acids but does not respond to acidic stimuli. Students often memorize T1R receptors as a group without distinguishing that sour taste uses an entirely different receptor family (TRPP channels), leading to incorrect selection of T1R options. D. T1R2 — T1R2 pairs with T1R3 to form the sweet taste receptor, responding to sugars, artificial sweeteners, and some amino acids. Like T1R1, it is a metabotropic G-protein-coupled receptor. The NBE trap here is listing all T1R variants to confuse students who have not internalized that sour taste is fundamentally different—an ionotropic mechanism via TRPP3, not a G-protein pathway.

High-Yield Facts

Mnemonics

T1R = Sweet & Umami (NOT Sour) T1R = Taste 1 Receptor (metabotropic, G-protein). T1R2+T1R3 = Sweet; T1R1+T1R3 = Umami. Sour is separate (TRPP3). Use when you see T1R options—immediately exclude them for sour taste questions. TRPP3 = Proton-gated (Sour) TRPP3 = Transient Receptor Potential (ionotropic, ion-channel). Directly gated by H⁺. Remember: Protons → Perfect for sour. Use when identifying the molecular basis of acidic taste.

NBE Trap

NBE pairs all four taste receptor genes (T1R1, T1R2, T1R3, TRPP3) to test whether students understand that sour taste uses a fundamentally different receptor class (ionotropic TRPP3) compared to the metabotropic T1R family. Students who memorize "T1R receptors" without distinguishing their specific roles will incorrectly choose a T1R option.

Clinical Pearl

In Indian patients with chronic acid reflux or GERD, repeated exposure to gastric acid can desensitize TRPP3 receptors on the esophageal and pharyngeal mucosa, contributing to altered taste perception and dysphagia—a bedside clue that TRPP3 is the primary acid sensor in the upper GI tract.

_Reference: Guyton & Hall Textbook of Medical Physiology (Chapter on Special Senses); Ganong's Review of Medical Physiology (Taste and Smell)_