Receptor Biology

TRPV1 is a non-selective cation channel expressed in nociceptor neurons throughout the peripheral nervous system. It is the principal sensor for noxious heat (>43°C) and for the chemical pungency of capsaicin, resiniferatoxin, and related vanilloids. Channel opening triggers calcium influx, depolarization, and the downstream sensation of heat and pain.

The TAS1R2/TAS1R3 heterodimer is a class C G-protein-coupled receptor (GPCR) that mediates sweet taste. It responds to all known sweet compounds including sugars, amino acids (e.g., glycine), sweet proteins (thaumatin, monellin), and synthetic sweeteners (aspartame, sucralose). The two subunits form a large extracellular venus flytrap domain that closes upon ligand binding.

Umami is mediated by a heterodimer of TAS1R1 and TAS1R3, which responds primarily to L-glutamate (and is potentiated by 5′-ribonucleotides such as IMP and GMP). Independently, metabotropic glutamate receptor 1 (mGluR1) in the oral cavity also contributes to umami perception, particularly the deep, long-lasting character of kokumi-adjacent richness. The two pathways are synergistic.

TAS2R38 is a bitter taste receptor that specifically recognizes phenylthiocarbamide (PTC), propylthiouracil (PROP), and cruciferous glucosinolates such as sinigrin and glucoraphanin — the compounds that make broccoli and Brussels sprouts bitter. The PAV/AVI haplotype polymorphism explains why ~30% of the human population are non-tasters for these compounds. IFI-B engineering targets suppression of this receptor's activation, either by reducing glucosinolate pathway flux in brassica hosts or by competitive antagonism.

The calcium-sensing receptor (CaSR) is a class C GPCR best known for regulating parathyroid hormone secretion in calcium homeostasis. Its role in flavor was established when γ-glutamyl peptides — particularly γ-Glu-Val-Gly — were identified as potent kokumi-inducing CaSR agonists. Kokumi is not a primary taste but a taste-modulating sensation: it enhances and prolongs sweetness, saltiness, and umami without contributing flavor of its own. The sensation is described as mouthfulness, continuity, and thickness.

TRPM8 is the principal cold and menthol receptor in the mammalian peripheral nervous system. It is activated by temperatures below approximately 26°C and by cooling compounds including menthol, icilin, and geraniol. Cool-burn sensation — the simultaneous feeling of coolness and heat found in some Scoville Splice constructs — exploits the fact that TRPM8 and TRPV1 can be co-activated at intermediate temperatures or by specific compound mixtures. The TRPM8/TRPV1 ratio determines the balance between the cool and hot components of the sensation.

TRPA1 is a non-selective cation channel activated by electrophilic compounds — molecules that covalently modify cysteine residues on the channel’s N-terminal ankyrin repeat domain. This includes allyl isothiocyanate (the pungent compound in mustard and wasabi), allicin (garlic), cinnamaldehyde (cinnamon), acrolein, and environmental irritants. TRPA1 pungency has a nasal-clearing, sharp character distinctly different from the thermal burn of capsaicin — it is perceived primarily in the nasal cavity and the back of the throat.

Astringency is mechanistically distinct from the other seven IFI axes: it is not receptor-mediated but results from the cross-linking of proanthocyanidins (condensed tannins) with proline-rich salivary proteins. This cross-linking reduces salivary lubrication, producing the characteristic dry, puckering sensation in red wine, strong tea, unripe fruit, and dark chocolate. The primary compounds are procyanidins and prodelphinidins — polymers of flavan-3-ol monomers such as catechin and epicatechin.