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Review
. 2022 Apr;87(4):1449-1465.
doi: 10.1111/1750-3841.16101. Epub 2022 Mar 17.

Molecular insights into human taste perception and umami tastants: A review

Affiliations
Review

Molecular insights into human taste perception and umami tastants: A review

Johan Diepeveen et al. J Food Sci. 2022 Apr.

Abstract

Understanding taste is key for optimizing the palatability of seaweeds and other non-animal-based foods rich in protein. The lingual papillae in the mouth hold taste buds with taste receptors for the five gustatory taste qualities. Each taste bud contains three distinct cell types, of which Type II cells carry various G protein-coupled receptors that can detect sweet, bitter, or umami tastants, while type III cells detect sour, and likely salty stimuli. Upon ligand binding, receptor-linked intracellular heterotrimeric G proteins initiate a cascade of downstream events which activate the afferent nerve fibers for taste perception in the brain. The taste of amino acids depends on the hydrophobicity, size, charge, isoelectric point, chirality of the alpha carbon, and the functional groups on their side chains. The principal umami ingredient monosodium l-glutamate, broadly known as MSG, loses umami taste upon acetylation, esterification, or methylation, but is able to form flat configurations that bind well to the umami taste receptor. Ribonucleotides such as guanosine monophosphate and inosine monophosphate strongly enhance umami taste when l-glutamate is present. Ribonucleotides bind to the outer section of the venus flytrap domain of the receptor dimer and stabilize the closed conformation. Concentrations of glutamate, aspartate, arginate, and other compounds in food products may enhance saltiness and overall flavor. Umami ingredients may help to reduce the consumption of salts and fats in the general population and increase food consumption in the elderly.

Keywords: TAS1R1/TAS1R3; flavor; protein; taste receptor; umami.

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Conflict of interest statement

The authors declare that there is no conflict of interest.

Figures

FIGURE 1
FIGURE 1
Anatomy of the human tongue. On the surface of the tongue (upper left) four types of lingual papillae (circumvallate, fungiform, filiform, and foliate) can be distinguished as indicated. A cross‐section of a circumvallate papilla is shown upper right. A schematic representation of a taste bud with basal, gustatory, and transitional cells is shown lower right. Taste hairs of the gustatory cells are accessible to saliva via taste pores (OpenStax Anatomy & Physiology, 2016)
FIGURE 2
FIGURE 2
Phylogenetic relationship of the five classes of G Protein‐Coupled Receptors (GPCRs). Based on an alignment of 826 human GPCRs, Lv et al. (2016) produced a phylogenetic tree that is shown in a compiled way. The number of members within each class is indicated between brackets. The taste receptor families TAS1R and GRM belong to the glutamate class whereas the TAS2R family belongs to the Frizzled & Taste 2 class
FIGURE 3
FIGURE 3
The umami taste receptor dimer TAS1R1+TAS1R3 in the plasma membrane and the intracellular G‐protein heterotrimer. The taste receptor heterodimer consists of TAS1R1 and TAS1R3, each with seven transmembrane protein segments (trans membrane domain, TMD) that anchor them in the taste cell membrane. Each monomer has two lobes on the extracellular side, together known as the Venus flytrap domain (VFTD). These N‐terminal lobes are attached to the TMD via a cysteine‐rich domain (CRD). The lobes, TMD, and CRD all function as possible ligand binding sites. Black jagged lines represent membrane attachment of the G proteins α and γ that reside with G protein β on the cytoplasmic side. The G protein trimer interacts with the C‐termini of the receptor proteins
FIGURE 4
FIGURE 4
Taste transduction pathways for sweet, umami, and bitter taste. Tastant binding enables the GPCR dimer (top left) to cause GTP‐driven dissociation of gustducin subunit Gα‐ from subunits Gβ3/Gγ13. This triggers phospholipase Cβ2 (PLCβ2) to hydrolyze phosphatidylinositol 4,5‐bisphosphate (PIP2) into the second messenger inositol 1,4,5‐triphosphate (IP3) and diacylglycerol (DAG). The release of Ca2+ from the endoplasmic reticulum via the type 3 IP3 receptor (IP3R3) raises cytosolic calcium [Ca2+]i and targets the channel protein TRPM5. Na+ inflow through voltage‐gated sodium channels (VGNa+) results in cell depolarization and the action potential triggers the opening of calcium homeostasis modulators 1 and 3 (CALHM1 and 3), through which adenosine triphosphate (ATP), produced by atypical mitochondria, is released onto gustatory afferent nerve fibers. Drawing inspired by Luddi et al. (2019)
FIGURE 5
FIGURE 5
l‐Glutamate and the relationship between umami taste and the chemical structures of related compounds. A. The linear molecule of glutamate forms an eclipsed conformation promoted by electrostatic interaction of CO2 and NH3 + which allows structural attachment to the umami taste receptor, facilitated by the three radicals of the α‐amino, γ‐carboxyl, and α‐hydrogen atoms (underlined).B. The three structures on the left elicit umami taste, whereas the three related structures on the right do not (based on Komata, 1990)
FIGURE 6
FIGURE 6
Umami taste synergism. Synergism of glutamate and inosine monophosphate (IMP) yields an inverted U shape when taste intensity (y‐axis) is plotted against compound proportion (x‐axis). Based on data from Yamaguchi et al. (1967)

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