Bitter Taste Receptors Critical Review: A Focus on Tas2r38 Research Paper

Discovery of Phenylthiocarbamide

Phenylthiocarbamide (herein referred to as PTC) is a compound that was discovered by an organic chemist. The chemist who discovered this compound is Arthur Fox from the United Kingdom.¹ The discovery occurred in 1931 when the chemist “accidentally released a plume of fine crystalline powder in his laboratory”¹ (par. 1). The compound has a number of unique properties. However, the most conspicuous property of this substance is its bitter taste. Interestingly, Fox determined that not everyone could experience the bitter taste of this compound. Different people detected the taste of this compound differently. For example, there were those who found it to be bitter, while others found it to be mildly bitter. In addition, other people could not detect any bitterness at all.

The compound is still fascinating to researchers in this field, a number of years after its discovery. The researchers are fascinated by the ability of this compound to be detected by the taste buds of certain individuals and remain undetectable by others. Research has shown that the detection of PTC through taste buds is inherited.¹ Before DNA technologies gained popularity in the medical field, PTC tasting was used as some sort of paternity test.¹To this end, the compound is of profound interest in the field of genetics.

Mendelian Genetics and PTC Compound

The subject of inherited characteristics among species has existed in medical research for many years. The mechanisms of inheritance were originally explained by Gregor Johann Mendel in 1866.² The researcher remains one of the most prominent geneticists to date. Through his research and publications, it was determined that the gene was the unit responsible for the hereditary phenomenon. In order to come up with his theory, Mendel conducted experiments using Pisum sativum. The plant is commonly referred to as the garden pea.

Mendel was able to determine that, indeed, all species posses small units of ‘inheritance’. The proposals made by Mendel became significant principles of research on matters pertaining to inheritance.² The assumptions he made laid the foundation for transmission genetics.

Isolation of the Gene Responsible for Binding PTC

As already indicated above, the ability to taste PTC is a characteristic that can be inherited. The ‘inheritability’ of this ability can be explained using the principles proposed by Mendel. It follows that there is a gene responsible for this trait.¹ TAS2R is the gene responsible for the aforementioned hereditary pattern. Essentially, TAS2R “has been mapped to the q-arm of chromosome 7 in humans”¹ (par. 3). The polymorphism exhibited by the said gene results from three mutations. Consequently three different taste responses are reported as explained by Fox in the discourse above.

According to researchers, an individual who can taste the bitterness in PTC possesses a “PAV dominant allele”¹ (par. 3). On the other hand, individuals who cannot taste the bitterness of the compound possess a recessive allele designated as ‘the AVI’. However, according to Fox, there is a third category of individuals who detect moderate bitterness of this compound. Such individuals are in possession of PAV/AVI homozygotes.¹

The Structure of TAS2R38 Gene

The Taste Receptor 2 Member 38 (TAS2R38) is a gene that is protein in nature.³ The gene enables an individual to detect bitter tasting compounds like PTC. TAS2R38 is programmed into a G receptor that is protein- coupled. For the gene to realize this programming, the aforementioned receptor has to be a seven-transmembrane. In this case, the bitter compound, PTC, is the receptor’s ligand. In the process, the compound activates the receptor.

SNPs and Taste

Single-nucleotide polymorphism (herein referred to as SNP) is closely related to individual DNA structures. The polymorphism is a variation of an individual’s DNA sequence.² The variation is brought about by a number of factors. One of the factors is the difference between the genomes of a given species. The difference between the genomes is brought about by a subsequent variation in their common sequence. A case in point is DNA fragments from two different people, ranging from CCTAAG to CCTAGG. In such an instance, the difference in nucleotides is brought about by the presence of 2 alleles. The distribution of this phenomenon among humans is evident. The distribution helps one appreciate the variations in human population. To this end, some scholars have found that a common allele may be rare in certain groups of individuals.² What this means is that a trait that may be regarded as common in the general population may be rare in some sections of the same population.

As already indicated in this paper, studies have shown that some people can detect the bitter taste of PTC. However, there are those who are unable to perceive the same.² SNP can be used to explain this variation. The polymorphism helps researchers to appreciate such variations owing to the disparities in the nucleotides of a given genome.³ As such, the ability to detect the bitter taste of PTC depends on the nucleotides of individual genomes.

SNPs Structure and the Binding Ability of PTC

The TAS2R38 is responsible for the bitter taste experienced when one comes into contact with PTC.³ An analysis of SNP’s nucleotide helps reveal the receptor responsible for this observation. Studies have determined that the G protein-coupled receptor is the one responsible for the experience. In this regard, the PTC acts as the ligand.⁴ Binding, in this case, results from a process known as molecular docking (see figure 1). Such a process (also known as binding) must occur at a given site. In this case, the receptor activation occurs on helices 6 and 7. It is believed that N103 residue is an active component of PTC binding. The shape and size of the binding cavity are defined by a number of factors. They include W99, M100, and S259. The binding process largely relies on receptor activation. M100, V296, and F255 play a key role in this activation.

Distribution of Tasting in a Given Population

In any given population, the number of people who are unable to taste PTC’s bitterness is usually higher than the number of those who can taste it.⁵ The implication is that the prevalence of the non-tasting allele is quite high. It is interesting to note that human senses are wired to relate bitter tasting substances to toxins. As such, one would expect that many people will be able to detect bitter substances like PTC. However, the case is different due to natural selection. As a result of this selection, there is some form of balance between the number of individuals who can detect the bitterness and those who cannot.

References

Wallace S. Phenylthiocarbamide. Chemistry World Web site. Web.

Carlson A. Mendel’s Legacy: The Origin of Classical Genetics. New York: Cold Spring Habor Laboratory Press; 2004.

TAS2R38: Taste Receptor, Type 2, Member 38 [Homo Sapiens (human)]. NCBI Web.

Bitter Taste Receptor Polymorphism and Human Aging. NCBI Web.

Kim UK, Drayna D. Genetics of individual differences in bitter taste perception: lessons from the PTC gene. Clin Genet. 2004; 67: 275-280.