Donna C. Sullivan, PhD Division of Infectious Diseases University of Mississippi Medical Center Sweet Sour Bitter Salty Umami (the taste of monosodium glutamate). Sweet: Identify energy-rich nutrients Umami: Recognize amino acids Salt: Ensures proper dietary electrolyte balance Sour and bitter: Warn against the intake of potentially noxious or poisonous chemicals The inability to taste certain compounds is usually due to simple, recessive Mendelian inheritance. Dozens of taste and odorant receptors have been cloned and sequenced in the last 20 years. The TAS2R28 gene encodes a bitter taste receptor that enables humans to taste the compound PTC. The PTC (TAS2R28) gene has a single coding exon, for a polypeptide chain with 333 amino acids. PTC taste receptor, continued Three common single nucleotide polymorphisms (SNPs) are associated with PTC sensitivity. Each SNP results in a change to the amino acid sequence of the PTC receptor. Table 1. Polymorphisms within the PTC gene Position Position SNP Amino Acid (bp) (amino acid) Allele Encoded 145 49 C or G Pro or Ala 785 262 C or T Ala or Val 886 296 G or A Val or Ile Table 2. SNP haplotypes of the PTC gene within two study groups (named for the first letter of the amino acid present at positions 49, 262 and 296) Haplotype European Freq. East Asian Freq. PAV 49% 70% AVI 47% 30% AAV (from recomb. at aa 49) 3% - A later screen identified two additional haplotypes, PVI and AAI, which were found only in individuals of sub-Saharan African ancestry. The AVI haplotype was found in all populations except Southwest Native Americans (Kim et al., 2003). Table 3. Genotype association with taste phenotypes (by haplotypes) Genotype (diploid) AVI/AVI (73) AVI/AAV (21) */PAV (170) Nontasters 81% 52% 2% Tasters 19% 48% 98% *= PAV, AVI or AAV. The total number of PTC genotypes observed was 5, as no AAV homozygotes were observed in the study group (Kim et al, 2003). More Fun Informaton about PTC Receptor PTC taste sensitivity displays a broad and continuous distribution (e.g., it behaves like a quantitative trait). On average, PTC taste sensitivity is highest for the PAV/PAV (taster) homozygotes, slightly but significantly lower for the PAV heterozygotes, and lowest by far for the AVI/AVI (non-taster) homozygotes. More rare AVI/AAV heterozygotes have a mean PTC score slightly, but significantly, higher than the AVI/AVI homozygotes. All non-human primates examined to date are homozygous for the PAV (taster) haplotype. Thus, the AVI nontaster haplotype arose after humans diverged from the most recent common primate ancestor. There are non-taster chimps: same gene, but different mutation than humans => molecular convergent evolution!! PCR amplification and restriction digestion identifies the G-C polymorphism in the TAS2R38 gene. The “C” allele, on the right, is digested by HaeIII and correlates with PTC tasting.