Sour-To-Sweet Miracle Mechanism Revealed

Miraculin, the glycoprotein found in the so-called miracle berries of the West African plant Richadella dulcifica, is famous for its short-lived ability to make sour foods taste sweet. But until now, the molecular mechanism of its mystifying taste-modifying activity was unknown. Using calcium imaging to assay molecularly engineered cells, a research team led by Keiko Abe and Takumi Misaka of the University of Tokyo confirmed that miraculin exerts its sour-to-sweet influence by binding to the human sweet-taste receptor protein, hT1R2-hT1R3 (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1016644108). At neutral pH, the researchers found that miraculin leaves a flat taste by binding to the receptor and preventing other sweeteners, such as aspartame and sucrose, from accessing the cell membrane protein. Under the acidic pH conditions of sour foods, however, miraculin triggers the receptor, suggesting that, when protonated, the berry glycoprotein changes conformation, switching on the receptor and signaling a sweet taste. Further mutation and molecular docking studies revealed that miraculin binds to the amino-terminal domain of the hT1R2 subunit. The researchers say this is intriguing, given that the sweeteners inhibited by miraculin activate the receptor at other sites.