Three years ago, a friend and I were eating a slice of lime. If we had had any normal sense of taste, the sour fruit would have made us squint our eyes and twist our faces. Instead, we just sucked the juice from the fruit without twitching a muscle. To mock sourness some more, we had a sip of vinegar. My friend and I were 'taste tripping'.
Moments before, we had let a tablet dissolve in our mouths. The tablet itself had no taste, but it did form a thin layer of film on our tongues. The packaging said that the active ingredient was a mysterious protein called miraculin. The effects of this protein were miraculous indeed: for more than an hour, sour tasted sweet.
Miraculin was first extracted in 1968, from a berry that grows in West Africa. The local population knew about the effects of the berry for much longer. They chewed on the pulp of the fruit to make stale and sour maize bread more tasty, for example. At the time, scientists didn't know exactly how miraculin worked, but they did have a hunch. "It is believed that the protein binds to receptors of the taste buds and modifies their function", is what they wrote. Now, forty years after the initial isolation of miraculin, researchers from Japan and France have proved them right.
The research team discovered that miraculin binds to the human sweet taste receptor (hT1R2-hT1R3). Most molecules that bind the sweet taste receptor, such as sugar and aspartame, induce a sweet sensation, but this is not the case for miraculin. Miraculin only activates the sweet taste receptor in a sour environment. This explains why vinegar tastes as sweet as syrup once your entire tongue is covered with miraculin. In a neutral environment the sweet taste receptor doesn't respond to miraculin at all.
Miraculin is different from other sweeteners in this regard. Aspartame always activates the sweet taste receptor for example, no matter how acidic or neutral the environment is.
The researchers found that in some cases, miraculin competes with these sweeteners. They first exposed the receptors to miraculin and then applied aspartame in a neutral environment. The receptors remained silent. However, something strange happened when the researchers made the environment a bit more sour. As soon as the researchers added aspartame, the receptor's response skyrocketed, increasing to levels way beyond a normal aspartame response.
This is speculation on my part, but this means it should be interesting to try some goat's milk in combination with miraculin. According to this table, goat's milk is slightly acidic, so during a taste trip, it should taste sweeter than it normally does.
The team also determined to which part of the receptor miraculin binds. For these experiments they exploited the fact that miraculin has no effect on the sweet taste receptors of mice. By swapping and mixing pieces of human and mouse receptor, they found that a small part of the human T1R2 protein was all it took for miraculin to activate the receptor. This region is different from where most sweeteners bind the receptor, so it's unlikely that miraculin directly competes with them.
Instead, the researchers think that when miraculin binds the sweet taste receptor, it changes its shape in such a way that it becomes impossible for other sweeteners to bind. Then, when the environment becomes more sour, miraculin changes shape itself, reactivating the receptor. Two amino acids in the miraculin protein are particularly important for shapeshifting. They are histidines, which gain protons in acidic conditions. This changes the overall charge of miraculin, which could lead to a change of shape.
This is not the entire story of what miraculin does to our sense of taste. In addition to making sour taste sweet, it also seems to neutralize any eye-twitching sourness. Keiko Abe, one of the main authors of the study, said: "Miraculin weakens sourness, so it is likely that miraculin blocks the function of some sour taste receptor." They are currently investigating whether this is the case.
It is easy to take the way we perceive our environment for granted, but even small protein like miraculin can turn our senses on their head. This all goes to show how much we depend on the proteins that form our cells and the molecules that cross our way. Indeed, cats and rodents have a mutation in their T1R2 gene, which makes it impossible for them to taste sweetness at all. So just remember: when life turns sour, just apply some miracle proteins. They will sweeten your day.
Note: Ed Yong from the excellent Not Exactly Rocket Science has also written about miraculin. Check out his take on the story here.
Miracle berries by Ohkubo.
Ayako Koizumi, Asami Tsuchiya, Ken-ichiro Nakajima, Keisuke Ito, Tohru Terada, Akiko Shimizu-Ibuka, Loc Briand, Tomiko Asakura, Takuma Misaka, & Keiko Abe (2011). Human sweet taste receptor mediates acid-induced sweetness of miraculin Proceedings of the National Academy of Sciences : 10.1073/pnas.1016644108