Weird facts about protein receptors
Why does mint taste cold? Iona Twaddell answers these questions and more
Receptors are proteins embedded in cell membranes that detect something, be it a chemical, vibration or light. If the target substance (called a ligand) binds to the channels, they will open and allow chemicals to flow in and/or out of the cell, setting off a series of reactions. But sometimes the signal from the receptor is ambiguous, which leads to some interesting phenomena.
Why does mint taste cold?
Mint-flavoured things tend to taste cold. This is because of receptors in the mouth that respond to mint – or, more specifically, to menthol, a chemical in mints.
The receptor of interest is a protein called TRPM8 (transient receptor potential cation channel, subfamily M, member 8). It is an ion channel which, when open, allows sodium ions and calcium ions to enter, causing an action potential (an electrical signal down a neuron). Menthol causes the TRPM8 channel to open.
But TRPM8 is also a temperature receptor that opens in response to low temperatures. When you eat something containing menthol, the TRPM8 channel opens, but this signal is indistinguishable from the signal that would be produced if the receptor sensed cold. Therefore, when it detects menthol, the receptor signals ‘cold’, making mint feel cold.
Why do chillies taste hot?
The reason chillies taste hot is very similar to the reason mint tastes cold. It is due to a receptor called TRPV1 (transient receptor potential cation channel, subfamily V, member 1).
Chillies contain the chemical capsaicin, which activates and opens the TRPV1 channel. But guess what else activates the TRPV1 channel? Heat. Therefore, when the capsaicin in the chilli opens the channel, there is no signal telling the brain why the channel opened, so the brain thinks it’s because of heat.
Very strong capsaicin is also used in things like pepper spray for self-defence. Although pepper spray causes a burning sensation, it does not cause lasting damage (unless it is sprayed in the eyes) – it just tricks your receptors into thinking it is burning you. Capsaicin is also being investigated as a painkiller.
How can a lemon taste sweet?
A lemon tastes sour. This fact is indisputable. But what if you were able to make a lemon activate sweet receptors? This is what the miracle berry (Synsepalum dulcificum) does. The miracle berry contains a molecule called miraculin, which doesn’t taste sweet by itself but binds to the sweet receptors on the tongue.
Sweet is one of the five basic tastes our taste receptors are sensitive to. The other basic tastes are salty, sour, bitter and umami (a savoury taste that is quite meaty or ‘brothy’, caused by the presence of the amino acid glutamate).
At neutral pH (pH7), miraculin blocks the receptors so sweetness can’t be tasted. At a low pH (when acidic foods such as lemons are eaten), there are extra protons in the mouth, because all acids contain protons. These protons bind to miraculin, changing its shape and activating the sweet receptors, which signal ‘sweet’ to the brain.
Therefore, with miraculin, sour food that creates a low pH environment will activate sweet receptors and taste sweet. The miraculous effect lasts up to an hour, until the saliva washes away the miraculin.
The berry has been used in West Africa as a sweetener for hundreds of years. It could provide a way to sweeten foods without adding unhealthy sugar, which would be especially useful for people with diabetes.
Other plants also cause this ‘taste tripping’ sensation. One is Gymnema sylvestre, which has the opposite effect to the miracle berry. It contains gymnemic acid, which blocks the sweet receptor, abolishing the sensation of sweetness.
Why does hot water sometimes feel cold?
Have you ever put your hand in hot water and noticed how, at first, it feels ice cold? This is ‘paradoxical cold’ and occurs because our receptors get mixed up. Some receptors are sensitive to cold, and others are sensitive to heat. They only respond in one way and to one type of temperature, so even if a cold receptor is accidentally stimulated by heat it will still send a cold signal to the brain.
It seems that some cold receptors can be activated by both low temperatures and very high temperatures, and this is what happens sometimes when you run your hand under hot water. The hot water causes a signal so strong that some cold receptors are activated and send a cold signal to the brain, so the water feels cold. This normally rights itself quickly and you realise you’ve actually got your hand under something hot (and are getting burned).
Illustration © Glen McBeth
Questions for discussion
- Why would sweetening food with miracle berry instead of sugar be beneficial for people with diabetes and/or those trying to lose weight?
- Why does ground pepper also feel hot? And why does it make you sneeze?