''Hmm, let's see. I detect a rather unusual grape variety with a very fragrant and grassy aroma. There are some nutty, crunchy and crisp flavours with a honeyed, ripe style of fruit. If I'm not mistaken, this must be a Trincadeira das Pratas from 1993..."
Professional wine tasters can often identify the vineyard and the vintage of a wine from the smell and taste of a little sip (although they would probably sneer at the above example from a supermarket leaflet). Exactly what happens on their tongues and palates when they are doing their job has so far remained a mystery. Of the five human senses, taste is probably the least understood. The mass of genetic information flooding in with the human genome project, however, has now helped scientists to start closing this gap in their knowledge. In the long term, an improved understanding of these issues could also lead to applications that would help us to enjoy a healthier lifestyle.
The experience one normally calls taste is in fact a combination of our senses of taste and smell - that's why many things taste indistinguishable with a blocked nose. As for taste in the narrower sense, the trouble already starts with the number of basic tastes that the taste buds in our tongues and palates can distinguish. Textbooks will tell you there are four of them: salty, sour, sweet, bitter.
However, scientists discovered a new taste only last year. They found that a certain taste component, which is contained in meat and soy sauce and an important part of the experience of a Chinese takeaway, is independent of the four mentioned above, bringing the total to five. A pioneering Japanese researcher who investigated this nearly a century ago called it umami, which is derived from the Japanese words for taste and tasty.
The perceptions of salty and sour are relatively easy to explain, as each only involves a single type of charged atomic particles (called ions), which most cells easily recognise. Umami is about one kind of molecule only: glutamate. This is an amino acid found in most proteins, so this taste presumably evolved because it allowed us to recognise food that was rich in protein.
One can easily imagine that taste cells in charge of detecting the umami component have on their surface a certain kind of molecule, called a receptor, which accommodates the glutamate molecule like a lock recognises the right key, and then triggers some onward signalling action which ultimately leads to the taste perception in the brain. (Similarly, the hot sensation provoked by chilli peppers has been traced back to a single type of molecule, but this one triggers pain receptors rather than taste).
But sweet and bitter have remained elusive, because there are many different substances that trigger these taste sensations without necessarily resembling each other. Sweet taste obviously tells us that there is food rich in sugar or other carbohydrates, which can supply the body with a lot of energy.
There are many different kinds of sugars, but they have certain chemical structures in common which could serve as keys to unlock the sweet sensation. However, scientists are puzzled by the observation that some natural proteins are more than a thousand time more efficient in sweetening than ordinary sugar. Presumably they can bind the parts that sugars cannot reach.
The situation is equally confusing for bitter substances. They can have many different structures which to a chemist have nothing whatsoever in common, but still all taste the same. Often these are substances (like cyanides) that are poisonous or chemically related to some kind of poison, so the natural role of bitter taste is probably a warning signal. Children naturally avoid bitter food, and when we favour certain kinds of bitter taste, it is most likely an acquired taste. With so many different bitter substances as keys, there had to be many different locks as well, so it was all the more surprising that none could be found. In fact, until last year, not a single receptor for any taste had been identified.
By analogy to other sensory perceptions, and to the well-studied way that hormones work, scientists expect that the taste receptors will pass the information on to a protein from a certain large family, the G proteins. They represent a kind of switchboard in the cell, which is known to be involved in many very important signalling processes. In 1992, the group of Robert Margolskee at the Roche Research Center in Nutley, New Jersey, identified one G protein they believed to be involved in taste signalling, and named it gustducin (derived from the name of the G protein involved in vision, transducin, plus gustatory, for taste-related).
But when, eventually, a pair of receptors were found in 1999, they turned up only in cells lacking gustducin. It was a situation a bit like digging a tunnel from both ends but failing to meet in the middle: rather than building up a complete pathway for the information to be passed on from the outside of the taste cells to the nerves forwarding it to the brain, researchers now had two loose ends: a pair of taste receptors without the corresponding G protein, and a taste-related G protein lacking a receptor.
Things only started to get better when the teams of Charles Zuker at the University of California in San Diego and of Nicholas Ryba at the National Institutes of Health in Bethesda, Maryland, began using genome data for the search of taste receptors. They decided to study people who have inherited a gene that makes them unable to taste a certain bitter substance called PROP. Although the gene had not yet been identified, the researchers knew that it had to be in a certain region of chromosome 5.
When the sequences of this area became available last year, they used computer-assisted searches to check whether it had any genes coding for receptors that look like the type that can talk to G proteins. They found one, which they called T2R1, and then went on searching for genes similar to this one. The search resulted in 19 further candidate taste receptor genes, which implies that this family could have up to 80 members throughout the genome. Unlike the receptors found in 1999, these are plausible partners for gustducin, as the proteins are produced in the same cells.
Indeed, Ryba, Zuker and their co-workers have shown that the T2R family not only look like taste receptors but that they recognise one bitter substance each. To prove that suitable keys actually turned the lock, the scientists used cell cultures derived from mouse taste cells. This was only possible because the corresponding area of the mouse genome is very similar to the human one, and the mouse receptors could be readily identified and matched to the human analogues.
Ground-breaking research findings in a particular field are like buses, only worse - you wait for years or even decades, and then you have five in a row. Thus, independently of Zuker's group, a team at Harvard University also reported having found a family of bitter taste receptors as a result of screening through the available sequences of mouse and human DNA. And a group at Yale found molecules which are probably taste receptors in the fruit fly, and another paper reported a receptor for umami. Separate teams at the universities of Texas and Florida reported for the first time the detailed effects of specific regions within the brain that respond to an intake of sugar.
Obviously, the time has finally come for a better understanding of taste perception. Apart from putting an end to the embarrassing lack of information about such a fundamental aspect of daily life, a better understanding of taste may have practical benefits. That famous bitter medicine could be defused by addition of an anti-bitter agent, rather than trying to cover it up with a lot of sugar. Healthy food could be made to taste better than unhealthy stuff. If you don't like what you're eating, maybe you could retune the taste to something better. Wine buffs, of course, will shiver at the thought that in the future a cheap little wine could be manipulated in a way that would even fool their well-trained taste buds.
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