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Will science ever come up with a non-fattening sweetener that can satisfy a sweet tooth? As this ScienCentral News video explains, figuring out how sweet receptors in our taste buds work could be the key to better sweeteners.
Sweet Sensation
Artificial sweeteners have come a long way from the days of bitter-tasting saccharine. But still, "What is, I think, pretty clear to anybody who's ever tried to diet, most of the sweeteners don't taste exactly like sugar," says biophysicist Mariana Max, who uses sugar in the lab, but not in the lunchroom, where she sticks to a low-carb diet.
Max, and her group at Mount Sinai School of Medicine in New York, want to figure out how some five hundred different sweet compounds can provoke the sensation of sweet taste by binding to protein molecules, called "receptors" in our taste cells, and somehow triggering a cascade of events that relays the sweet signals to our brains "To the point where your brain can say, 'oh, there's something sweet in my mouth'," Max says. "One of the things we're trying to understand is how all of these different compounds can bind and activate this single receptor. The more we understand about how each of these individual sweeteners interacts with and binds to the receptor, the better able we will be to design sweeteners that taste more like sugar."
The upper surface of our tongues is covered in a concentration of taste buds each of which contains 50 to 100 taste cells representing all five taste sensations: salty, sour, sweet, bitter and umami (savoriness such as the taste of monosodium glutamate (MSG)). These cells respond to food in the way they do because the shape of their surface receptor proteins fit and bind to different taste sensation compounds. The receptor proteins straddle the outer membrane of cell and send taste signals into the cell.
Computer model of lactisole binding to sweet receptor image: Marianna Max
To figure out the structure of the human sweet taste receptor, Maxs group is using genetic and chemical detective work to understand different pieces of it. Max describes the part of the receptor protein on the outside of the cell, which binds to many sweet compounds, as very large and in the shape of a clamshell. When sweet compounds bind to it, the receptor changes shape to kind of close the clamshell, but they don't know why. "How does the sweetener bind?" asked Max. "How does the binding cause the receptor to change its shape in a way that then leads to activation and excitation of the taste cell and eventually you go 'oh, thats really sweet, I really like it'?"
To learn more about the receptor, the researchers studied a compound called lactisole, that doesn't taste sweet, but instead is able to block sweet taste in humans, though not in rodents. "So if you were to put it in your mouth at the same time you drink a glass of lemonade, it would only taste sour. You wouldnt taste the sweetness at all," Max explains, adding, "this blocker of sweet taste blocked all the sweet compounds that we tried."
"So, what we were trying to do is to figure out how this compound is able to antagonize the taste of sweet. We knew that it was dependent on part of the human taste receptor, and we wanted to know what part," explains Max. They created combinations of human and mouse taste cells in the lab so that they could find the part of the receptor that the lactisole binds to, preventing the cells from becoming excited when sweet compounds were added.
As they reported in the Journal of Biological Chemistry, they found that, "lactisole is binding in a part of the receptor that's very far away from where the sugars bind," says Max. This led them to what they believe is the actual "business end" of the receptor that actually communicates with machinery inside the cell. "It would be like getting closer to the point of the light bulb where the electricity passes through the light bulb and turns it on. Rather than being so far away at the switch, you're closer to the actual mechanism of what lights up the light bulb," she explains. "The 'business' part of the receptor is the part that's binding lactisole, and occasionally some other types of artificial sweeteners."
The group then tested this finding using a what's called homology modeling - using the 3-dimensional structure of a known receptor as a template for a computer to generate a 3-D structural model of the receptor. The automated computer program then repeatedly "threw" a molecule of lactisole at the site to see where it would fit. The results confirmed their lab results, giving them. "a lot of confidence that we actually had the right spot," Max says.
While Max isn't in the business of designing better sweeteners, she hopes food chemists will use her discoveries to do just that. Although she believes that they will certainly be able to come up with better artificial sweeteners than those currently available, she doubts sweeteners will ever be exactly like sugar. "Part of the problem is that sugar has properties that affect not only the taste but the texture of foods," she says.
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