FoundationalConceptsNeuroscience

molecules. The particular molecular shapes of the various sugar mol-
ecules permit them to bind as ligands to the sweet GPCRs, shape-shift
the proteins, and initiate a signal. The functional form of the sweet
taste receptor protein appears to be a dimer of two GPCRs—that is, the
two GPCRs are linked (by a noncovalent interaction) to form the func-
tional sweet receptor.
Sweet taste is generally experienced as pleasurable and often elicits
a desire to consume more of whatever it is that tastes sweet. Sweet-
tasting things are not very common in the nature. Ripe fruits are gen-
erally sweet, and they appear only during limited times of the year.
Until very recently in the history of life on Earth, consuming a whole
lot of whatever it was that tasted sweet was almost certainly a healthy
thing to do, providing energy and other nutritional goodies. However,
in our sugar-saturated contemporary society, this is no longer the
case, and overconsumption of sugary foods has led to serious health
problems for many people. Unfortunately, this trend toward increas-
ing consumption of unhealthy quantities of sugar is continuing to
grow worldwide.
Unlike the thirty or so GPCRs that allow a variety of molecular
shapes to be associated with the taste of bitter, the two GPCRs as-
sociated with sweet receptor cells limit the taste of sweetness toa
smaller diversity of molecular shapes. Indeed, the several sweet-
tasting sugar molecules all share many features of molecular shape.
There are, however, molecules that are not sugars and possess a va-
riety of shapes, and yet still taste sweet. Some of these molecules are
really, really sweet, far more potent than sucrose in their sweetness.
They constitute the so-called nonnutritive (because they have little
or no caloric or other nutritional value) or synthetic (because they are
products of the synthetic chemical industry) sweeteners, and they are
used to sweeten foods without adding calories. They are also some-