Sweetness lies in the curly loops of a complex protein molecule. Chemists at the University of California at Berkeley have determined the three-dimensional structure of what they believe is the world’s sweetest compound, a substance derived from an African berry and called thaumatin I. It is one of the first taste-bearing proteins to be found — and how sweet it is! About 100,000 times sweeter than sugar, thaumatin I tastes sweet at concentrations as small as 1 molecule in 100 million. With X-ray crystallography, Sung-Hou Kim and colleagues have determined the arrangement of this molecule’s protein chain. The molecule has two different features: a series of slats called a beta sheet (a structure commonly found in proteins) and regions of complicated loops.
It is the loops that have attracted the scientists’ speculation. They resemble parts of other proteins, such as snake venom toxins and ragweed pollen allergens, that bind to specific receptors. Kim and colleagues report that antibodies that bind to the looped regions eliminate the sweetness of the proteins. They are now determining more specifically which loop areas are crucial in creating a sweet taste.