Protein's shape may give extra-sugary taste.

"Short and sweet" best describes brazzein, a protein found in a West African fruit. Just 54 amino acids long and 2,000 times sweeter than sugar, brazzein is one of only six proteins known to taste sugary to humans and other primates.

Now, researchers at the University of Wisconsin-Madison have determined brazzein's three-dimensional structure, a finding that may eventually shed light on what gives these proteins their flavor. Presumably, it is their shape that allows them to bind to sweet taste receptors on the tongue. Scientists already know the structures of two other sweet proteins, thaumatin and monellin (SN: 5/10/97, p. 284), but they don't seem to resemble each other.

Brazzein doesn't appear to resemble either thaumatin or monellin. In fact, its structure has more in common with some scorpion toxins and plant defense proteins. That similarity "doesn't really tell us much" about why brazzein tastes sweet, says Wisconsin's Goran B. Hellekant.

It may, however, say something about how the protein evolved, he notes. Brazzein's resemblance to proteins that plants use to defend themselves against microbes suggests that its precursor once had the same function in the fruit. Through mutations, the original protein may have lost its defense capability and become sweet, Hellekant speculates. He, John L. Markley, and their colleagues report the findings in the June Nature Structural Biology.

"This is an important piece of work in the context of protein sweeteners," says Joseph Brand, associate director of the Monell Chemical Senses Center in Philadelphia. Comparing the structure of brazzein, which is smaller and more rigid, to those of thaumatin and monellin may allow researchers to understand how all three bind to sweet taste receptors, he adds.

Brazzein's rigidity originates from four links called disulfide bridges that lock in the protein's three-dimensional shape. The bridges make brazzein very stable to heat, a property the food industry finds attractive. Once unfolded, brazzein loses its flavor, as do the other sweet proteins. The researchers next intend to shuffle the protein's sections to see how their arrangement affects the taste.

Hellekant envisions that the food industry could use brazzein as an additive like sugar or aspartame or, via genetic engineering, transfer it into fruits and vegetables. Because brazzein comes from a plant, Pentadiplandra brazzeana BailIon, it should be easier to incorporate into crops than aspartame, an artificial sweetener, he suggests.

African monkeys and apes find the intensely sweet fruit irresistible, Hellekant says. Perhaps one day, brazzein will make other foods just as tasty to humans.