NutraSweet (aspartame) is not a carbohydrate but rather a modified dipeptide. What is the biochemical explanation for why this molecule is 500 times sweeter in human taste tests than the monosaccharide galactose?

Aspartame is found in Nutra sweet which is a modified dipeptide rather than a carbohydrate and they are many times sweeter than monosaccharides galactose because they binds very tightly with the heterodimer GPCR receptors present on the tongue which is made up of proteins called taste receptor type 1 member 2 and taste receptor type 1 member 3.

This tight binding to aspartame generates a neuronal output that is sent to the brain and the brain interprets it as a super sweet compound. It's decrease off rate also contribute to its more sweetening effect.

Therefore the type of binding to taste receptors explains why aspartame is 500 times sweeter in human taste test.