Steviol glycosides, natural sweeteners extracted from Stevia rebaudiana, are widely used as sucrose substitutes due to their high sweetness and low caloric value. Among them, Rebaudioside M (Reb M) is regarded as a next-generation, high-value steviol glycoside product because of its intense sweetness and superior taste profile. However, the natural abundance of Reb M in Stevia is extremely low.

Efficient biosynthetic methods are needed to meet market demand. Until now, the key enzyme catalyzing the conversion of Rebaudioside D (Reb D) to Reb M in the biosynthetic pathway has not been identified, and it is generally assumed to be UGT76G1. However, UGT76G1 exhibits strict regioselectivity for the C13 position of steviol glycosides, while its catalytic activity at the C19 position is very weak.

In a study published in the Proceedings of the National Academy of Sciences on September 17, a team led by Prof. Yin Heng from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences identified the key glycosyltransferase that catalyzes the conversion of Reb D to Reb M, and revealed the molecular mechanism underlying its substrate regioselectivity.