Now, UC Riverside researchers have identified a promising enzyme for that task, UGT76G4. This enzyme is key to producing Reb M, a highly valued all natural sweetener made from the Stevia plant. Clinton P. Kearney et al. studied the effects high-Reb M producing cultivar of Stevia rebaudiana (Bert.) This cultivar is renowned for producing the plant derived steviol glycosides, a class of natural sweeteners known for their high sweetness intensity and low caloric value, which has made them popular sugar alternatives.
UGT76G4 turned out to be a natural variant of the previously known enzyme UGT76G1. UGT76G1 was previously thought to be the sole enzyme to convert Rebaudioside D (Reb D) into Reb M. This assumption has yet to be proven. The discovery of UGT76G4 has painted a much clearer picture of the enzymatic process. This indicates that this enzyme catalyzes the production of Reb D to Reb M in the steviol glycosides biosynthetic pathway.
Interestingly, UGT76G4 has a unique conformational conformation of the substrate binding pocket in comparison to UGT76G1. This structural variation is key to the enzyme’s regioselectivity. It further increases its efficiency in synthesizing Reb M. In prior studies, UGT76G1 was found to exclusively modify the C13 position of steviol glycosides. It shows very limited catalytic activity at the C19 position. In contrast, UGT76G4 possesses a crucial residue that promotes effective Reb M biosynthesis.
Moreover, scientists have performed mechanistic studies of the molecular basis influencing substrate regioselectivity of UGT76G4. Their discovery elucidates how this enzyme accomplishes high catalytic efficiency. Prof. Yin and Yu Wang, along with their research team, recently published their results in the Proceedings of the National Academy of Sciences. They underscored how their effort can ultimately pave the way for better quality Reb M production.
Reb M has excellent properties, but its natural abundance in Stevia is extremely low. To bring down the costs and make it commercially viable, we have to go enzymatic conversion. The discovery of UGT76G4 has promising implications for increasing the production efficiency of this high-demand sweetener. This would go a long way in pleasing the increasingly avid consumer demand for natural sugar substitutes.