Regio-selective glycosylation of mogroside IIIE by glycosyltransferase

Mogrosides, the main sweet components isolated from Siraitia grosvenorii, are a family of cucurbitane-type tetracyclic triterpenoid saponins. Given that the high sweetness, low calorie and excellent taste, mogrosides have become the important resource for the development of natural non-nutritive sweeteners. As reported, 11α-hydroxyl group in the structural skeleton of mogrosides was closely related to sweetness and taste, but it had not been confirmed experimentally. In this work, we used mogroside IIIE as a model compound, which was 300 times sweeter than 5% sucrose and tasted better, and modified its 11α-hydroxyl group through glycosyltransferase to elucidate the relations between structure and sweetness of mogroside compounds. The glycosyltransferase HXSW-GT-2 was obtained to regio-selectively glycosylate the 11α-hydroxyl group of mogroside IIIE through the screening of glycosyltransferase library. And then, the soluble expression of HXSW-GT-02 in Escherichia coli was efficiently achieved by optimizing the induction conditions. Subsequently, the yield of glycosylated mogroside IIIE(MG-IIIE-Glu)was increased to > 85% through optimizing reaction pH, temperature, UDP-G dosage and biocatalyst loading. The product MG-IIIE-Glu was bio-prepared at a 0. 5 L scale and the final purity was 97. 8%. A “mouth feel” test showed that MG-IIIE-Glu had no sweetness and displayed obvious bitterness through the comparison with mogroside IIIE and 5% sucrose. In conclusion, the function of the 11α-hydroxyl group of mogrosides in sweetness and taste was preliminarily elucidated which would be beneficial for the structural modification and development of mogroside sweeteners.