β-Glucosidase and β-Galactosidase-Mediated Transglycosylation of Steviol Glycosides Utilizing Industrial Byproducts.

Anastasia Zerva,Nikolaos S Thomaidis,Evangelos Topakas,Anastasia S Kritikou,Koar Chorozian

doi:10.3389/fbioe.2021.685099

Abstract

Stevia rebaudiana Bertoni is a plant cultivated worldwide due to its use as a sweetener. The sweet taste of stevia is attributed to its numerous steviol glycosides, however, their use is still limited, due to their bitter aftertaste. The transglycosylation of steviol glycosides, aiming at the improvement of their taste, has been reported for many enzymes, however, glycosyl hydrolases are not extensively studied in this respect. In the present study, a β-glucosidase, MtBgl3a, and a β-galactosidase, TtbGal1, have been applied in the transglycosylation of two steviol glycosides, stevioside and rebaudioside A. The maximum conversion yields were 34.6 and 33.1% for stevioside, while 25.6 and 37.6% were obtained for rebaudioside A conversion by MtBgl3a and TtbGal1, respectively. Low-cost industrial byproducts were employed as sugar donors, such as cellulose hydrolyzate and acid whey for TtbGal1- and MtBgl3a- mediated bioconversion, respectively. LC-HRMS analysis identified the formation of mono- and di- glycosylated products from stevioside and rebaudioside A. Overall, the results of the present work indicate that both biocatalysts can be exploited for the design of a cost-effective process for the modification of steviol glycosides.

Highlights

Stevia rebaudiana Bertoni is a plant native to south America, cultivated worldwide due to its use as a low-caloric sweetener
Stevia stems and leaves are rich in diterpenoid glycosides, mainly stevioside and rebaudioside A, but many more minor glycosides have been isolated
Cyclodextrin glucanotransferases (CGTases, 1,4α-d-glucan: 1,4-α-d-glucopyranosyltransferase, cyclizing, EC 2.4.1.19) are the most common enzymes used for steviol glycosides transglycosylation, introducing α-1,4 glycosidic bonds with very high yields, exceeding 70% (Li et al, 2013; Yu et al, 2015; Zhang et al, 2015)

Summary

Introduction

Stevia rebaudiana Bertoni is a plant native to south America, cultivated worldwide due to its use as a low-caloric sweetener. The chemical structure of stevia glycosides includes the diterpenoid steviol, glycosylated through an ester bond at the C13 or C19 with one or more glucose moieties, connected with β-1,2 or β-1,3 glycosidic bond (Nguyen et al, 2019). Steviol glycoside-containing products are not so well received by a large number of Enzymatic Transglycosylation of Steviol Glycosides consumers, due to their lingering bitter aftertaste. Rebaudioside A, due to an additional sugar moiety, is unable to interact with the receptor, and has less lingering bitter taste than stevioside. The hydrolysis of α- 1,4 bonds by human amylases can increase the caloric value of the modified steviosides, resulting in a final product with low appeal to target consumer groups, e.g., diabetics (Gerwig et al, 2016). The increase is expected to be very low, or even negligible, considering the low dose of steviol glucosides required to obtain the desired level of sweetness

Results

Discussion

Conclusion