Improving maltodextrin specificity for enzymatic synthesis of 2-O-d-glucopyranosyl-l-ascorbic acid by site-saturation engineering of subsite-3 in cyclodextrin glycosyltransferase from Paenibacillus macerans

Enhancing 2-O-α-D-glucopyranosyl-L-ascorbic acid synthesis by weakening the acceptor specificity of CGTase toward glucose and maltose

2-O-α-D-glucopyranosyl-L-ascorbic acid (AA-2G) is a stable derivative of L-ascorbic acid (L-AA), which has been widely used in food and cosmetics industries. Sugar molecules, such as glucose and maltose produced by cyclodextrin glycosyltransferase (CGTase) during AA-2G synthesis may compete with L-AA as the acceptors, resulting in low AA-2G yield. Multiple sequence alignment combined with structural simulation analysis indicated that residues at positions 191 and 255 of CGTase may be responsible for the difference in substrate specificity. To investigate the effect of these two residues on the acceptor preference and the AA-2G yield, five single mutants Bs F191Y, Bs F255Y, Bc Y195F, Pm Y195F and Pm Y260F of three CGTases from Bacillus stearothermophilus NO2 (Bs), Bacillus circulans 251 (Bc) and Paenibacillus macerans (Pm) were designed for AA-2G synthesis. Under optimal conditions, the AA-2G yields of the mutants Bs F191Y and Bs F255Y AA-2G were 34.3% and 7.9% lower than that of Bs CGTase, respectively. The AA-2G yields of mutant Bc Y195F, Pm Y195F and Pm Y260F were 45.8%, 36.9% and 12.6% higher than those of wild-type CGTases, respectively. Kinetic studies revealed that the residues at positions 191 and 255 of the three CGTases were F, which decreased glucose and maltose specificity and increased L-AA specificity. This study not only proposes for the first time that the AA-2G yield can be improved by weakening the acceptor specificity of CGTase toward sugar byproducts, but also provides new insight on the modification of CGTase that catalyze the double-substrate transglycosylation reaction.

Producing 2-O-α-D-glucopyranosyl-L-ascorbic acid by modified cyclodextrin glucosyltransferase and isoamylase

In this study, site saturation mutagenesis was performed on the - 3 (R44, D86, S90, and D192) and - 6 subsite (Y163, G175, G176, and N189) of Bacillus stearothermophilus NO2 cyclodextrin glucosyltransferase to enhance its specificity for the donor substrate maltodextrin for 2-O-α-D-glucopyranosyl-L-ascorbic acid (AA-2G) preparation. The AA-2G yields produced by the mutants S90D, G176H, and S90D/G176H were 181, 171, and 185 g/L, respectively. Our previous study found that the mutant K228R/M230L also increased the AA-2G yield. Therefore, the mutants S90D, G176H, S90D/G176H, and K228R/M230L were further used to generate combinatorial mutants. Among these mutants, the highest AA-2G yield (217 g/L) was produced by S90D/K228R/M230L with 500 g/L maltodextrin as the glucosyl donor, which was 56 g/L higher than that produced by wild-type CGTase. In addition, AA-2G was prepared by adding isoamylase to hydrolyze α-1,6 glucosidic linkages in maltodextrin that could not be utilized by CGTase to improve the utilization rate of maltodextrin. The addition of isoamylase reduced the concentration of maltodextrin from 500 to 350 g/L, while the AA-2G yield remained high (208 g/L). The preparation of AA-2G by complexing isoamylase with mutant S90D/K228R/M230L reduced the maltodextrin concentration by 150 g/L, while the AA-2G yield increased by 47 g/L than preparation with wild-type CGTase alone, which laid a foundation for the large-scale preparation of AA-2G. KEY POINTS: • Mutants exhibited improved maltodextrin specificity. • Mutant S90D/K228R/M230L produced high yield of AA-2G with maltodextrin as substrate. • AA-2G was first synthesized by a combination of isoamylase and CGTase.

Current studies on the enzymatic preparation 2-O-α-d-glucopyranosyl-l-ascorbic acid with cyclodextrin glycosyltransferase

2-O-α-d-glucopyranosyl-l-ascorbic acid (AA-2G) is one of the most important l-ascorbic acid derivatives because of its resistance to reduction and oxidation and its easy degradation by α-glucosidase to release l-ascorbic acid and glucose. Thus, AA-2G has commercial uses in food, medicines and cosmetics. This article presents a review of recent studies on the enzymatic production of AA-2G using cyclodextrin glycosyltransferase. Reaction mechanisms with different donor substrates are discussed. Protein engineering, physical and biological studies of cyclodextrin glycosyltransferase are introduced from the viewpoint of effective AA-2G production. Future prospects for the production of AA-2G using cyclodextrin glycosyltransferase are reviewed.

Enhanced 2- O-α-d-Glucopyranosyl-l-ascorbic Acid Synthesis through Iterative Saturation Mutagenesis of Acceptor Subsite Residues in Bacillus stearothermophilus NO2 Cyclodextrin Glycosyltransferase

Low synthesis yields of the l-ascorbic acid (l-AA) derivative 2- O-α-d-glucopyranosyl-l-ascorbic acid (AA-2G) limit its application in the food industry. In this work, the AA-2G synthesis yield of Bacillus stearothermophilus NO2 cyclodextrin glycosyltransferase (CGTase) was improved. Nine residues within 10 Å of the catalytic residue Glu²⁵³ displaying ≤30% conservation and located in the acceptor subsite were selected for iterative saturation mutagenesis. The best mutant, K228R/M230L, produced a higher AA-2G yield with maltodextrin as the glucosyl donor than that produced by its parent wild-type. The l-AA K_m values of the mutant K228R/M230L decreased by 35%, whereas the k_cat/ K_m increased by 2.69-fold. Kinetic analysis indicated that K228R/M230L displayed enhanced l-AA specificity. These results demonstrate that acceptor subsite residues play an important role in acceptor substrate specificity. Mutant K228R/M230L afforded the highest AA-2G concentration (211 g L^-1, 624 mM) reported to date after optimization of the reaction conditions.

Application of Transgalactosylation Activity of β-Galactosidase from Kluyveromyces lactis for the Synthesis of Ascorbic Acid Galactoside

In view of a commonly known beneficial role and low stability of ascorbic acid, many efforts are constantly undertaken to produce its improved derivatives. This paper presents results on the synthesis of ascorbic acid galactoside using transgalactosylation properties of β-galactosidase from Kluyveromyces lactis and lactose as a donor of galactosyl moiety. The purpose of this study was to determine the influence of selected factors (concentration and molar ratio of substrates, amount of the enzyme preparation, pH of the solution, presence of different ions) on the course of transgalactosylation reaction. Research has shown that approx. 2.5% dry matter (d.m.; 12.7 g/L) of ascorbic acid galactoside is formed under favourable conditions (50% (w/v) substrates, sodium ascorbate and lactose at the molar ratio of 1.9:1, enzyme dose of 28,600 U/100 g lactose, pH = 7.0). The addition of Mg²⁺ or K⁺ ions to the reaction medium caused an increase in the final product content (even up to approx. 3.4% d.m., 17.2 g/L), while Na⁺ or Mn²⁺ had an adverse impact on the yield. The gathered data may be valuable for cosmetic or food industry.

Position 228 in Paenibacillus macerans cyclodextrin glycosyltransferase is critical for 2-O-d-glucopyranosyl-l-ascorbic acid synthesis

The markedly stable l-ascorbic acid (L-AA) derivative 2-O-d-glucopyranosyl-l-ascorbic acid (AA-2G) has been widely used in the fields of food, medicine, cosmetics, and husbandry. Cyclodextrin glycosyltransferase (CGTase) is considered suitable for the large-scale production of AA-2G. In this work, Paenibacillus macerans CGTase was used to produce AA-2G and the production was 13.5g/l. An amino-acid sequence alignment of α-, β-, and α⁄β-CGTase indicated that the Phe at position 228 of P. macerans CGTase was different from the amino acids at this position in other CGTases (Met, Val, or Ile). In addition, the CGTases from Anaerobranca gottschalkii and Bacillus circulans 251, which have Val and Met at position 228, were shown to produce 28.9 and 35.7g/l AA-2G, respectively, which verified the importance of this position for AA-2G synthesis. Subsequently, P. macerans CGTase mutants F228M and F228V were constructed and shown to produce 24.8g/l and 24.0g/l AA-2G, respectively, which are 84% and 78% higher than that of wild-type P. macerans CGTase, respectively. Kinetic analysis of AA-2G synthesis showed that affinities of the two mutants for L-AA and the catalytic efficiencies increased. Meanwhile, the mutants had lower cyclization activity but higher disproportionation activities, which is beneficial for AA-2G synthesis. All these results indicated that amino acid at position 228 of P. macerans CGTase is crucial to AA-2G synthesis.

Fusion of self-assembling amphipathic oligopeptides with cyclodextrin glycosyltransferase improves 2-O-D-glucopyranosyl-L-ascorbic acid synthesis with soluble starch as the glycosyl donor

In this study, we fused six self-assembling amphipathic peptides (SAPs) with cyclodextrin glycosyltransferase (CGTase) from Paenibacillus macerans to catalyze 2-O-D-glucopyranosyl-L-ascorbic acid (AA-2G) production with cheap substrates, including maltose, maltodextrin, and soluble starch as glycosyl donors. The results showed that two fusion enzymes, SAP5-CGTase and SAP6-CGTase, increased AA-2G yields to 2.33- and 3.36-fold that of wild-type CGTase when soluble starch was used as a substrate. The cyclization activities of these enzymes decreased, while disproportionation activities increased. Enzymatic characterization of the two fusion enzymes was performed, and kinetics analysis of AA-2G synthesis confirmed the enhanced soluble starch specificity of SAP5-CGTase and SAP6-CGTase compared to that in the wild-type CGTase. As revealed by structure modeling of the fusion and wild-type CGTases, enhanced substrate-binding capacity may result from the increased number of hydrogen bonds present after fusion. This study demonstrates an effective protein fusion approach to improving the substrate specificity of CGTase for AA-2G synthesis. Fusion enzymes, especially SAP6-CGTase, are promising starting points for further development through protein engineering.

Recent advances in discovery, heterologous expression, and molecular engineering of cyclodextrin glycosyltransferase for versatile applications

Cyclodextrin glycosyltransferase (CGTase) is an important enzyme with multiple functions, in particular the production of cyclodextrins. It is also widely applied in baking and carbohydrate glycosylation because it participates in various types of catalytic reactions. New applications are being found with novel CGTases being isolated from various organisms. Heterologous expression is performed for the overproduction of CGTases to meet the requirements of these applications. In addition, various directed evolution techniques have been applied to modify the molecular structure of CGTase for improved performance in industrial applications. In recent years, substantial progress has been made in the heterologous expression and molecular engineering of CGTases. In this review, we systematically summarize the heterologous expression strategies used for enhancing the production of CGTases. We also outline and discuss the molecular engineering approaches used to improve the production, secretion, and properties (e.g., product and substrate specificity, catalytic efficiency, and thermal stability) of CGTase.

Iterative saturation mutagenesis of -6 subsite residues in cyclodextrin glycosyltransferase from Paenibacillus macerans to improve maltodextrin specificity for 2-O-D-glucopyranosyl-L-ascorbic acid synthesis

2-O-d-Glucopyranosyl-l-ascorbic acid (AA-2G), a stable l-ascorbic acid derivative, is usually synthesized by cyclodextrin glycosyltransferase (CGTase), which contains nine substrate-binding subsites (from +2 to -7). In this study, iterative saturation mutagenesis (ISM) was performed on the -6 subsite residues (Y167, G179, G180, and N193) in the CGTase from Paenibacillus macerans to improve its specificity for maltodextrin, which is a cheap and easily soluble glycosyl donor for AA-2G synthesis. Site saturation mutagenesis of four sites-Y167, G179, G180, and N193-was first performed and revealed that four mutants-Y167S, G179R, N193R, and G180R-produced AA-2G yields higher than those of other mutant and wild-type CGTases. ISM was then conducted with the best positive mutant as a template. Under optimal conditions, mutant Y167S/G179K/N193R/G180R produced the highest AA-2G titer of 2.12 g/liter, which was 84% higher than that (1.15 g/liter) produced by the wild-type CGTase. Kinetics analysis of AA-2G synthesis using mutant CGTases confirmed the enhanced maltodextrin specificity and showed that compared to the wild-type CGTase, the mutants had no cyclization activity but high hydrolysis and disproportionation activities. A possible mechanism for the enhanced substrate specificity was also analyzed through structure modeling of the mutant and wild-type CGTases. These results indicated that the -6 subsite played crucial roles in the substrate binding and catalytic reactions of CGTase and that the obtained CGTase mutants, especially Y167S/G179K/N193R/G180R, are promising starting points for further development through protein engineering.