Molecular dynamic analysis of mutant Y195I α-cyclodextrin glycosyltransferase with switched product specificity from α-cyclodextrin to γ-cyclodextrin

A review on recent upgradation and strategies to enhance cyclodextrin glucanotransferase properties for its applications

Cyclodextrin glycosyltransferase (CGTase) is a significant extracellular enzyme with diverse functions. CGTase is widely used in production of cyclic α-(1,4)-linked oligosaccharides (cyclodextrins) from starch via transglycosylation reaction. Recent discoveries of novel CGTases from different microorganisms have expanded its applications but natural CGTase have lower yield, leading to heterologous expression for increased production to meet various needs. Moreover, significant advancements in directed evolution approach have been explored to alter the molecular structure of CGTase to enhance its performance. This review comprehensively summarizes the strategies employed in heterologous expression to boost CGTase production and secretion in various host. It also outlines molecular engineering approaches aimed to improving CGTase properties, including product and substrate specificity, catalytic efficiency, and thermal stability. Additionally, a considerable stability against changes in temperature and organic solvents can be obtained by immobilization.

Engineering of cyclodextrin glycosyltransferase improves the conversion efficiency of rebaudioside A to glucosylated steviol glycosides and increases the content of short-chain glycosylated steviol glycoside

BACKGROUND

Compared with steviol glycosides, the taste of glucosylated steviol glycosides is better and more similar to that of sucrose. At present, cyclodextrin glucanotransferase (CGTase) is primarily used to catalyze the conversion of steviol glycosides to glucosylated steviol glycosides, with soluble starch serving as a glycosyl donor. The main disadvantages of enzymatic transglycosylation are the limited number of enzymes available, the low conversion rates that result in low yields, and the lack of selectivity in the degree of glycosylation of the products. In order to fill these gaps, the proteome of Alkalihalobacillus oshimensis (also named Bacillus oshimensis) was used for mining novel CGTases.

RESULTS

Here, CGTase-15, a novel β-CGTase with a wide pH adaptation range, was identified and characterized. The catalyzed product of CGTase-15 tasted better than that of the commercial enzyme (Toruzyme® 3.0 L). In addition, two amino acid sites, Y199 and G265, which play important roles in the conversion of steviol glycosides to glucosylated steviol glycosides were identified by site-directed mutagenesis. Compared with CGTase-15, CGTase-15-Y199F mutant significantly increased the conversion rate of rebaudioside A (RA) to glucosylated steviol glycosides. Compared with CGTase-15, the content of short-chain glycosylated steviol glycosides catalyzed by CGTase-15-G265A mutant was significantly increased. Moreover, the function of Y199 and G265 was verified in other CGTases. The above mutation pattern has also been applied to CGTase-13 (a CGTase discovered by our laboratory with great potential in the production of glycosylated steviol glycosides), confirming that the catalytic product of CGTase-13-Y189F/G255A mutant has a better taste than that of CGTase-13.

CONCLUSIONS

This is the first report on the improvement of the sensory profiles of glycosylated steviol glycosides through site-directed mutagenesis of CGTase, which is significant for the production of glycosylated steviol glycosides.

Application of cyclodextrinase in non-complexant production of γ-cyclodextrin

The production of γ-cyclodextrin usually includes the utilization of organic complexants. However, the non-complexant production of γ-cyclodextrin is always being explored due to the defects of organic complexants. However, in non-complexant production, the separation of γ-cyclodextrin from α- and β-cyclodextrin is still a challenge. Here, the selective hydrolysis ability of a cyclodextrinase designated PpCD (cyclodextrinase from Palaeococcus pacificus) on α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin was proved. The k_cat /K_m values of PpCD for α-cyclodextrin and β-cyclodextrin were roughly 12-fold and 5-fold higher than that of γ-cyclodextrin. It was proved that PpCD had selective hydrolysis ability and its γ-cyclodextrin purification performance was apparent on various simulated cyclodextrin mixtures with reported proportions derived from different CGTases. Besides, the hydrolysis temperature was optimized and it could be seen that 85°C was appropriate for the production of γ-cyclodextrin. In addition, the production of γ-cyclodextrin was achieved by using PpCD in the γ-CGTase reaction products.

Engineered cyclodextrin glucanotransferases from Bacillus sp. G-825-6 produce large-ring cyclodextrins with high specificity

Cyclodextrin glucanotransferases (CGTases) synthesize cyclic oligosaccharides (cyclodextrins, CD) from starch. A CGTase from Bacillus sp. G-825-6 was engineered by site-directed mutagenesis at two positions by the construction of the variants Y183W, Y183R, D358R, Y183W/D358R, and Y183R/D358R. Among CD composed of 7-12 glucose units (CD7-CD12), Y183W mainly produced CD8. Y183R had completely lost its ability to synthesize CD7, and CD8 and the larger CD were the only cyclic oligosaccharides produced. D358R also formed mainly CD8-CD12 during a reaction time of 24 hr. The double mutant Y183W/D358R showed combined characteristics of the single mutations with very low CD7 cyclization activity and an increased formation of the larger CD. The results show that CGTases synthesizing mainly CD8-CD12 can be constructed allowing a convenient production of larger CD in significant amounts as host molecules in supramolecular complexing reactions.

Enhanced stability of manganese superoxide dismutase by amino acid replacement designed via molecular dynamics simulation

In order to improve manganese-SOD stability, three mutations were constructed via site-directed mutagenesis, and the root mean square fluctuation (RMSF) and root mean square deviation (RMSD) were used as stability assessment indexes. The amino acids of V140, E155 and E215 from wild-type mouse Mn-SOD was replaced to L140, W155 and W215, and a recombinant plasmid containing DNA segment coding wild-type and mutant Mn-SOD protein was transformed into Escherichia coli BL21 for expression. The highest enzyme activity of the mutations-MnSOD was 2050 U/mg. In addition, the recombinant protein, TM-MnSOD^{V140L, E155W, E215W} exhibited higher working temperature and improved stability compared with the wild-type Mn-SOD. Furthermore, CD spectrum analysis of the improved mutants and wild-type enzyme showed that there was no significant change in their secondary structures. This study not only expands the scope of the application of enzymes, but also helps us understand the relationship between protein structure and function.

Development and Application of Cyclodextrin Hydrolyzing Mutant Enzyme Which Hydrolyzes β- and γ-CD Selectively

Cyclodextrins (CDs) are produced from starch by cyclodextrin glucanotransferase (CGTase), which has cyclization activity. Specifically, α-CD is an important biomolecule, as it is a molecular carrier and soluble dietary fiber used in the food industry. Upon inspection of the conserved regions of the glycoside hydrolase (GH) 13 family amylases, the amino acids K232 and H233 of CGTase were identified as playing an important role in enzyme reaction specificity. A novel CD hydrolyzing enzyme, cyclodextrin glycosyl transferase (CGTase)-alpha, was developed using site-directed mutagenesis at these positions. Action pattern analysis using various substrates revealed that CGTase-alpha was able to hydrolyze β- and γ-CD, but not α-CD. This selective CD hydrolyzing property was employed to purify α-CD from a CD mixture solution. The α-CD that remained after treatment with CGTase-alpha and exotype glucoamylase was purified using hydrophobic interaction chromatography with 99% purity.