Enhanced secretion of cyclodextrin glucanotransferase (CGTase) by Lactococcus lactis using heterologous signal peptides and optimization of cultivation conditions

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.

Optimization of Signal Peptide via Site-Directed Mutagenesis for Enhanced Secretion of Heterologous Proteins in Lactococcus lactis

Secretion efficiency of heterologous proteins in the Generally Regarded As Safe (GRAS) Lactococcus lactis is often reported to be insufficiently low due to limitations such as poor targeting and translocation by the signal peptide or degradation by the host proteases. In this study, the secretion efficiency in the host was enhanced through the utilization of a heterologous signal peptide (SP) SPK1 of Pediococcus pentosaceus. The SPK1 was subjected to site-directed mutations targeting its tripartite N-, H-, and C-domains, and the effect on secretion efficiency as compared to the wild-type SPK1 and native lactococcal USP45 was determined on a reporter nuclease (NUC) of Staphylococcus aureus. A Fluorescence Resonance Energy Transfer (FRET) analysis indicated that four out of eight SPK1 variants successfully enhanced the secretion of NUC, with the best mutant, SPKM19, showing elevated secretion efficiency up to 88% (or by 1.4-fold) and an improved secretion activity yield of 0.292 ± 0.122 U/mL (or by 1.7-fold) compared to the wild-type SPK1. Modifications of the SPK1 at the cleavage site C-domain region had successfully augmented the secretion efficiency. Meanwhile, mutations in the H-domain region had resulted in a detrimental effect on the NUC secretion. The development of heterologous SPs with better efficacy than the USP45 has been demonstrated in this study for enhanced secretion of heterologous production and mucosal delivery applications in the lactococcal host.

High-level production of γ-cyclodextrin glycosyltransferase in recombinant Escherichia coli BL21 (DE3): culture medium optimization, enzymatic properties characterization, and product specificity analysis

Purpose

γ-Cyclodextrin glycosyltransferase (γ-CGTase) catalyzes the biotransformation of low-cost starch into valuable γ-cyclodextrin (γ-CD), which is widely applied in biotechnology, food, and pharmaceutical industries. However, the low specificity and activity of soluble γ-CGTase increase the production cost of γ-CD, thereby limiting its applications. Therefore, the present study aimed at optimizing an economical medium for high production of γ-CGTase by the recombinant Escherichia coli (E. coli) BL21 (DE3) and evaluating its enzymatic properties and product specificity.

Methods

The γ-CGTase production was optimized using the combination of Plackett-Burman experimental design (PBD) and Box-Behnken design-response surface methodology (BBD-RSM). The hydrolysis and cyclization properties of γ-CGTase were detected under the standard assay conditions with buffers of various pHs and different reaction temperatures. The product specificity of γ-CGTase was investigated by high-performance liquid chromatography (HPLC) analysis of three CDs (α-, β-, γ-CD) in the biotransformation product of cassava starch.

Results

The γ-CGTase activity achieved 53992.10 U mL−1 under the optimum conditions with the significant factors (yeast extract 38.51 g L−1, MgSO4 4.19 mmol L−1, NiSO4 0.90 mmol L−1) optimized by the combination of PBD and BBD-RSM. The recombinant γ-CGTase exhibited favorable stability in a wide pH and temperature range and maintained both the hydrolysis and cyclization activity under the pH 9.0 and 50 °C. Further analysis of the products from cassava starch catalyzed by the γ-CGTase reported that the majority (90.44%) of product CDs was the γ form, which was nearly 11% higher than the wild enzyme. Cyclododecanone added to the transformation system could enhance the γ-CD purity to 98.72%, which is the highest purity value during the transformation process reported so far.

Conclusion

The yield of γ-CGTase activity obtained from the optimized medium was 2.83-fold greater than the unoptimized medium, and the recombinant γ-CGTase exhibited a favorable thermal and pH stability, and higher γ-cyclization specificity. These results will provide a fundamental basis for the high productivity and purity of γ-CD in the industrial scale.