Catalytic activity enhancement of 1,4-α-glucan branching enzyme by N-terminal modification

The 1,4-α-glucan branching enzyme (GBE, EC 2.4.1.18) has garnered considerable attention for its ability to increase the degree of branching of starch and retard starch digestion, which has great industrial applications. Previous studies have reported that the N-terminal domain plays an important role in the expression and stability of GBEs. To further increase the catalytic ability of Gt-GBE, we constructed five mutants in the N-terminal domain: L19R, L19K, L25R, L25K, and L25A. Specific activities of L25R and L25A were increased by 28.46% and 23.46%, respectively, versus the wild-type Gt-GBE. In addition, the α-1,6-glycosidic linkage ratios of maltodextrin samples treated with L25R and L25A increased to 5.71%, which were significantly increased by 19.96% compared with that of the wild-type Gt-GBE. The results of this study suggest that the N-terminal domain selective modification can improve enzyme catalytic activity, thus further increasing the commercial application of enzymes in food and pharmaceutical industries.

Characterization of the Glucan-Branching Enzyme GlgB Gene from Swine Intestinal Bacteria

Starch hydrolysis by gut microbiota involves a diverse range of different enzymatic activities. Glucan-branching enzyme GlgB was identified as the most abundant glycosidase in Firmicutes in the swine intestine. GlgB converts α-(1→4)-linked amylose to form α-(1→4,6) branching points. This study aimed to characterize GlgB cloned from a swine intestinal metagenome and to investigate its potential role in formation of α-(1→4,6)-branched α-glucans from starch. The branching activity of purified GlgB was determined with six different starches and pure amylose by quantification of amylose after treatment. GlgB reduced the amylose content of all 6 starches and amylose by more than 85% and displayed a higher preference towards amylose. The observed activity on raw starch indicated a potential role in the primary starch degradation in the large intestine as an enzyme that solubilizes amylose. The oligosaccharide profile showed an increased concentration of oligosaccharide introduced by GlgB that is not hydrolyzed by intestinal enzymes. This corresponded to a reduced in vitro starch digestibility when compared to untreated starch. The study improves our understanding of colonic starch fermentation and may allow starch conversion to produce food products with reduced digestibility and improved quality.

Biochemical characteristics and potential application of a thermostable starch branching enzyme from Bacillus licheniformis

Slowly digestible starch (SDS) has attracted increasing attention for its function of preventing metabolic diseases. Based on transglycosylation, starch branching enzymes (1,4-α-glucan branching enzymes, GBEs, EC 2.4.1.18) can be used to regulate the digestibility of starch. In this study, a GBE gene from Bacillus licheniformis (bl-GBE) was cloned, expressed, purified, and characterized. Sequence analysis and structural modeling showed that bl-GBE belong to the glycoside hydrolase 13 (GH13) family, with which its active site residues were conserved. The bl-GBE was highly active at 80 °C and a pH range of 7.5-9.0, and retained 90% of enzyme activity at 70 °C for 16 h. bl-GBE also showed high substrate specificity (80.88 U/mg) on potato starch. The stability and the changes of the secondary structure of bl-GBE at different temperature were determined by circular dichroism (CD) spectroscopy. The CD data showed a loss of 20% of the enzyme activity at high temperatures (80 °C), due to the decreased content of the α -helix in the secondary structure. Furthermore, potato starch treated with bl-GBE (300 U/g starch) showed remarkable increase in stability, solubility, and significant reduction viscosity. Meanwhile, the slowly digestible starch content of bl-GBE modified potato starch increased by 53.03% compared with native potato starch. Our results demonstrated the potential applications of thermophilic bl-GBE in food industries.

Starch Branching Enzyme 1 Is Important for Amylopectin Synthesis and Cyst Reactivation in Toxoplasma gondii

Toxoplasma gondii (T. gondii) bradyzoites facilitate chronic infections that evade host immune response. Furthermore, reactivation in immunocompromised individuals causes severe toxoplasmosis. The presence of abundant granules containing the branched starch amylopectin is major characteristic of bradyzoites that is nearly absent from tachyzoites that drive acute disease. T. gondii genome encodes to potential Starch branching enzyme 1 (SBE1) that creates branching during amylopectin biosynthesis. However, the physiological function of the amylopectin in T. gondii remains unclear. In this study, we generated a SBE1 knockout parasites and revealed that deletion of SBE1 caused amylopectin synthesis defects while having no significant impact on the growth of tachyzoites under normal culture conditions in vitro as well as virulence and brain cyst formation. Nevertheless, SBE1 knockout decreased the influx of exogenous glucose and reduced tachyzoites proliferation in nutrition-deficient conditions. Deletion of SBE1 together with the α-amylase (α-AMY), responsible for starch digestion, abolished amylopectin production and attenuated virulence while restoring brain cyst formation. In addition, cysts with defective amylopectin metabolism showed abnormal morphology and were avirulent to mice. In conclusion, SBE1 is essential for the synthesis of amylopectin, which serves as energy storage during the development and reactivation of bradyzoites. IMPORTANCE Toxoplasmosis has become a global, serious public health problem due to the extensiveness of the host. There are great differences in the energy metabolism in the different stages of infection. The most typical difference is the abundant accumulation of amylopectin granules in bradyzoites, which is almost absent in tachyzoites. Until now, the physiological functions of amylopectin have not been clearly elucidated. We focused on starch branching enzyme 1 (SBE1) in the synthesis pathway to reveal the exact physiological significance of amylopectin. Our study clarified the role of SBE1 in the synthesis pathway and amylopectin in tachyzoites and bradyzoites, and demonstrated that amylopectin, as an important carbon source, was critical to parasites growth under an unfavorable environment and the reactivation of bradyzoites to tachyzoites. The findings obtained from our study provides a new avenue for the development of Toxoplasma vaccines and anti-chronic toxoplasmosis drugs.

Rational Design of Disulfide Bonds for Enhancing the Thermostability of the 1,4-α-Glucan Branching Enzyme from Geobacillus thermoglucosidans STB02

Disulfide bonds play crucial roles in thermostabilization, recognition, or activation of proteins. They are vital in maintaining the respective conformations of globular structures, thereby enhancing thermostability. Bioinformatic approaches provide practical strategies to build disulfide bonds based on structural information. We constructed nine mutants by rational analysis of the 1,4-α-glucan branching enzyme (EC 2.4.1.18) from Geobacillus thermoglucosidans STB02, which catalyzes the synthesis of α-1,6-glucosidic bonds by acting on α-(1,4) and/or α-(1,6) glucosidic linkages. Four of the mutations enhanced thermostability, and five of them had adverse or negligible effects on stability. Circular dichroism spectra and intrinsic fluorescence analysis showed that introducing disulfide bonds might only affect secondary structures. The results also demonstrated that the distances of Cα carbons and thiol groups, as well as the sequence between the two cysteines, need to be considered when designing disulfide bonds.

Enzymatic properties of an efficient glucan branching enzyme and its potential application in starch modification

Glucan branching enzymes (GBEs, EC 2.4.1.18) catalyze the formation of α-1,6-linked branch in starch, which is important for the starch modification with prospective properties. In this study, the aqGBE gene encoding an efficient glucan branching enzyme was cloned from Aquabacterium sp. strain A7-Y and successfully expressed in Escherichia coli BL21 (DE3). The specific activity of the purified recombinant enzyme rAqGBE was 2850 U/mg with potato starch as the optimal substrate, and the K_m and V_max values of rAqGBE were 1.18 mg/mL and 588.2 μmol/min/mg, respectively. Enzymological characterization showed that rAqGBE exhibits its optimal activity under the condition of 40 °C and pH 7.0, respectively, which is independent of calcium ions. Otherwise, rAqGBE-treated potato starch showed different chain length distribution compared with control, the numbers of short chains (degree of polymerization, DP < 7) and long chains (DP > 25) increased from 4.5% to 9.6% and 6.1%-15.7% after enzymatic treatment, respectively. In starch anti-ageing assay, with minimum usage of 0.8 mg rAqGBE per g starch, the rAqGBE-treated potato starch exhibited reduced retrogradation properties. Our results indicate that the branching enzyme AqGBE may therefore be a promising tool for the enzymatic modification of starch.

Importance of C-Terminal Extension in Thermophilic 1,4-α-Glucan Branching Enzyme from Geobacillus thermoglucosidans STB02

By sequence comparison, the majority of 1,4-α-glucan-branching enzymes (GBEs) consist of an N-terminal carbohydrate-binding domain, a TIM-barrel catalytic domain, and a C-terminal all-beta domain. Among these structures, the GBE from Geobacillus thermoglucosidans STB02 uniquely has a highly charged 26-amino-acid C-terminal extension, whose functional roles are the least understood. In this research, the functional significance of the C-terminal domain in GBE from G. thermoglucosidans STB02 and its extension were assessed using a C-terminal deletion analysis. Mutants lacking of more than 7 residues of the C-terminal all-beta domain could not be detected in lysates of their Escherichia coli expression strains, suggesting that an intact all-beta domain is required for structural stability. In contrast, truncation of the C-terminal extension resulted in greater stability and solubility than the wild type, as well as a lower sensitivity to the presence of added metal ions. Comparison of this mutant with the wild type suggests that the interaction of metal ions with the C-terminal extension influences performance of this enzyme.

Evolutionary Stability of Salt Bridges Hints Its Contribution to Stability of Proteins

The contribution of newly designed salt bridges to protein stabilization remains controversial even today. In order to solve this problem, we investigated salt bridges from two aspects: spatial distribution and evolutionary characteristics of salt bridges. Firstly, we analyzed spatial distribution of salt bridges in proteins, elucidating the basic requirements of forming salt bridges. Then, from an evolutionary point of view, the evolutionary characteristics of salt bridges as well as their neighboring residues were investigated in our study. The results demonstrate that charged residues appear more frequently than other neutral residues at certain positions of sequence even under evolutionary pressure, which are able to form electrostatic interactions that could increase the evolutionary stability of corresponding amino acid regions, enhancing their importance to stability of proteins. As a corollary, we conjectured that the newly designed salt bridges with more contribution to proteins, not only, are qualified spatial distribution of salt bridges, but also, are needed to further increase the evolutionary stability of corresponding amino acid regions. Based on analysis, the 8 mutations were accordingly constructed in the 1,4-α-glucan branching enzyme (EC 2.4.1.18, GBE) from Geobacillus thermoglucosidans STB02, of which 7 mutations improved thermostability of GBE. The enhanced thermostability of 7 mutations might be a result of additional salt bridges on residue positions that at least one of amino acids positions is conservative, improving their contribution of stabilization to proteins.

Effects of potassium fertilization on potato starch physicochemical properties

Potato starch serves as an excellent raw material or food additive in the food industry. With the advancement of the potato staple food strategy in China, improving the potato starch yield and quality has attracted more and more attention. Potassium is an essential nutrient for potato due to its direct effects on the yield and quality of potato tubers. Here, the effects of three different potassium levels on potato starch physicochemical properties were evaluated by field experiments. With increasing potassium fertilization rates, the amylose content, phosphorus content and particle size decreased, thereby resulting in low gelatinization temperature, breakdown and setback viscosity, and high swelling power, relative crystallinity and transparency. Our study indicated that enhanced potassium fertilization improved the resistance to heat and shear stress and decreased the retrogradation of starch, and the 270 kg/ha potassium fertilization rate could obtain the highest tuber and starch production with desirable starch physicochemical properties. The integrated results also provide some novel insights into the management of the fertilization conditions to obtain native starches with special properties.

Met349 Mutations Enhance the Activity of 1,4-α-Glucan Branching Enzyme from Geobacillus thermoglucosidans STB02

1,4-α-Glucan branching enzyme (GBE, EC 2.4.1.18) is used to increase the number of α-1,6 branch points in starch and glycogen. On the basis of a multiple sequence alignment of the GBEs from a variety of bacteria, residue 349 (Geobacillus thermoglucosidans STB02 numbering) in region III is generally methionine in bacteria with higher identity, while it is threonine or serine in bacteria with lower identity. Four mutants (M349T, M349S, M349H, and M349Y) were constructed by site-directed mutagenesis and characterized. M349T and M349S showed 24.5% and 21.1% increases in specific activity compared with that of wild-type GBE, respectively. In addition, M349T and M349S displayed 24.2% and 17.6% enhancements in the α-1,6-glycosidic linkage ratio of potato starch samples, respectively. However, M349Y displayed a significant reduction in activity. Moreover, the mutations at M349 have a negligible effect on substrate specificity. Thus, M349T and M349S are more suitable for industrial applications than wild-type GBE.