Properties of the Glucan Branching Enzyme of the Hyperthermophilic BacteriumAquifex aeolicus

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.

Differentiated structure of synthetic glycogen-like particle by the combined action of glycogen branching enzymes and amylosucrase

Glycogen-like particles (GLPs) were built up from sucrose by applying de novo one-pot enzymatic process of amylosucrase (ASase; 6 U·mL^-1) and glycogen branching enzymes (GBEs; 0.001 and 0.005 U·mL^-1). Due to different chain-length transferring patterns of GBEs, structurally differentiated GLPs were synthesized. Yields of GLPs synthesized at pH 7.0 and 30 °C were improved by increasing the GBE/ASase ratio. Branching degrees of GLPs obviously was increased along with the ratio of GBEs, of which result was directly supported by shortened branch-chain length with greater GBE activity. Long branch chains seemed to play as efficient acceptor molecules to bind newly transferred branch chains especially at lower ratio of GBE/ASase, resulting in greater molecular weight and size of GLP with higher proportion of them. Molecular weight, size, and density of GLPs were ranged from 7.37 × 10⁵ to 1.94 × 10⁸ g·mol^-1, from 23.70 to 52.65 nm, and from 7.99 to 374.32 g·mol^-1·nm^-3, respectively. By increasing GBE/ASase ratio, more compact GLP architecture was fabricated due to increased weight and reduced size with exception of a unique GBE. GLPs were efficiently synthesized by two different glycosyltransferases, and their chemical structures were controllable by source and ratio of GBEs due to their different branch-chain transferring specificity.

Structural factors governing starch digestion and glycemic responses and how they can be modified by enzymatic approaches: A review and a guide

Starch is the most abundant glycemic carbohydrate in the human diet. Consumption of starch-rich food products that elicit high glycemic responses has been linked to the occurrence of noncommunicable diseases such as cardiovascular disease and diabetes mellitus type II. Understanding the structural features that govern starch digestibility is a prerequisite for developing strategies to mitigate any negative health implications it may have. Here, we review the aspects of the fine molecular structure that in native, gelatinized, and gelled/retrograded starch directly impact its digestibility and thus human health. We next provide an informed guidance for lowering its digestibility by using specific enzymes tailoring its molecular and three-dimensional supramolecular structure. We finally discuss in vivo studies of the glycemic responses to enzymatically modified starches and relevant food applications. Overall, structure-digestibility relationships provide opportunities for targeted modification of starch during food production and improving the nutritional profile of starchy foods.

GH13 Glycogen branching enzymes can adapt the substrate chain length towards their preferences via α-1,4-transglycosylation

Glycogen branching enzymes (GBEs; 1,4-α-glucan branching enzyme; E.C. 2.4.1.18) have so far been described to be capable of both α-1,6-transglycosylation (branching) and α-1,4-hydrolytic activity. The aim of the present study was to elucidate the mode of action of three distantly related GBEs from the glycoside hydrolase family 13 by in depth analysis of the activity on a well-defined substrate. For this purpose, the GBEs from R. marinus (RmGBE), P. mobilis (PmGBE1), and B. fibrisolvens (BfGBE) were incubated with a highly pure fraction of a linear substrate of 18 anhydroglucose units. A well-known and characterized branching enzyme from E. coli (EcGBE) was also taken along. Analysis of the chain length distribution over time revealed that, next to hydrolytic and branching activity, all three GBEs were capable of generating chains longer than the substrate, clearly showing α-1,4-transglycosylation activity. Furthermore, the GBEs used those elongated chains for further branching. The sequential activity of elongation and branching enabled the GBEs to modify the substrate to a far larger extent than would have been possible with branching activity alone. Overall, the three GBEs acted ambiguous on the defined substrate. RmGBE appeared to have a strong preference towards transferring chains of nine anhydroglucose units, even during elongation, with a comparably low activity. BfGBE generated an array of elongated chains before using the chains for introducing branches while PmGBE1 exhibited a behaviour intermediate of the other two enzymes. On the basis of the mode of action revealed in this research, an updated model of the mechanism of GBEs was proposed now including the α-1,4-transglycosylation activity.

Structural elements determining the transglycosylating activity of glycoside hydrolase family 57 glycogen branching enzymes

Glycoside hydrolase family 57 glycogen branching enzymes (GH57GBE) catalyze the formation of an α-1,6 glycosidic bond between α-1,4 linked glucooliogosaccharides. As an atypical family, a limited number of GH57GBEs have been biochemically characterized so far. This study aimed at acquiring a better understanding of the GH57GBE family by a systematic sequence-based bioinformatics analysis of almost 2500 gene sequences and determining the branching activity of several native and mutant GH57GBEs. A correlation was found in a very low or even no branching activity with the absence of a flexible loop, a tyrosine at the loop tip, and two β-strands.

Expression and characterization of 1,4-α-glucan branching enzyme from Microvirga sp. MC18 and its application in the preparation of slowly digestible starch

Nutraceuticals containing modified starch with increased content of slowly-digestible starch (SDS) may reduce the prevalence of obesity, diabetes and cardiovascular diseases due to its slow digestion rate. Enzymatic methods for the preparation of modified starch have attracted increasing attention because of their low environmental impact, safety and specificity. In this study, the efficient glucan branching enzyme McGBE from Microvirga sp. MC18 was identified, and its relevant properties as well as its potential for industrial starch modification were evaluated. The purified McGBE exhibited the highest specificity for potato starch, with a maximal specific activity of 791.21 U/mg. A time-dependent increase in the content of α-1,6 linkages from 3.0 to 6.0% was observed in McGBE-modified potato starch. The proportion of shorter chains (degree of polymerization, DP < 13) increased from 29.2 to 63.29% after McGBE treatment, accompanied by a reduction of the medium length chains (DP 13-24) from 52.30 to 35.99% and longer chains (DP > 25) from 18.51 to 0.72%. The reduction of the storage modulus (G') and retrogradation enthalpy (ΔHr) of potato starch with increasing treatment time demonstrated that McGBE could inhibit the short- and long-term retrogradation of starch. Under the optimal conditions, the SDS content of McGBE-modified potato starch increased by 65.8% compared to native potato starch. These results suggest that McGBE has great application potential for the preparation of modified starch with higher SDS content that is resistant to retrogradation.

Highly branched corn starch: Preparation, encapsulation, and release of ascorbic acid

The aim of this study was to prepare a supporting carrier, namely highly branched corn starch (HBCS), and to investigate its encapsulation property with ascorbic acid (AA). High amylose corn starch was converted into HBCS via dual enzymatic modification by successively using α-amylase and glycogen branching enzyme. The results showed that the ratio of α-1, 6 linkage of HBCS increased by 1.93%, and a short-to-medium chain length distribution with a compact branched conformation was formed, which suggested HBCS could be a potential highly branched carrier. The HBCS-AA inclusion complex was formed as confirmed by differential scanning calorimetry. The release of AA conformed to the pseudo-Fickian diffusion mechanism and followed the first-order kinetics. Meanwhile, the photostability and thermostability of the embedded AA were moderately enhanced. These findings suggest that HBCS provides new insights into the preparation of wall materials and can be potentially used to deliver AA into food systems.

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.

Recombinant expression and characterization of the glycogen branching enzyme from Vibrio vulnificus and its application in starch modification

Resistant starch (RS) is helpful in controlling and preventing metabolic syndrome relevant diseases. However, the RS content of natural starch and modified starch produced by enzymatic method is generally low. To solve this problem, we selected the glycogen branching enzyme from Vibrio vulnificus (VvGBE) and investigated its application. Firstly, it was expressed in E. coli with the enzyme activity was 53.33 U/mL, and its optimum temperature and pH was 35 °C and 7.5, respectively. The half-life of VvGBE at 35 °C was 10 h, and the enzyme was most stable at pH 9.5. When we used the recombinant enzyme to treat corn starch, the content of RS increased by 19.41%, which was higher than that achieved with other enzymes. More specially, the conversion of slowly digestible starch to RS, which was only demonstrated in chemical modification, was accomplished. The fine structure of the modified starch was further investigated. Results showed that the number of short chains (DP < 13) increased to 90.58%, and the α-1,6 linkages ratio increased from 7.19% to 15.64%. The increase of short chains and α-1,6 linkages may contribute to high RS content. This study can provide a reference for the development of modified starch with lower digestibility.

Biofabrication, structure and characterization of an amylopectin-based cyclic glucan

A novel amylopectin-based cyclic architecture was fabricated, arising from microbial branching enzyme treated waxy rice starch. The recombinant enzyme had a molecular weight of 72.0 kDa, and exhibited optimum activity at pH 7.0 and 75 °C. During the cyclization reaction catalyzed by a branching enzyme, the molecular weight of amylopectin rapidly decreased for the initial 2 h, and then very slowly decreased, tapering off at approximately 1.8 × 105 g mol-1 at 12 h. The number of A-chain fractions greatly increased, whereas the percentage of B-chain fractions decreased after enzymatic modification, accompanied by more α-1, 6 linkage formation. The core ring structure as a glucoamylase-resistant fraction had a number-average degree of polymerization of 21, which was constructed by 19 glucose units linked with, 2 glucosyl stubs at the O-6-position of the cyclic glucan through α-1,4 and α-1,6 linkages. Similar to large-ring cyclodextrin with equal glucose units, this cyclic glucan had a cavity geometry with two-circular loops and short stubs in perpendicular planes. Moreover, this cyclic glucan could complex with iodine for the host-guest formation. These results revealed the potential application of the amylopectin-based cyclic glucan as a good delivery system to encapsulate and protect bioactive ingredients.

Characterization of the GH13 and GH57 glycogen branching enzymes from Petrotoga mobilis SJ95 and potential role in glycogen biosynthesis

Glycogen is a highly branched α-glucan polymer widely used as energy and carbon reserve by many microorganisms. The branches are introduced by glycogen branching enzymes (EC 2.4.1.18), that are classified into glycoside hydrolase families 13 (GH13) and 57 (GH57). Most microorganisms have typically only a single glycogen branching enzyme (gbe) gene. Only a few microorganisms carry both GH13 and GH57 gbe genes, such as Petrotoga mobilis and Mycobacterium tuberculosis. Here we report the basic characteristics of the GH13 and GH57 GBE of P. mobilis, both heterologously expressed in E. coli. The GH13 GBE has a considerably higher branching activity towards the linear α-glucan amylose, and produces a highly branched α-glucan with a high molecular weight which is very similar to glycogen. The GH57 GBE, on the contrary, makes a much smaller branched α-glucan. While the GH13 GBE acts as a classical glycogen branching enzyme involved in glycogen synthesis, the role of GH57 GBE remains unclear.

Expression and Biochemical Characterization of a Thermostable Branching Enzyme from

The branching enzyme (EC 2.4.1.18) catalyzes the formation of α-1,6 branch points in starch. In this study, the <i>Geobacillus thermoglucosidans</i> gene-encoding branching enzyme was expressed in <i>Escherichia coli </i>BL21 (DE3) and the protein was isolated and characterized. <i>G. thermoglucosidans </i>branching enzyme is a thermostable enzyme with an optimal reaction temperature of nearly 60°C and a half-life at 65°C of approximately 1.1 h. The activity of the recombinant enzyme is optimal at pH 7.5, with broad stability between pH 5.5 and 9.0. Its thermostability, relatively broad pH stability and optimal temperature near the temperature at which starch begins to gelatinize may make it easy to use in industrial production. Furthermore, the enzyme is activated by Mg²⁺, Ba²⁺, K⁺ and Na⁺ in a concentration-dependent manner and dramatically inhibited by Ni²⁺ and Co²⁺. Its substrate dependence, using amylopectin as the substrate, could be adequately fitted using the Michaelis-Menten equation, yielding a<i> K</i>_m of 0.99 mg/ml. High-performance anion exchange chromatography results showed that the chain length distribution of branching enzyme-treated waxy corn starch is indistinguishable from that of the branching enzyme-treated common corn starch. This enzyme may therefore be a promising tool for the enzymatic modification of starch.

The unique branching patterns of Deinococcus glycogen branching enzymes are determined by their N-terminal domains

Glycogen branching enzymes (GBE) or 1,4-alpha-glucan branching enzymes (EC 2.4.1.18) introduce alpha-1,6 branching points in alpha-glucans, e.g., glycogen. To identify structural features in GBEs that determine their branching pattern specificity, the Deinococcus geothermalis and Deinococcus radiodurans GBE (GBE(Dg) and GBE(Dr), respectively) were characterized. Compared to other GBEs described to date, these Deinococcus GBEs display unique branching patterns, both transferring relatively short side chains. In spite of their high amino acid sequence similarity (88%) the D. geothermalis enzyme had highest activity on amylose while the D. radiodurans enzyme preferred amylopectin. The side chain distributions of the products were clearly different: GBE(Dg) transferred a larger number of smaller side chains; specifically, DP5 chains corresponded to 10% of the total amount of transferred chains, versus 6.5% for GBE(Dr). GH13-type GBEs are composed of a central (beta/alpha) barrel catalytic domain and an N-terminal and a C-terminal domain. Characterization of hybrid Deinococcus GBEs revealed that the N2 modules of the N domains largely determined substrate specificity and the product branching pattern. The N2 module has recently been annotated as a carbohydrate binding module (CBM48). It appears likely that the distance between the sugar binding subsites in the active site and the CBM48 subdomain determines the average lengths of side chains transferred.

Biosynthesis of storage compounds by Rhodococcus jostii RHA1 and global identification of genes involved in their metabolism

Background

Members of the genus Rhodococcus are frequently found in soil and other natural environments and are highly resistant to stresses common in those environments. The accumulation of storage compounds permits cells to survive and metabolically adapt during fluctuating environmental conditions. The purpose of this study was to perform a genome-wide bioinformatic analysis of key genes encoding metabolism of diverse storage compounds by Rhodococcus jostii RHA1 and to examine its ability to synthesize and accumulate triacylglycerols (TAG), wax esters, polyhydroxyalkanoates (PHA), glycogen and polyphosphate (PolyP).

Results

We identified in the RHA1 genome: 14 genes encoding putative wax ester synthase/acyl-CoA:diacylglycerol acyltransferase enzymes (WS/DGATs) likely involved in TAG and wax esters biosynthesis; a total of 54 genes coding for putative lipase/esterase enzymes possibly involved in TAG and wax ester degradation; 3 sets of genes encoding PHA synthases and PHA depolymerases; 6 genes encoding key enzymes for glycogen metabolism, one gene coding for a putative polyphosphate kinase and 3 putative exopolyphosphatase genes. Where possible, key amino acid residues in the above proteins (generally in active sites, effectors binding sites or substrate binding sites) were identified in order to support gene identification. RHA1 cells grown under N-limiting conditions, accumulated TAG as the main storage compounds plus wax esters, PHA (with 3-hydroxybutyrate and 3-hydroxyvalerate monomers), glycogen and PolyP. Rhodococcus members were previously known to accumulate TAG, wax esters, PHAs and polyP, but this is the first report of glycogen accumulation in this genus.

Conclusion

RHA1 possess key genes to accumulate diverse storage compounds. Under nitrogen-limiting conditions lipids are the principal storage compounds. An extensive capacity to synthesize and metabolize storage compounds appears to contribute versatility to RHA1 in its responses to environmental stresses.

Potential and utilization of thermophiles and thermostable enzymes in biorefining

In today's world, there is an increasing trend towards the use of renewable, cheap and readily available biomass in the production of a wide variety of fine and bulk chemicals in different biorefineries. Biorefineries utilize the activities of microbial cells and their enzymes to convert biomass into target products. Many of these processes require enzymes which are operationally stable at high temperature thus allowing e.g. easy mixing, better substrate solubility, high mass transfer rate, and lowered risk of contamination. Thermophiles have often been proposed as sources of industrially relevant thermostable enzymes. Here we discuss existing and potential applications of thermophiles and thermostable enzymes with focus on conversion of carbohydrate containing raw materials. Their importance in biorefineries is explained using examples of lignocellulose and starch conversions to desired products. Strategies that enhance thermostablity of enzymes both in vivo and in vitro are also assessed. Moreover, this review deals with efforts made on developing vectors for expressing recombinant enzymes in thermophilic hosts.

Heterologous expression and characterization of a novel branching enzyme from the thermoalkaliphilic anaerobic bacterium Anaerobranca gottschalkii

The gene encoding the branching enzyme (BE) from the thermoalkaliphilic, anaerobic bacterium Anaerobranca gottschalkii was fused with a twin arginine translocation protein secretory-pathway-dependent signal sequence from Escherichia coli and expressed in Staphylococcus carnosus. The secreted BE was purified using hydrophobic interaction and gel filtration chromatography. The monomeric enzyme (72 kDa) shows maximal activity at 50 degrees C and pH 7.0. With amylose the BE displays high transglycosylation and extremely low hydrolytic activity. The conversion of amylose and linear dextrins was analysed by applying high-performance anion exchange chromatography and quantitative size-exclusion chromatography. Amylose (10(4)-4 x 10(7) g/mol) was converted to a major extent to products displaying molecular masses of 10(4)-4 x 10(5) g/mol, indicating that the enzyme could be applicable for the production of starch or dextrins with narrow molecular mass distributions. The majority of the transferred oligosaccharides, determined after enzymatic hydrolysis of the newly synthesized alpha-1,6 linkages, ranged between 10(3) and 10(4) g/mol, which corresponds to a degree of polymerisation (DP) of 6-60. The minimal donor chain length is DP 16. Furthermore, the obtained results support the hypotheses of a random endocleavage mechanism of BE and the occurrence of interchain branching.