Structure of maize BZR1-type β-amylase BAM8 provides new insights into its noncatalytic adaptation

Plant β-amylase (BAM) proteins play an essential role in growth, development, stress response, and hormone regulation. Despite their typical (β/α)₈ barrel structure as active catalysts in starch breakdown, catalytically inactive BAMs are implicated in diverse yet elusive functions in plants. The noncatalytic BAM7/8 contain N-terminal BZR1 domains and were shown to be involved in the regulation of brassinosteroid signaling and possibly serve as sensors of yet an uncharacterized metabolic signal. While the structures of several catalytically active BAMs have been reported, structural characterization of the catalytically inactive BZR1-type BAMs remain unknown. Here, we determine the crystal structure of β-amylase domain of Zea mays BAM8/BES1/BZR1-5 and provide comprehensive insights into its noncatalytic adaptation. Using structural-guided comparison combined with biochemical analysis and molecular dynamics simulations, we revealed conformational changes in multiple distinct highly conserved regions resulting in rearrangement of the binding pocket. Altogether, this study adds a new layer of understanding to starch breakdown mechanism and elucidates the acquired adjustments of noncatalytic BZR1-type BAMs as putative regulatory domains and/or metabolic sensors in plants.

A simplified method of determining the internal structure of amylopectin from barley starch without amylopectin isolation

To determine the internal structure of barley starch without amylopectin isolation, whole starch was hydrolyzed using β-amylase to remove the linear amylose and obtain β-limit dextrins (β-LDs). The β-LDs were treated with extensive α-amylase to prepare α-limit dextrins (α-LDs), and the α-LDs were further hydrolyzed with β-amylase into building blocks. The chain-length distribution of β-LD and building block composition were analyzed by size-exclusion chromatography and anion-exchange chromatography. The internal structure of the barley whole starches had similar pattern to barley amylopectins analyzed by conventional methods. The starch of barley amo1-mutated varieties contained more short internal B-chains and less long internal B-chains than that of other varieties. The starch from amo1-mutated varieties had more large building blocks than that from waxy varieties. The simplified method presented in this study can effectively characterize starch internal structure that relates to physicochemical properties of starch, although some details of amylopectin structure are not assessable.

Improvement of gluten-free steamed bread quality by partial substitution of rice flour with powder of Apios americana tuber

This study investigated the effect of powder made from tubers of the legume Apios americana (Apios) as a rice flour substitute in the making of gluten-free steamed bread. The carbohydrates of Apios powder were mainly starch and sucrose, and included legume-specific raffinose and stachyose. Apios powder contained almost no α-amylase but had a high level of β-amylase activity. Substitution of rice flour with Apios powder delayed the hardening of bread on storage and helped to maintain cohesiveness. Apios powder-substituted bread had higher maltose content than unsubstituted control bread due to β-amylase activity in the Apios powder. Bread substituted with 10% Apios powder had a significantly higher degree of gelatinization than the control even after storage, most likely due to lower amounts of recrystallized amylose as determined by differential scanning calorimetry. These results demonstrate Apios powder as promising a new food ingredient for improving the quality of gluten-free rice bread.

Denaturant Induced Equilibrium Unfolding and Conformational Transitional Studies of Germinated Fenugreek β-Amylase Revealed Molten Globule like State at Low pH

BACKGROUND

β-Amylase (EC 3.2.1.2) is a maltogenic enzyme, which releases β-maltose from the non-reducing end of the substrates. The enzyme plays important roles for the production of vaccine, maltiol and maltose rich syrups. Apart from these applications the enzyme protects cells from abiotic as well as oxidative damage. The enzyme is βwell characterized in βplants and microbes and crystal structures of β-amylases βhave been βobtained from sweet potato, soybean and Bacillus cereus.

OBJECTIVE

Find out correlation between structural and functional stability induced by change in pH, temperature and chaotropes.

METHODS

Activity, intrinsic fluorescence, extrinsic fluorescence, near- and far- ultraviolet circular dichroism spectroscopic measurements were performed.

RESULTS

Peaks about 208 nm and 222 nm obtained by near-ultraviolet circular dichroism correspond to α-helix whereas peak at 215 nm shows presence of β-sheet. At pH 2.0, absence of tertiary structures, exposed of hydrophobic regions and presence of substantial secondary structures, revealed the existence of molten globule like state. Temperature induced denaturation studies showed that the enzyme was stable up to 75 ºC and the process was found to be irreversible in nature. Chaotropes dependent equilibrium unfolding studies revealed that at low concentration of chaotropes, ellipticity and intrinsic fluorescence βintensity were βdecreased βwhereas βenzymatic activity remained unchanged, which revealed fenugreek β-amylase is multi-domains enzyme and catalytic βdomain βis more βstable compare to non-catalytic domain. Moreover, the transition was sigmoidal and non-coincidental.

CONCLUSION

Results indicate the probable existence of intermediate states that might perform significant role in physiological process and biotechnological applications.

Immobilization of fenugreek β-amylase onto functionalized tungsten disulfide nanoparticles using response surface methodology: Its characterization and interaction with maltose and sucrose

In this communication, fenugreek β-amylase was immobilized onto functionalized tungsten disulfide nanoparticles through cross-linker glutaraldehyde and successful immobilization was confirmed by SEM, AFM and FTIR spectroscopy. To make the process economical and efficient, optimization of independent variables was carried out using Box-Behnken design of response surface methodology. Approximately similar predicted (85.6%) and experimental (84.2%) immobilization efficiency revealed that the model is suitable for design of space. Optimum temperature was calculated to be 60 °C. After immobilization, an increased K_m (2.12 times) and a decreased V_max (0.58 times), indicated inaccessibility of active site residues to the substrate. The immobilized enzyme retained 77% relative activity after 10 uses whereas 40% residual activity was obtained after 120 days. An increased half-life with concomitantly decreased kinetic rate constant revealed that the immobilized enzyme is more stable at a higher temperature and the process followed first-order kinetics (R² > 0.93). The limit of detection for maltose and sucrose fluorescence biosensor was found to be 0.052 and 0.096 mM, respectively. Thermodynamic parameters such as changes in Gibbs free energy (ΔG < 0), enthalpy (ΔH > 0) and entropy (ΔS >0) revealed that the process is spontaneous and endothermic, driven by hydrophobic interactions. Thermo-stability data at higher temperature for the immobilized enzyme makes it a suitable candidate for industrial applications in the production of maltose in food and pharmaceutical industries. Furthermore, fluorescence biosensor could be used to detect and quantify maltose and sucrose to maintain the quality of industrial products.

Encapsulation of β-amylase in water-oil-water enzyme emulsion liquid membrane (EELM) bioreactor for enzymatic conversion of starch to maltose

The β-amylase was encapsulated in emulsion liquid membrane (ELM), which acted as a reactor for conversion of starch to maltose. The membrane phase was consisted of surfactant (span 80), stabilizer (polystyrene), carrier for maltose transport (methyl cholate) and solvent (xylene). The substrate starch in feed phase entered into the internal phase by the process of diffusion and hydrolyzed to maltose by encapsulated β-amylase. Methyl cholate present in the membrane acts as a carrier for the product maltose, which helps in transport of maltose to feed phase from internal aqueous phase. The residual activity of β-amylase after the five-reaction cycle was found to decrease to ∼70%, which indicated possibility to recycle the components of the emulsion and enzyme. The pH and temperature of the encapsulated enzyme were found to be optimum at 5.5 and 60 °C, respectively. The novelty of the present work lies in the development of Enzyme Emulsion Liquid Membranes (EELM) bioreactor for the hydrolysis of starch into maltose mediated by encapsulated β-amylase. The attempt has been made for the first time for the successful encapsulation of β-amylase into EELM. The best results gave the highest residual enzyme activity (94.1%) and maltose production (29.13 mg/mL).

The evolution of functional complexity within the β-amylase gene family in land plants

Background

β-Amylases (BAMs) are a multigene family of glucan hydrolytic enzymes playing a key role not only for plant biology but also for many industrial applications, such as the malting process in the brewing and distilling industries. BAMs have been extensively studied in Arabidopsis thaliana where they show a surprising level of complexity in terms of specialization within the different isoforms as well as regulatory functions played by at least three catalytically inactive members. Despite the importance of BAMs and the fact that multiple BAM proteins are also present in other angiosperms, little is known about their phylogenetic history or functional relationship.

Results

Here, we examined 961 β-amylase sequences from 136 different algae and land plant species, including 66 sequenced genomes and many transcriptomes. The extraordinary number and the diversity of organisms examined allowed us to reconstruct the main patterns of β-amylase evolution in land plants. We identified eight distinct clades in angiosperms, which results from extensive gene duplications and sub- or neo-functionalization. We discovered a novel clade of BAM, absent in Arabidopsis, which we called BAM10. BAM10 emerged before the radiation of seed plants and has the feature of an inactive enzyme. Furthermore, we report that BAM4 – an important protein regulating Arabidopsis starch metabolism – is absent in many relevant starch-accumulating crop species, suggesting that starch degradation may be differently regulated between species.

Conclusions

BAM proteins originated sometime more than 400 million years ago and expanded together with the differentiation of plants into organisms of increasing complexity. Our phylogenetic analyses provide essential insights for future functional studies of this important class of storage glucan hydrolases and regulatory proteins.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1395-2) contains supplementary material, which is available to authorized users.

Modification of the digestibility of extruded rice starch by enzyme treatment (β-amylolysis): An in vitro study

The rate and extent of starch hydrolysis in the digestive tract impacts blood glucose levels, which may influence an individual's susceptibility to diabetes and obesity. Strategies for decreasing starch digestibility are therefore useful for developing healthier foods. β-amylase is an exo-hydrolase that specifically cleaves α-1,4 glycosidic linkages of gelatinized starches. In this study, starch granules were disrupted by extrusion under different feed moisture conditions, and then subjected to β-amylolysis. The degree of starch gelatinization increased with increasing feed moisture content during extrusion, leading to faster β-amylolysis. The hydrolysis of in vitro starch digestion study was reduced for extruded samples treated with β-amylase, which was attributed to an increase in resistant starch (RS) after β-amylase treatment. Indeed, X-ray diffraction (XRD) indicated that the crystalline structure in the extruded starch was either partially or fully lost after β-amylase treatment. Similarly, Fourier transform infrared (FTIR) analysis indicated there was a higher level of amorphous regions in the starch after β-amylase treatment. Overall, our results suggest that enzymatic treatment of extruded starch with β-amylolysis reduces the ratio of crystalline-to-amorphous regions, which increases the level of resistant starch, thereby slowing down digestion. These results have important implications for the development of healthier starch-based foods.

Structural insights on starch hydrolysis by plant β-amylase and its evolutionary relationship with bacterial enzymes

The conversion of starch to maltose is catalysed in plants by β-amylase. The enzymatic mechanism has been well-characterized for the soybean and barley enzymes, which utilise a glutamic acid-glutamate pair. In the present study, we present a surprise observation of maltotetraose at the active site, the presence of which elucidates the clear role of Thr344 as a conformational "switch" between substrate binding and product release during hydrolysis. This observation is confirmed by the selection of maltotetraose by the crystallized enzyme although that carbohydrate was present in only trace amounts. The conformation of the residues in the substrate-binding site changed upon substrate binding, leading to the movement of threonine, glutamic acid, and the loop conformation, elucidating a missing link in the existing mechanism. By aligning our substrate-free and maltotetraose-bound structures with other existing structures, the sequence of events from substrate binding to hydrolysis can be visualized. Apart from this, the evolutionary relationship among β-amylases of bacterial and amyloplastic origin could be established. The presence of a sugar-binding domain in the bacterial enzyme and its absence in the plant counterpart could be attributed to a carbohydrate-rich environment. Interestingly, cladogram analysis indicates the presence of N-terminal additions in some plant β-amylases. Based on sequence similarity, we postulate that the role of such additions is important for the regulation of enzymatic activity, particularly under stress conditions.

Assessment of malting characteristics of different Indian barley cultivars

The impact of malting on composition and malt quality parameters such as diastatic power, α-amylase activity, β-amylase activity, hot water extract and β-glucan content were investigated in five different Indian barley cultivars. Protein content of grains increased significantly after malting. Soluble protein content of unmalted grain, which ranged from 3.20-3.93% increased after malting to 4.26-4.85%. Diastatic power of mature grain varied across genotype and their level increased (58.98-81.05 to 115.93-142.45 DP°) after malting. Diastatic power correlated very strongly with protein content (r = 0.90) and strongly with β-amylase activity (r = 0.74). α-amylase, which was low (0.042-0.189 Ceralpha Unit/g) initially in unmalted grain, was synthesized during germination to the range of 149.42-223.78 Ceralpha Unit/g. The correlation between diastatic power and α-amylase was very weak (r = - 0.04). The levels of β-amylase in unmalted grain was in the range of 13.97-18.26; that amount got reduced after malting to 12.55-15.97 Betamyl-3 U/g. β-amylase had a strong positive correlation (r = 0.85) with grain protein. Malted grain which had higher protein content showed very strong negative correlation (r = - 0.86) with hot water extract value. β-glucan content reduced 70-80% from the initial level, across genotypes.

Covalent immobilization of peanut β-amylase for producing industrial nano-biocatalysts: A comparative study of kinetics, stability and reusability of the immobilized enzyme

Stability of enzymes is an important parameter for their industrial applicability. Here, we report successful immobilization of β-amylase (bamyl) from peanut (Arachis hypogaea) onto Graphene oxide-carbon nanotube composite (GO-CNT), Graphene oxide nanosheets (GO) and Iron oxide nanoparticles (Fe₃O₄). The Box-Behnken Design of Response Surface Methodology (RSM) was used which optimized parameters affecting immobilization and gave 90%, 88% and 71% immobilization efficiency, respectively, for the above matrices. β-Amylase immobilization onto GO-CNT (bamyl@GO-CNT) and Fe₃O₄ (bamyl@Fe₃O₄), resulted into approximately 70% retention of activity at 65 °C after 100 min of exposure. We used atomic force microscopy (AFM), scanning and transmission electron microscopy (SEM and TEM), Fourier transformed infrared (FT-IR) spectroscopy and fluorescence microscopy for characterization of free and enzyme bound nanostructures (NS). Due to the non-toxic nature of immobilization matrices and simple but elegant immobilization procedure, these may have potential utility as industrial biocatalysts for production of maltose.

Combined small angle X-ray solution scattering with atomic force microscopy for characterizing radiation damage on biological macromolecules

BACKGROUND

Synchrotron radiation facilities are pillars of modern structural biology. Small-Angle X-ray scattering performed at synchrotron sources is often used to characterize the shape of biological macromolecules. A major challenge with high-energy X-ray beam on such macromolecules is the perturbation of sample due to radiation damage.

RESULTS

By employing atomic force microscopy, another common technique to determine the shape of biological macromolecules when deposited on flat substrates, we present a protocol to evaluate and characterize consequences of radiation damage. It requires the acquisition of images of irradiated samples at the single molecule level in a timely manner while using minimal amounts of protein. The protocol has been tested on two different molecular systems: a large globular tetremeric enzyme (β-Amylase) and a rod-shape plant virus (tobacco mosaic virus). Radiation damage on the globular enzyme leads to an apparent increase in molecular sizes whereas the effect on the long virus is a breakage into smaller pieces resulting in a decrease of the average long-axis radius.

CONCLUSIONS

These results show that radiation damage can appear in different forms and strongly support the need to check the effect of radiation damage at synchrotron sources using the presented protocol.

Pea starch (Pisum sativum L.) with slow digestion property produced using β-amylase and transglucosidase

Starches extracted from wrinkled (WP) and smooth (SP) peas were treated using β-amylase (B) alone and also with a combination of β-amylase and transglucosidase (BT). After enzymatic treatment, the proportions of slowly digested starch in WP-B, WP-BT, SP-B and SP-BT samples were increased by 6%, 9%, 9% and 12%, respectively. Starches treated by a combination of β-amylase and transglucosidase exhibited a smaller amount of longer amylopectin chains, a larger amount of short amylopectin chains, and higher branching fraction. The branching fraction was significantly increased, with an increase of 8%, 10%, 13% and 14% for WP-B, WP-BT, SP-B and SP-BT, respectively. The maximum absorbance and iodine binding of enzyme-treated starches were reduced compared with their native starch parents. The C-type crystalline structure completely disappeared after enzymatic treatment. The results support previous findings that increases in the amount of shorter amylopectin chains and branch fraction are likely to contribute to the slow digestion of starch.

Identification of active site residues of Fenugreek β-amylase: chemical modification and in silico approach

The amino acid sequence of Fenugreek β-amylase is not available in protein data bank. Therefore, an attempt has been made to identify the catalytic amino acid residues of enzyme by employing studies of pH dependence of enzyme catalysis, chemical modification and bioinformatics. Treatment of purified Fenugreek β-amylase with EDAC in presence of glycine methyl ester and sulfhydryl group specific reagents (IAA, NEM and p-CMB), followed a pseudo first-order kinetics and resulted in effective inactivation of enzyme. The reaction with EDAC in presence of NTEE (3-nitro-l-tyrosine ethylester) resulted into modification of two carboxyl groups per molecule of enzyme and presence of one accessible sulfhydryl group at the active site, per molecule of enzyme was ascertained by titration with DTNB. The above results were supported by the prevention of inactivation of enzyme in presence of substrate. Based on MALDI-TOF analysis of purified Fenugreek β-amylase and MASCOT search, β-amylase of Medicago sativa was found to be the best match. To further confirm the amino acid involved in catalysis, homology modelling of β-amylase of M. sativa was performed. The sequence alignment, superimposition of template and target models, along with study of interactions involved in docking of sucrose and maltose at the active site, led to identification of Glu187, Glu381 and Cys344 as active site residues.

Slow glucose release property of enzyme-synthesized highly branched maltodextrins differs among starch sources

Seven types of starch (waxy corn, normal corn, waxy rice, normal rice, waxy potato, normal potato, and tapioca) were selected to produce slowly digestible maltodextrins by enzymatic modification using a previously developed procedure. Branching enzyme (BE) alone and in combination with β-amylase (BA) were used to increase the amount of α-1,6 branching points, which are slowly hydrolyzed by mucosal α-glucosidases in the small intestine. The enzymatic treatments of all starches resulted in a reduction of the debranched linear chain length distribution and weight-average molecular weight. After α-amylolysis of the enzymatically synthesized-maltodextrins, the proportion of branched α-limit dextrins increased, and consequently a reduction in rate of glucose release by rat intestinal α-glucosidases in vitro. Among the samples, enzyme-modified waxy starches had a more pronounced effect on an increase in the slow digestion property than normal starches. These enzyme-modified maltodextrins show potential as novel functional foods by slowing digestion rate to attain extended glucose release.