Three-dimensional structure of an intact glycoside hydrolase family 15 glucoamylase from Hypocrea jecorina

Carbohydrate binding modules: Compact yet potent accessories in the specific substrate binding and performance evolution of carbohydrate-active enzymes

Carbohydrate binding modules (CBMs) are independent non-catalytic domains widely found in carbohydrate-active enzymes (CAZymes), and they play an essential role in the substrate binding process of CAZymes by guiding the appended catalytic modules to the target substrates. Owing to their precise recognition and selective affinity for different substrates, CBMs have received increasing research attention over the past few decades. To date, CBMs from different origins have formed a large number of families that show a variety of substrate types, structural features, and ligand recognition mechanisms. Moreover, through the modification of specific sites of CBMs and the fusion of heterologous CBMs with catalytic domains, improved enzymatic properties and catalytic patterns of numerous CAZymes have been achieved. Based on cutting-edge technologies in computational biology, gene editing, and protein engineering, CBMs as auxiliary components have become portable and efficient tools for the evolution and application of CAZymes. With the aim to provide a theoretical reference for the functional research, rational design, and targeted utilization of novel CBMs in the future, we systematically reviewed the function-related characteristics and potentials of CAZyme-derived CBMs in this review, including substrate recognition and binding mechanisms, non-catalytic contributions to enzyme performances, module modifications, and innovative applications in various fields.

Aspergillus clavatus UEM 04: An efficient producer of glucoamylase and α-amylase able to hydrolyze gelatinized and raw starch

The best amylolytic activity production by Aspergillus clavatus UEM 04 occurred in submersed culture, with starch, for 72 h, at 25 °C, and 100 rpm. Exclusion chromatography partially purified two enzymes, which ran as unique bands in SDS-PAGE with approximately 84 kDa. LC-MS/MS identified a glucoamylase (GH15) and an α-amylase (GH13_1) as the predominant proteins and other co-purified proteins. Zn2+, Cu2+, and Mn2+ activated the glucoamylase, and SDS, Zn2+, Fe3+, and Cu2+ inhibited the α-amylase. The α-amylase optimum pH was 6.5. The optimal temperatures for the glucoamylase and α-amylase were 50 °C and 40 °C, and the Tm was 53.1 °C and 56.3 °C, respectively. Both enzymes remained almost fully active for 28-32 h at 40 °C, but the α-amylase thermal stability was calcium-dependent. Furthermore, the glucoamylase and α-amylase KM for starch were 2.95 and 1.0 mg/mL, respectively. Still, the Vmax was 0.28 μmol/min of released glucose for glucoamylase and 0.1 mg/min of consumed starch for α-amylase. Moreover, the glucoamylase showed greater affinity for amylopectin and α-amylase for maltodextrin. Additionally, both enzymes efficiently degraded raw starch. At last, glucose was the main product of glucoamylase, and α-amylase produced mainly maltose from gelatinized soluble starch hydrolysis.

Microbial glucoamylases: structural and functional properties and biotechnological uses

Glucoamylases (GAs) are one of the principal groups of enzymes involved in starch hydrolysis and belong to the glycosylhydrolase family. They are classified as exo-amylases due to their ability to hydrolyze α-1,4 glycosidic bonds from the non-reducing end of starch, maltooligosaccharides, and related substrates, releasing β-D-glucose. Structurally, GAs possess a characteristic catalytic domain (CD) with an (α/α)6 fold and exhibit five conserved regions within this domain. The CD may or may not be linked to a non-catalytic domain with variable functions depending on its origin. GAs are versatile enzymes with diverse applications in food, biofuel, bioplastic and other chemical industries. Although fungal GAs are commonly employed for these purposes, they have limitations such as their low thermostability and an acidic pH requirement. Alternatively, GAs derived from prokaryotic organisms are a good option to save costs as they exhibit greater thermostability compared to fungal GAs. Moreover, a group of cold-adapted GAs from psychrophilic organisms demonstrates intriguing properties that make them suitable for application in various industries. This review provides a comprehensive overview of the structural and sequential properties as well as biotechnological applications of GAs in different industrial processes.

The Toxoplasma glucan phosphatase TgLaforin utilizes a distinct functional mechanism that can be exploited by therapeutic inhibitors

Toxoplasma gondii is an intracellular parasite that generates amylopectin granules (AGs), a polysaccharide associated with bradyzoites that define chronic T. gondii infection. AGs are postulated to act as an essential energy storage molecule that enable bradyzoite persistence, transmission, and reactivation. Importantly, reactivation can result in the life-threatening symptoms of toxoplasmosis. T. gondii encodes glucan dikinase and glucan phosphatase enzymes that are homologous to the plant and animal enzymes involved in reversible glucan phosphorylation and which are required for efficient polysaccharide degradation and utilization. However, the structural determinants that regulate reversible glucan phosphorylation in T. gondii are unclear. Herein, we define key functional aspects of the T. gondii glucan phosphatase TgLaforin (TGME49_205290). We demonstrate that TgLaforin possesses an atypical split carbohydrate-binding-module domain. AlphaFold2 modeling combined with hydrogen-deuterium exchange mass spectrometry and differential scanning fluorimetry also demonstrate the unique structural dynamics of TgLaforin with regard to glucan binding. Moreover, we show that TgLaforin forms a dual specificity phosphatase domain-mediated dimer. Finally, the distinct properties of the glucan phosphatase catalytic domain were exploited to identify a small molecule inhibitor of TgLaforin catalytic activity. Together, these studies define a distinct mechanism of TgLaforin activity, opening up a new avenue of T. gondii bradyzoite biology as a therapeutic target.

Genomic and proteomic analysis of Tausonia pullulans reveals a key role for a GH15 glucoamylase in starch hydrolysis

Basidiomycetous yeasts remain an almost unexplored source of enzymes with great potential in several industries. Tausonia pullulans (Tremellomycetes) is a psychrotolerant yeast with several extracellular enzymatic activities reported, although the responsible genes are not known. We performed the genomic sequencing, assembly and annotation of T. pullulans strain CRUB 1754 (Perito Moreno glacier, Argentina), a gene survey of carbohydrate-active enzymes (CAZymes), and analyzed its secretome by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) after growth in glucose (GLU) or starch (STA) as main carbon sources. T. pullulans has 7210 predicted genes, 3.6% being CAZymes. When compared to other Tremellomycetes, it contains a high number of CAZy domains, and in particular higher quantities of glucoamylases (GH15), pectinolytic enzymes (GH28) and lignocellulose decay enzymes (GH7). When the secretome of T. pullulans was analyzed experimentally after growth in starch or glucose, 98 proteins were identified. The 60% of total spectral counts belonged to GHs, oxidoreductases and to other CAZymes. A 65 kDa glucoamylase of family GH15 (TpGA1) showed the highest fold change (tenfold increase in starch). This enzyme contains a conserved active site and showed extensive N-glycosylation. This study increases the knowledge on the extracellular hydrolytic enzymes of basidiomycetous yeasts and, in particular, establishes T. pullulans as a potential source of carbohydrate-active enzymes. KEY POINTS: • Tausonia pullulans genome harbors a high number of genes coding for CAZymes. • Among CAZy domains/families, the glycoside hydrolases are the most abundant. • Secretome analysis in glucose or starch as main C sources identified 98 proteins. • A 65 kDa GH15 glucoamylase showed the highest fold increase upon culture in starch.

CBM20CP, a novel functional protein of starch metabolism in green algae

Ostreococcus tauri is a picoalga that contains a small and compact genome, which resembles that of higher plants in the multiplicity of enzymes involved in starch synthesis (ADP-glucose pyrophosphorylase, ADPGlc PPase; granule bound starch synthase, GBSS; starch synthases, SSI, SSII, SSIII; and starch branching enzyme, SBE, between others), except starch synthase IV (SSIV). Although its genome is fully sequenced, there are still many genes and proteins to which no function was assigned. Here, we identify the OT_ostta06g01880 gene that encodes CBM20CP, a plastidial protein which contains a central carbohydrate binding domain of the CBM20 family, and a coiled coil domain at the C-terminus that lacks catalytic activity. We demonstrate that CBM20CP has the ability to bind starch, amylose and amylopectin with different affinities. Furthermore, this protein interacts with OsttaSSIII-B, increasing its binding to starch granules, its catalytic efficiency and promoting granule growth. The results allow us to postulate a functional role for CBM20CP in starch metabolism in green algae. KEY MESSAGE: CBM20CP, a plastidial protein that has a modular structure but lacks catalytic activity, regulates the synthesis of starch in Ostreococcus tauri.

Evaluation of the roles of hydrophobic residues in the N-terminal region of archaeal trehalase in its folding

Thermoplasma trehalase Tvn1315 is predicted to be composed of a β-sandwich domain (BD) and a catalytic domain (CD) based on the structure of the bacterial GH15 family glucoamylase (GA). Tvn1315 as well as Tvn1315 (Δ5), in which the 5 N-terminal amino acids are deleted, could be expressed in Escherichia coli as active enzymes, but deletion of 10 residues (Δ10) led to inclusion body formation. To further investigate the role of the N-terminal region of BD, we constructed five mutants of Δ5, in which each of the 5th to 10th residues of the N-terminus of Tvn1315 was mutated to Ala. Every mutant protein could be recovered in soluble form, but only a small fraction of the Y9A mutant was recovered in the soluble fraction. The Y9A mutant recovered in soluble form had similar specific activity to the other proteins. Subsequent mutation analysis at the 9th position of Tvn1315 in Δ5 revealed that aromatic as well as bulky hydrophobic residues could function properly, but residues with hydroxy groups impaired the solubility. Similar results were obtained with mutants based on untruncated Tvn1315. When the predicted BD, Δ5BD, Δ10BD, and BD mutants were expressed, the Δ10BD protein formed inclusion bodies, and the BD mutants behaved similarly to the Δ5 and full-length enzyme mutants. These results suggest that the hydrophobic region is involved in the solubilization of BD during the folding process. Taken together, these results indicate that the solubility of CD depends on BD folding. KEY POINTS: • N-terminal hydrophobic region of the BD is involved in the protein folding. • The N-terminal hydrophobic region of the BD is also involved in the BD folding. • BD is able to weakly interact with the insoluble β-glucan.

Carbohydrate Binding Modules: Diversity of Domain Architecture in Amylases and Cellulases From Filamentous Microorganisms

Enzymatic degradation of abundant renewable polysaccharides such as cellulose and starch is a field that has the attention of both the industrial and scientific community. Most of the polysaccharide degrading enzymes are classified into several glycoside hydrolase families. They are often organized in a modular manner which includes a catalytic domain connected to one or more carbohydrate-binding modules. The carbohydrate-binding modules (CBM) have been shown to increase the proximity of the enzyme to its substrate, especially for insoluble substrates. Therefore, these modules are considered to enhance enzymatic hydrolysis. These properties have played an important role in many biotechnological applications with the aim to improve the efficiency of polysaccharide degradation. The domain organization of glycoside hydrolases (GHs) equipped with one or more CBM does vary within organisms. This review comprehensively highlights the presence of CBM as ancillary modules and explores the diversity of GHs carrying one or more of these modules that actively act either on cellulose or starch. Special emphasis is given to the cellulase and amylase distribution within the filamentous microorganisms from the genera of Streptomyces and Aspergillus that are well known to have a great capacity for secreting a wide range of these polysaccharide degrading enzyme. The potential of the CBM and other ancillary domains for the design of improved polysaccharide decomposing enzymes is discussed.

Starch-binding domains as CBM families-history, occurrence, structure, function and evolution

The term "starch-binding domain" (SBD) has been applied to a domain within an amylolytic enzyme that gave the enzyme the ability to bind onto raw, i.e. thermally untreated, granular starch. An SBD is a special case of a carbohydrate-binding domain, which in general, is a structurally and functionally independent protein module exhibiting no enzymatic activity but possessing potential to target the catalytic domain to the carbohydrate substrate to accommodate it and process it at the active site. As so-called families, SBDs together with other carbohydrate-binding modules (CBMs) have become an integral part of the CAZy database (http://www.cazy.org/). The first two well-described SBDs, i.e. the C-terminal Aspergillus-type and the N-terminal Rhizopus-type have been assigned the families CBM20 and CBM21, respectively. Currently, among the 85 established CBM families in CAZy, fifteen can be considered as families having SBD functional characteristics: CBM20, 21, 25, 26, 34, 41, 45, 48, 53, 58, 68, 69, 74, 82 and 83. All known SBDs, with the exception of the extra long CBM74, were recognized as a module consisting of approximately 100 residues, adopting a β-sandwich fold and possessing at least one carbohydrate-binding site. The present review aims to deliver and describe: (i) the SBD identification in different amylolytic and related enzymes (e.g., CAZy GH families) as well as in other relevant enzymes and proteins (e.g., laforin, the β-subunit of AMPK, and others); (ii) information on the position in the polypeptide chain and the number of SBD copies and their CBM family affiliation (if appropriate); (iii) structure/function studies of SBDs with a special focus on solved tertiary structures, in particular, as complexes with α-glucan ligands; and (iv) the evolutionary relationships of SBDs in a tree common to all SBD CBM families (except for the extra long CBM74). Finally, some special cases and novel potential SBDs are also introduced.

Purification and biochemical characterization of extracellular glucoamylase from Paenibacillus amylolyticus strain

In the present study, glucoamylase produced from a soil bacterium Paenibacillus amylolyticus NEO03 was cultured under submerged fermentation conditions. The extracellular enzyme was purified by starch adsorption chromatography and further by gel filtration, with 2.73-fold and recovery of 40.02%. The protein exhibited molecular mass of ∼66,000 Da as estimated by SDS-PAGE and depicted to be a monomer. The enzyme demonstrated optimum activity at pH range 6.0-7.0 and temperature range 30-40 °C. Glucoamylase was mostly activated by Mn²⁺ metal ions and depicted no dependency on Ca²⁺ ions. The enzyme preferentially hydrolyzed all the starch substrates. High substrate specificity was demonstrated towards soluble starch and kinetic values K_m and V_max were 2.84 mg/ml and 239.2 U/ml, respectively. The products of hydrolysis of soluble starch were detected by thin layer chromatography which showed only _D -glucose, indicating a true glucoamylase. The secreted glucoamylase from P. amylolyticus strain possesses properties suitable for saccharification processes such as biofuel production.

Heterologous expression and biochemical characterization of a novel cold-active α-amylase from the Antarctic bacteria Pseudoalteromonas sp. 2-3

α-Amylase is an endo-acting enzyme which catalyzes random hydrolysis of starch. These enzymes are used in various biotechnological processes including the textile, paper, food, biofuels, detergents and pharmaceutical industries. The use of active enzymes at low temperatures has a high potential because these enzymes would avoid the demand for heating during the process thereby reducing costs. In this work, the gene of α-amylase from Pseudoalteromonas sp. 2-3 (Antarctic bacteria) has been sequenced and expressed in Escherichia coli BL21(DE3). The ORF of the α-amylase gene cloned into pET22b(+) is 1824 bp long and codes for a protein of 607 amino acid residues including a His₆-tag. The mature protein has a calculated molecular mass of 68.8 kDa. Recombinant α-amylase was purified with Ni-NTA affinity chromatography. The purified enzyme is active on potato starch with a K_m of 6.94 mg/ml and V_max of 0.27 mg/ml*min. The pH optimum is 8.0 and the optimal temperature is 20 °C. This enzyme was strongly activated by Ca²⁺; results consistent with other α-amylases. To the best of our knowledge, this enzyme has the lowest temperature optimum so far reported for α-amylases.

Structural insight into industrially relevant glucoamylases: flexible positions of starch-binding domains

Glucoamylases are one of the most important classes of enzymes in the industrial degradation of starch biomass. They consist of a catalytic domain and a carbohydrate-binding domain (CBM), with the latter being important for the interaction with the polymeric substrate. Whereas the catalytic mechanisms and structures of the individual domains are well known, the spatial arrangement of the domains with respect to each other and its influence on activity are not fully understood. Here, the structures of three industrially used fungal glucoamylases, two of which are full length, have been crystallized and determined. It is shown for the first time that the relative orientation between the CBM and the catalytic domain is flexible, as they can adopt different orientations independently of ligand binding, suggesting a role as an anchor to increase the contact time and the relative concentration of substrate near the active site. The flexibility in the orientations of the two domains presented a considerable challenge for the crystallization of the enzymes.

The structure of the large regulatory α subunit of phosphorylase kinase examined by modeling and hydrogen-deuterium exchange

Phosphorylase kinase (PhK), a 1.3 MDa regulatory enzyme complex in the glycogenolysis cascade, has four copies each of four subunits, (αβγδ)₄ , and 325 kDa of unique sequence (the mass of an αβγδ protomer). The α, β and δ subunits are regulatory, and contain allosteric activation sites that stimulate the activity of the catalytic γ subunit in response to diverse signaling molecules. Due to its size and complexity, no high resolution structures have been solved for the intact complex or its regulatory α and β subunits. Of PhK's four subunits, the least is known about the structure and function of its largest subunit, α. Here, we have modeled the full-length α subunit, compared that structure against previously predicted domains within this subunit, and performed hydrogen-deuterium exchange on the intact subunit within the PhK complex. Our modeling results show α to comprise two major domains: an N-terminal glycoside hydrolase domain and a large C-terminal importin α/β-like domain. This structure is similar to our previously published model for the homologous β subunit, although clear structural differences are present. The overall highly helical structure with several intervening hinge regions is consistent with our hydrogen-deuterium exchange results obtained for this subunit as part of the (αβγδ)₄ PhK complex. Several low exchanging regions predicted to lack ordered secondary structure are consistent with inter-subunit contact sites for α in the quaternary structure of PhK; of particular interest is a low-exchanging region in the C-terminus of α that is known to bind the regulatory domain of the catalytic γ subunit.

Purification and biochemical characterization of a novel mesophilic glucoamylase from Aspergillus tritici WZ99

Glucoamylase, cleaving the nonreducing end of starch releasing glucose, is an important enzyme in starch processing. The optimal temperature for industrial glucoamylase activity is 60-70°C, which is not compatible with the optimal growth temperature for Saccharomyces cerevisiae. In this study, 26 fungal strains producing amylolytic activities that were more active at 30°C than at 60°C were isolated from 151 environmental samples. Fungal strain WZ99, producing extracellular amylolytic activities with the lowest optimal temperature at 40°C, was identified as Aspergillus tritici by analysis of morphological and molecular data. An extracellular glucoamylase was purified from A. tritici WZ99. The optimal pH of the enzyme was 4.0-5.0 and optimal temperature was 45°C. The glucoamylase was stable at pH 4.5-10.0 and below 40°C. Metal ions at four concentrations did not inhibit the enzyme activity. The glucoamylase contained a catalytic domain belonging to glycosyl hydrolase family 15 and thus was named as AtriGA15A. The enzyme shared the highest identity of 54% with a glucoamylase from Rasamsonia emersonii. This glucoamylase showing excellent comprehensive enzymatic characteristics might have potential applications in starch-based bioethanol production and starch processing.

High-resolution structure of a lytic polysaccharide monooxygenase from Hypocrea jecorina reveals a predicted linker as an integral part of the catalytic domain

For decades, the enzymes of the fungus Hypocrea jecorina have served as a model system for the breakdown of cellulose. Three-dimensional structures for almost all H. jecorina cellulose-degrading enzymes are available, except for HjLPMO9A, belonging to the AA9 family of lytic polysaccharide monooxygenases (LPMOs). These enzymes enhance the hydrolytic activity of cellulases and are essential for cost-efficient conversion of lignocellulosic biomass. Here, using structural and spectroscopic analyses, we found that native HjLPMO9A contains a catalytic domain and a family-1 carbohydrate-binding module (CBM1) connected via a linker sequence. A C terminally truncated variant of HjLPMO9A containing 21 residues of the predicted linker was expressed at levels sufficient for analysis. Here, using structural, spectroscopic, and biochemical analyses, we found that this truncated variant exhibited reduced binding to and activity on cellulose compared with the full-length enzyme. Importantly, a 0.95-Å resolution X-ray structure of truncated HjLPMO9A revealed that the linker forms an integral part of the catalytic domain structure, covering a hydrophobic patch on the catalytic AA9 module. We noted that the oxidized catalytic center contains a Cu(II) coordinated by two His ligands, one of which has a His-brace in which the His-1 terminal amine group also coordinates to a copper. The final equatorial position of the Cu(II) is occupied by a water-derived ligand. The spectroscopic characteristics of the truncated variant were not measurably different from those of full-length HjLPMO9A, indicating that the presence of the CBM1 module increases the affinity of HjLPMO9A for cellulose binding, but does not affect the active site.

Purification and characterization of a novel cold adapted fungal glucoamylase

Background

Amylases are used in various industrial processes and a key requirement for the efficiency of these processes is the use of enzymes with high catalytic activity at ambient temperature. Unfortunately, most amylases isolated from bacteria and filamentous fungi have optimal activity above 45 °C and low pH. For example, the most commonly used industrial glucoamylases, a type of amylase that degrades starch to glucose, are produced by Aspergillus strains displaying optimal activities at 45–60 °C. Thus, isolating new amylases with optimal activity at ambient temperature is essential for improving industrial processes. In this report, a glucoamylase secreted by the cold-adapted yeast Tetracladium sp. was isolated and biochemically characterized.

Results

The effects of physicochemical parameters on enzyme activity were analyzed, and pH and temperature were found to be key factors modulating the glucoamylase activity. The optimal conditions for enzyme activity were 30 °C and pH 6.0, and the K_m and k_cat using soluble starch as substrate were 4.5 g/L and 45 min⁻¹, respectively. Possible amylase or glucoamylase encoding genes were identified, and their transcript levels using glucose or soluble starch as the sole carbon source were analyzed. Transcription levels were highest in medium supplemented with soluble starch for the potential glucoamylase encoding gene. Comparison of the structural model of the identified Tetracladium sp. glucoamylase with the solved structure of the Hypocrea jecorina glucoamylase revealed unique structural features that may explain the thermal lability of the glucoamylase from Tetracladium sp.

Conclusion

The glucoamylase secreted by Tetracladium sp. is a novel cold-adapted enzyme and its properties should render this enzyme suitable for use in industrial processes that require cold-active amylases, such as biofuel production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0693-x) contains supplementary material, which is available to authorized users.

Functional dissection of the N-terminal sequence of Clostridium sp. G0005 glucoamylase: identification of components critical for folding the catalytic domain and for constructing the active site structure

Clostridium sp. G0005 glucoamylase (CGA) is composed of a β-sandwich domain (BD), a linker, and a catalytic domain (CD). In the present study, CGA was expressed in Escherichia coli as inclusion bodies when the N-terminal region (39 amino acid residues) of the BD was truncated. To further elucidate the role of the N-terminal region of the BD, we constructed N-terminally truncated proteins (Δ19, Δ24, Δ29, and Δ34) and assessed their solubility and activity. Although all evaluated proteins were soluble, their hydrolytic activities toward maltotriose as a substrate varied: Δ19 and Δ24 were almost as active as CGA, but the activity of Δ29 was substantially lower, and Δ34 exhibited little hydrolytic activity. Subsequent truncation analysis of the N-terminal region sequence between residues 25 and 28 revealed that truncation of less than 26 residues did not affect CGA activity, whereas truncation of 26 or more residues resulted in a substantial loss of activity. Based on further site-directed mutagenesis and N-terminal sequence analysis, we concluded that the 26XaaXaaTrp28 sequence of CGA is important in exhibiting CGA activity. These results suggest that the N-terminal region of the BD in bacterial GAs may function not only in folding the protein into the correct structure but also in constructing a competent active site for catalyzing the hydrolytic reaction.

Unique carbohydrate binding platforms employed by the glucan phosphatases

Glucan phosphatases are a family of enzymes that are functionally conserved at the enzymatic level in animals and plants. These enzymes bind and dephosphorylate glycogen in animals and starch in plants. While the enzymatic function is conserved, the glucan phosphatases employ distinct mechanisms to bind and dephosphorylate glycogen or starch. The founding member of the family is a bimodular human protein called laforin that is comprised of a carbohydrate binding module 20 (CBM20) followed by a dual specificity phosphatase domain. Plants contain two glucan phosphatases: Starch EXcess4 (SEX4) and Like Sex Four2 (LSF2). SEX4 contains a chloroplast targeting peptide, dual specificity phosphatase (DSP) domain, a CBM45, and a carboxy-terminal motif. LSF2 is comprised of simply a chloroplast targeting peptide, DSP domain, and carboxy-terminal motif. SEX4 employs an integrated DSP-CBM glucan-binding platform to engage and dephosphorylate starch. LSF2 lacks a CBM and instead utilizes two surface binding sites to bind and dephosphorylate starch. Laforin is a dimeric protein in solution and it utilizes a tetramodular architecture and cooperativity to bind and dephosphorylate glycogen. This chapter describes the biological role of glucan phosphatases in glycogen and starch metabolism and compares and contrasts their ability to bind and dephosphorylate glucans.

Efficient hydrolysis of raw starch and ethanol fermentation: a novel raw starch-digesting glucoamylase from Penicillium oxalicum

BACKGROUND

Starch is a very abundant and renewable carbohydrate and is an important feedstock for industrial applications. The conventional starch liquefaction and saccharification processes are energy-intensive, complicated, and not environmentally friendly. Raw starch-digesting glucoamylases are capable of directly hydrolyzing raw starch to glucose at low temperatures, which significantly simplifies processing and reduces the cost of producing starch-based products.

RESULTS

A novel raw starch-digesting glucoamylase PoGA15A with high enzymatic activity was purified from Penicillium oxalicum GXU20 and biochemically characterized. The PoGA15A enzyme had a molecular weight of 75.4 kDa, and was most active at pH 4.5 and 65 °C. The enzyme showed remarkably broad pH stability (pH 2.0-10.5) and substrate specificity, and was able to degrade various types of raw starches at 40 °C. Its adsorption ability for different raw starches was consistent with its degrading capacities for the corresponding substrate. The cDNA encoding the enzyme was cloned and heterologously expressed in Pichia pastoris. The recombinant enzyme could quickly and efficiently hydrolyze different concentrations of raw corn and cassava flours (50, 100, and 150 g/L) with the addition of α-amylase at 40 °C. Furthermore, when used in the simultaneous saccharification and fermentation of 150 g/L raw flours to ethanol with the addition of α-amylase, the ethanol yield reached 61.0 g/L with a high fermentation efficiency of 95.1 % after 48 h when raw corn flour was used as the substrate. An ethanol yield of 57.0 g/L and 93.5 % of fermentation efficiency were achieved with raw cassava flour after 36 h. In addition, the starch-binding domain deletion analysis revealed that SBD plays a very important role in raw starch hydrolysis by the enzyme PoGA15A.

CONCLUSIONS

A novel raw starch-digesting glucoamylase from P. oxalicum, with high enzymatic activity, was biochemically, molecularly, and genetically identified. Its efficient hydrolysis of raw starches and its high efficiency during the direct conversion of raw corn and cassava flours via simultaneous saccharification and fermentation to ethanol suggests that the enzyme has a number of potential applications in industrial starch processing and starch-based ethanol production.

A thermostable glucoamylase from Bispora sp. MEY-1 with stability over a broad pH range and significant starch hydrolysis capacity

Background

Glucoamylase is an exo-type enzyme that converts starch completely into glucose from the non-reducing ends. To meet the industrial requirements for starch processing, a glucoamylase with excellent thermostability, raw-starch degradation ability and high glucose yield is much needed. In the present study we selected the excellent Carbohydrate-Activity Enzyme (CAZyme) producer, Bispora sp. MEY-1, as the microbial source for glucoamylase gene exploitation.

Methodology/Principal Findings

A glucoamylase gene (gla15) was cloned from Bispora sp. MEY-1 and successfully expressed in Pichia pastoris with a high yield of 34.1 U/ml. Deduced GLA15 exhibits the highest identity of 64.2% to the glucoamylase from Talaromyces (Rasamsonia) emersonii. Purified recombinant GLA15 was thermophilic and showed the maximum activity at 70°C. The enzyme was stable over a broad pH range (2.2–11.0) and at high temperature up to 70°C. It hydrolyzed the breakages of both α-1,4- and α-1,6-glycosidic linkages in amylopectin, soluble starch, amylose, and maltooligosaccharides, and had capacity to degrade raw starch. TLC and H¹-NMR analysis showed that GLA15 is a typical glucoamylase of GH family 15 that releases glucose units from the non-reducing ends of α-glucans. The combination of Bacillus licheniformis amylase and GLA15 hydrolyzed 96.14% of gelatinized maize starch after 6 h incubation, which was about 9% higher than that of the combination with a commercial glucoamylase from Aspergillus niger.

Conclusion/Significance

GLA15 has a broad pH stability range, high-temperature thermostability, high starch hydrolysis capacity and high expression yield. In comparison with the commercial glucoamylase from A. niger, GLA15 represents a better candidate for application in the food industry including production of glucose, glucose syrups, and high-fructose corn syrups.

The structure of a glycoside hydrolase family 81 endo-β-1,3-glucanase

Endo-β-1,3-glucanases catalyze the hydrolysis of β-1,3-glycosidic linkages in glucans. They are also responsible for rather diverse physiological functions such as carbon utilization, cell-wall organization and pathogen defence. Glycoside hydrolase (GH) family 81 mainly consists of β-1,3-glucanases from fungi, higher plants and bacteria. A novel GH family 81 β-1,3-glucanase gene (RmLam81A) from Rhizomucor miehei was expressed in Escherichia coli. Purified RmLam81A was crystallized and the structure was determined in two crystal forms (form I-free and form II-Se) at 2.3 and 2.0 Å resolution, respectively. Here, the crystal structure of a member of GH family 81 is reported for the first time. The structure of RmLam81A is greatly different from all endo-β-1,3-glucanase structures available in the Protein Data Bank. The overall structure of the RmLam81A monomer consists of an N-terminal β-sandwich domain, a C-terminal (α/α)6 domain and an additional domain between them. Glu553 and Glu557 are proposed to serve as the proton donor and basic catalyst, respectively, in a single-displacement mechanism. In addition, Tyr386, Tyr482 and Ser554 possibly contribute to both the position or the ionization state of the basic catalyst Glu557. The first crystal structure of a GH family 81 member will be helpful in the study of the GH family 81 proteins and endo-β-1,3-glucanases.

Functional role of β domain in the Thermoanaerobacter tengcongensis glucoamylase

Thermoanaerobacter tengcongensis MB4 glucoamylase (TteGA) contains a catalytic domain (CD), which is structurally similar to eukaryotic GA, and a β domain (BD) with ambiguous function. Firstly, BD is found to be essential to TteGA activity because CD alone could not hydrolyze soluble starch. However, starch hydrolysis activity, similar to that of intact TteGA, was restored to CD in the presence of BD. Secondly, BD is found to be an important helper in the correct folding of CD because CD was mainly expressed in the inclusion bodies on its own in Escherichia coli. By contrast, intact TteGA, BD, and CD combined with BD could be expressed as soluble proteins. Additionally, BD is essential to the thermostability of TteGA because CD displayed lower thermostability compared with the intact TteGA and exhibited enhanced thermostability in the presence of BD in vitro. Truncation of TteGA or mutagenesis of the residues that participate in the interdomain interaction at its BD also led to the reduced thermostability of TteGA.

The crystallization and structural analysis of cellulases (and other glycoside hydrolases): strategies and tactics

The three-dimensional (3-D) structures of cellulases, and other glycoside hydrolases, are a central feature of research in carbohydrate chemistry and biochemistry. 3-D structure is used to inform protein engineering campaigns, both academic and industrial, which are typically used to improve the stability or activity of an enzyme. Examples of classical protein engineering goals include higher thermal stability, reduced metal-ion dependency, detergent and protease resistance, decreased product inhibition, and altered specificity. 3-D structure may also be used to interpret the behavior of enzyme variants that are derived from screening or random mutagenesis approaches, with a view to establishing an iterative design process. In other areas, 3-D structure is used as one of the many tools to probe enzymatic catalysis, typically dovetailing with physical organic chemistry approaches to provide complete reaction mechanisms for enzymes by visualizing catalytic site interactions at different stages of the reaction. Such mechanistic insight is not only fundamentally important, impacting on inhibitor and drug design approaches with ramifications way beyond cellulose hydrolysis, but also provides the framework for the design of enzyme variants to use as biocatalysts for the synthesis of bespoke oligosaccharides. Here we review some of the strategies and tactics that may be applied to the X-ray structure solution of cellulases (and other carbohydrate-active enzymes). The general approach is first to decide why you are doing the work, then to establish correct domain boundaries for truncated constructs (typically the catalytic domain only), and finally to pursue crystallization of pure, homogeneous, and monodisperse protein with appropriate ligand and additive combinations. Cellulase-specific strategies are important for the delineation of domain boundaries, while glycoside hydrolases generally also present challenges and opportunities for the selection and optimization of ligands to both aid crystallization, and also provide structural and mechanistic insight. As the many roles for plant cell wall degrading enzymes increase, so does the need for rapid high-quality structure determination to provide a sound structural foundation for understanding mechanism and specificity, and for future protein engineering strategies.

The carbohydrate-binding module family 20--diversity, structure, and function

Starch-active enzymes often possess starch-binding domains (SBDs) mediating attachment to starch granules and other high molecular weight substrates. SBDs are divided into nine carbohydrate-binding module (CBM) families, and CBM20 is the earliest-assigned and best characterized family. High diversity characterizes CBM20s, which occur in starch-active glycoside hydrolase families 13, 14, 15, and 77, and enzymes involved in starch or glycogen metabolism, exemplified by the starch-phosphorylating enzyme glucan, water dikinase 3 from Arabidopsis thaliana and the mammalian glycogen phosphatases, laforins. The clear evolutionary relatedness of CBM20s to CBM21s, CBM48s and CBM53s suggests a common clan hosting most of the known SBDs. This review surveys the diversity within the CBM20 family, and makes an evolutionary comparison with CBM21s, CBM48s and CBM53s, discussing intrafamily and interfamily relationships. Data on binding to and enzymatic activity towards soluble ligands and starch granules are summarized for wild-type, mutant and chimeric fusion proteins involving CBM20s. Noticeably, whereas CBM20s in amylolytic enzymes confer moderate binding affinities, with dissociation constants in the low micromolar range for the starch mimic beta-cyclodextrin, recent findings indicate that CBM20s in regulatory enzymes have weaker, low millimolar affinities, presumably facilitating dynamic regulation. Structures of CBM20s, including the first example of a full-length glucoamylase featuring both the catalytic domain and the SBD, are summarized, and distinct architectural and functional features of the two SBDs and roles of pivotal amino acids in binding are described. Finally, some applications of SBDs as affinity or immobilization tags and, recently, in biofuel and in planta bioengineering are presented.