Glucanases are enzymes that break down large polysaccharides via hydrolysis. The product of the hydrolysis reaction is called a glucan, a linear polysaccharide made of up to 1200 glucose monomers, held together with glycosidic bonds.[1] Glucans are abundant in the endosperm cell walls of cereals such as barley, rye, sorghum, rice, and wheat.[1] Glucanases are also referred to as lichenases, hydrolases, glycosidases, glycosyl hydrolases, and/or laminarinases.[1] Many types of glucanases share similar amino acid sequences but vastly different substrates.[1] Of the known endo-glucanases, 1,3-1,4-β-glucanase is considered the most active.[1]
Glucanase | |||||||||
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Identifiers | |||||||||
EC no. | 3.2.1. | ||||||||
CAS no. | 9015-78-5 | ||||||||
Databases | |||||||||
IntEnz | IntEnz view | ||||||||
BRENDA | BRENDA entry | ||||||||
ExPASy | NiceZyme view | ||||||||
KEGG | KEGG entry | ||||||||
MetaCyc | metabolic pathway | ||||||||
PRIAM | profile | ||||||||
PDB structures | RCSB PDB PDBe PDBsum | ||||||||
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Glucanase | |||||||
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Identifiers | |||||||
Symbol | Eng1p | ||||||
CAS number | |||||||
PDB | 5GY3 | ||||||
RefSeq | WP_012967086.1 | ||||||
UniProt | A0A0J4VP90 | ||||||
Other data | |||||||
EC number | 3.2.1 | ||||||
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β-Glucanases are a diverse group of enzymes that catalyze the hydrolysis of β-glucans, which are polysaccharides composed of glucose monomers connected by β-glycosidic linkages. These enzymes play crucial roles in various biological processes across different organisms, including plants, fungi, and bacteria.[2]
In plants, β-glucanases are involved in cell wall metabolism, including the synthesis, remodeling, and turnover of cell wall components during different stages of growth and development.[2] They are classified into three main categories based on the type of glycosidic bond they cleave: β-1,4-glucanases, β-1,3-glucanases, and β-1,3-1,4-glucanases.[2] Plant β-glucanases also participate in defense mechanisms against pathogen attack by degrading the cell walls of invading microbes and generating signaling glucans that trigger global defense responses.[2][3]
Microbial β-glucanases are involved in the degradation of β-glucans present in their environment, allowing them to utilize these polysaccharides as a carbon source for growth and energy production.[2] These enzymes have potential applications in various industries, such as food, feed, and pharmaceuticals.[4]
β-Glucanases are classified into different glycoside hydrolase (GH) families based on their amino acid sequence similarities and structural features.[2] The main GH families containing β-glucanases are GH5, GH16, GH17, GH55, and GH81.[2] Each family has distinct substrate specificities, catalytic mechanisms, and evolutionary origins.[2][5]
The historical interest in β-glucanases can be traced back to their significant roles in plant and microbial biology, particularly in the context of cell wall remodeling, seed germination, and defense mechanisms. Early research efforts were directed towards isolating and characterizing these enzymes from various sources, including plants, fungi, and bacteria, which led to the discovery of their widespread presence and functional diversity.
For example, the study of β-glucanases in barley was a critical milestone in understanding the process of seed germination. In this context, β-glucanases break down the cell walls of the endosperm, facilitating the mobilization of nutrients necessary for the growth of the seedling.[2] This enzymatic activity is essential for the successful transition from seed to seedling, as it allows the developing plant to access stored carbohydrates.
In addition to their role in germination, β-glucanases are also implicated in the remodeling of cell walls during plant growth and development. They participate in the modification of cell wall structure, which is crucial for processes such as cell elongation, fruit ripening, and organ abscission.[6] The enzymes are involved in the degradation of cellulose and other polysaccharides containing 1,4-glycosidic bonds, which are key components of the plant cell wall.[2][6]
Furthermore, β-glucanases have been identified as important players in plant defense. They contribute to the formation of a local antimicrobial defense barrier by hydrolyzing β-glucans found in the walls of microbial pathogens. This enzymatic action not only helps to prevent the spread of infection but also generates signaling molecules that activate broader plant defense responses.[2]
The study of microbial β-glucanases has also revealed their role in the degradation of β-glucans in the environment, which allows microorganisms to utilize these polysaccharides as a source of carbon and energy.[4] This aspect of β-glucanase function has implications for the production of these enzymes for industrial applications.
The secondary and tertiary structures of β-glucanases involves the stacking of multiple β-sheets, each of which are made of several anti-parallel strands that bend and form a cleft crossing the active site of the enzyme.[1] This type of structure has been called the "jelly roll fold."
The functional formation of the enzyme-substrate complex is dictated by the induced-fit mechanism.[1]
β-Glucanases belong to the glycoside hydrolase (GH) families, which are classified into over 170 families in the CAZy database based on sequence similarities.[8] These families are further divided into subfamilies to account for more specific variations in sequence, structure, and substrate specificity. For β-glucanases, the GH16 and GH17 families are particularly important, with each family targeting specific linkages within β-glucans.[8]
The GH16 family is a large and taxonomically diverse group of enzymes that adopt a common β-jelly-roll fold and are active on a variety of terrestrial and marine polysaccharides. The GH16 family has been systematically divided into 23 robust subfamilies, supported by hidden Markov model and maximum likelihood molecular phylogenetic analyses.[8] These subfamilies exhibit key tertiary structural differences, particularly in active-site loops, which dictate substrate specificity across the GH16 evolutionary landscape.
The GH17 family, while not detailed in the provided search results, is likely to have similar diversity and specialization within its subfamilies, allowing for a range of substrate specificities and biological functions. The classification of β-glucanases into these families and subfamilies provides a roadmap for functional glycogenomics and guides bioinformatics and experimental structure-function analyses.[8]
The structural features of β-glucanases, particularly within the GH16 family, highlight the importance of the active site's architecture and the positioning of catalytic residues in determining enzyme specificity towards different β-glucan linkages. The GH16 family exhibits structural diversity in the active-site loops, which is a critical factor in determining the enzyme's substrate specificity.[8]
The GH16 family enzymes adopt a common β-jelly-roll fold and are active on a range of polysaccharides. The delineation of nearly 23,000 GH16 sequences into 23 robust subfamilies, supported by hidden Markov model and maximum likelihood molecular phylogenetic analyses, has highlighted key tertiary structural differences. These differences are predominantly manifested in active-site loops, dictating substrate specificity across the GH16 evolutionary landscape.[8] This structural diversity allows GH16 enzymes to target a variety of β-glucan linkages, reflecting their adaptability and functional versatility.
The active site of β-glucanases is designed to accommodate the β-glucan chain for catalysis. The specificity of these enzymes towards different β-glucan linkages is determined by the architecture of the active site and the positioning of catalytic residues within it. For instance, the depth and width of the substrate-binding groove, along with the nature of the amino acid residues that line this groove, play a crucial role in defining the enzyme's affinity and specificity for its substrate.[8]
Moreover, the catalytic mechanism of β-glucanases typically involves a pair of glutamic acid residues, one acting as a nucleophile and the other as an acid/base during the hydrolysis reaction. This dual role of glutamic acid residues is a common feature among glycoside hydrolases and is essential for the cleavage of the glycosidic bond in β-glucans.[8]
β-Glucanases are a diverse group of enzymes that catalyze the hydrolysis of various β-glucans, which are polysaccharides composed of glucose monomers linked by β-glycosidic bonds. The specificity of these enzymes towards different types of β-glucan linkages determines their biological roles and potential applications. Here, we explore the major substrates of β-glucanases and their significance.
β-(1,3)-Glucans are found in the cell walls of fungi and some plants. β-(1,3)-Glucanases are enzymes that specifically target and degrade these polysaccharides, playing a crucial role in biocontrol against fungal pathogens.[9] These enzymes can weaken the cell walls of invading fungi, limiting their spread and reducing the severity of infections. As such, β-(1,3)-glucanases have potential applications in the development of environmentally friendly antifungal agents for agricultural and medical purposes.
Cellulose, a major component of plant cell walls, consists of β-(1,4)-linked glucose units. Enzymes capable of hydrolyzing cellulose, known as cellulases, are widespread among bacteria, fungi, and other organisms.[9] This widespread distribution reflects the universal need to access glucose from plant biomass for energy and carbon sources. Cellulases, including β-(1,4)-glucanases, play a crucial role in the degradation of cellulosic materials, making them valuable in various industrial processes, such as biofuel production, textile manufacturing, and paper recycling.[4]
β-(1,6)-Glucans are less common polysaccharides found as minor components in the cell walls of some fungi and bacteria. β-(1,6)-Glucanases are enzymes that efficiently hydrolyze these glucans, releasing glucose as the primary product without forming oligosaccharides.[9] While their applications are less explored compared to other β-glucanases, these enzymes may have potential in the food and pharmaceutical industries, where the production of specific oligosaccharides is desirable.
Mixed-linkage β-glucans are polysaccharides found in the cell walls of grasses and cereals like barley and oats. These glucans consist of chains with both β-(1,3)- and β-(1,4)-glucosyl residues.[9] The physicochemical properties of these glucans, and thus their susceptibility to enzymatic hydrolysis, are influenced by the ratio and distribution of these linkages.[9] β-Glucanases that can efficiently degrade mixed-linkage glucans are of particular interest in the brewing and baking industries, where they can improve the quality and processing of cereal-based products.[10]
The diversity of β-glucan substrates highlights the versatility and importance of β-glucanases in various biological processes and industrial applications. Understanding the specificity of these enzymes towards different β-glucan linkages is crucial for harnessing their potential in biotechnology, agriculture, and medicine.
The main function of glucanase is to catalyze the hydrolysis of glycosidic bonds in polysaccharides. This function is not highly specific, and the enzymes distinguish among substrates mostly by the types of bonds present and α- or β- configuration.[11]
In 1953, Dr. D. E. Koshland proposed a double-displacement mechanism for this enzyme action.[12] The first step of his proposed mechanism is rate-limiting step independent of the concentration of the substrate and involves an amino acid nucleophile and an acid/base catalyst.[12] In this step, the nucleophile, with help from the acid residue, displaces the aglycone and forms a covalent glycosyl-enzyme intermediate.[12][1] The second step involves a water molecule, assisted by the conjugate base of the acid catalyst, rendering the free sugar while retaining an anomeric configuration of the molecule.[1]
Glucanases can also catalyze transglycosylation, resulting in new β-glycosidic bonds between donor and acceptor saccharides.[1] This reaction, which has the same region- and stereo-specificity as the hydrolysis reaction, involves either the direct reversal of hydrolysis (known as condensation) or kinetic control of a glycosyl donor substrate.[1]
The hydrolytic mechanism of β-glucanases, particularly those from Bacillus species such as Bacillus licheniformis, follows a double-displacement reaction. This involves two main steps: glycosylation and deglycosylation.[13] During glycosylation, a catalytic nucleophile attacks the anomeric carbon of the substrate, forming a covalent enzyme-substrate intermediate. This is followed by deglycosylation, where a water molecule, activated by a general acid-base, attacks the intermediate, completing the hydrolysis and releasing the product.[13]
In the case of Bacillus 1,3-1,4-β-glucanases, which are retaining endo-glycosidases of family 16 GH, the catalytic mechanism involves a catalytic triad. This triad consists of two glutamic acid residues, Glu134 and Glu138, and an aspartic acid residue, Asp136.[13] Glu134 serves as the enzyme nucleophile, Glu138 acts as the general acid-base, and Asp136 assists in catalysis by participating in both glycosylation and deglycosylation steps, contributing to the pKa modulation of Glu138 during the enzyme cycle.[13]
The binding-site cleft of Bacillus 1,3-1,4-β-glucanases is composed of several subsites, typically ranging from -4 to +2, with subsite -3 making the largest contribution to transition state stabilization.[14] The specificity of this subsite is crucial for both glycosidase and glycosynthase activities. For example, a D-galactosyl residue on the nonreducing end of a trisaccharide substrate can be accepted by the enzyme and binds at subsite -3 in the productive enzyme–substrate complex.[14] This specificity allows the enzyme to catalyze the hydrolysis of substrates with different glycosidic linkages, as well as to participate in transglycosylation reactions when the catalytic nucleophile is mutated.[14]
Upon mutation of the catalytic nucleophile Glu134 to alanine, the Bacillus licheniformis 1,3-1,4-β-glucanase becomes a highly efficient endo-glycosynthase, with strict specificity for the formation of β-1,4 glycosidic bonds.[13] This modified enzyme can be used to catalyze the transfer of glycosyl residues to acceptor molecules, a process known as transglycosylation, which is valuable for oligosaccharide synthesis.[13]
β-Glucanases are enzymes with a wide array of uses and applications across various fields, from agriculture to biotechnology and medicine.
In plants, β-glucanases are involved in the synthesis, remodeling, and turnover of cell wall components during multiple physiological processes. They play a role in cell growth, symplastic trafficking through plasmodesmata, and energy mobilization during rapid seedling growth.[2] Additionally, plant β-glucanases are crucial in regulating symbiotic and hostile plant-microbe interactions by degrading non-self glucan structures, thus contributing to antimicrobial defense and triggering global defense responses.[2]
β-Glucans have marked immunomodulatory and metabolic properties, which have led to their use in fighting cancer, reducing the risk of cardiovascular diseases, and controlling diabetes.[15] β-Glucanases are invaluable tools for studying β-glucans and have applications in numerous biotechnological and industrial processes, both alone and in conjunction with their natural substrates.[15]
β-Glucanases are used in the brewing industry to break down glucans that can cause viscosity problems in beer production.[16] They are also added to wines to improve organoleptic characteristics and are used in the food industry to obtain bioactive oligosaccharides and for the structural characterization of microbial cell walls.[17]
In agriculture, β-glucanases can be used as biocontrol agents against plant pathogens. They degrade the cell walls of fungi, which can help protect crops from diseases.[18] Additionally, the application of β-glucanases can enhance the degradation of plant cell walls, facilitating the colonization of plant tissues by beneficial microbes.[18]
β-Glucanases are added to animal feed to improve the nutritive value of high-fiber ingredients like alfalfa and rye for monogastric animals such as poultry.[19] While the effectiveness of these enzymes in improving the nutritive value of alfalfa-containing diets for laying hens is still under investigation, they have the potential to impact the quality of poultry products.[19]
The rational design of thermostability in bacterial 1,3-1,4-β-glucanases has been explored to enhance their performance under industrial conditions.[20] By understanding the structural features that contribute to enzyme stability, β-glucanases can be engineered for improved functionality in various applications.
β-Glucanases are generally considered safe for use in food and other applications. A safety evaluation of β-glucanases produced by Disporotrichum dimorphosporum did not raise any concerns regarding genotoxicity or systemic toxicity, with a high margin of exposure indicating safety for dietary consumption. However, a similarity search of amino acid sequences identified matches with allergens from mite proteins for one of the enzymes, suggesting potential allergenicity in sensitive individuals.[21] Despite this, the overall risk of allergic reactions is deemed low, although not entirely dismissible. It's noteworthy that while β-glucanases have been associated with respiratory sensitization in occupational settings, dietary exposure to these enzymes is unlikely to provoke allergic reactions.[21]
The stability and activity of β-glucanases are influenced by storage conditions. Short-term stability is typically achieved at refrigeration temperatures (2-8°C), which is recommended for maintaining enzyme activity over shorter periods. Long-term storage conditions may vary, and it's essential to refer to individual component labels for specific recommendations.[22] The stability of β-glucanases under different storage conditions, including temperature variations, has been studied, indicating that enzyme activity can decrease over time, especially at higher temperatures or if improperly stored.[22] For instance, β-glucanase activity was found to degrade by up to 22.5% at -20°C and 9.3% at -70°C over 63 days, highlighting the importance of low-temperature storage for preserving enzyme functionality[22].
Bacteria such as Escherichia coli, and Bacillus spp. produce 1,3-1,4-β-glucanases in order to degrade and use polysaccharides from their environment as an energy source.[1] These bacterial glucanases are an example of convergent evolution as they share similarity or relation with plant glucanase primary, secondary, or tertiary structure.[1] Glucanases have also been found to be secreted by fungi such as Trichoderma harzianum, Saccharomyces cerevisiae and the anaerobic fungi Orpinomyces and Neocallimastigomycota, found in the digestive tracts of herbivores.[1][23][24] T. harzianum is also used as a fungicide, which is linked to the ability of its β-gluanases to hydrolyze phytopathogenic fungi via a mycoparasitic attack.[24]
Barley 1,3-1,4-β-glucanases are heat inactivated during malting, which can cause the build-up of high molecular-weight glucans which in turn result in reduced extract yield, lower filtration rates, and even gelatinous precipitates in the finished product. As a remedy, heat-resistant bacterial 1,3-1,4-β-glucanases are added.[1]
Used in enological practices during the aging process of wine, particularly when aged on lees with microxygenation. The enzyme aids in autolysis of yeast cells to release polysaccharides and mannoproteins, which is believed to aid in the color and texture of the wine.
In the production of feedstuff for broiler chickens and piglets, it has been found that β-glucanases improve digestibility of barley-based diets.[1]