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Catalytic properties, functional attributes and industrial applications of β -glucosidases

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ABSTRACT

β-Glucosidases are diverse group of enzymes with great functional importance to biological systems. These are grouped in multiple glycoside hydrolase families based on their catalytic and sequence characteristics. Most studies carried out on β-glucosidases are focused on their industrial applications rather than their endogenous function in the target organisms. β-Glucosidases performed many functions in bacteria as they are components of large complexes called cellulosomes and are responsible for the hydrolysis of short chain oligosaccharides and cellobiose. In plants, β-glucosidases are involved in processes like formation of required intermediates for cell wall lignification, degradation of endosperm’s cell wall during germination and in plant defense against biotic stresses. Mammalian β-glucosidases are thought to play roles in metabolism of glycolipids and dietary glucosides, and signaling functions. These enzymes have diverse biotechnological applications in food, surfactant, biofuel, and agricultural industries. The search for novel and improved β-glucosidase is still continued to fulfills demand of an industrially suitable enzyme. In this review, a comprehensive overview on detailed functional roles of β-glucosidases in different organisms, their industrial applications, and recent cloning and expression studies with biochemical characterization of such enzymes is presented for the better understanding and efficient use of diverse β-glucosidases.

No MeSH data available.


Proposed “retaining” mechanism for hydrolysis of β-glycosidic bond by β-glucosidase: (1) during first glycosylation step, a conserved glutamate residue acts as nucleophile and attacks on the glycosidic bonds or cellobiose and other oligosaccharides formed by the hydrolytic action of other enzyme of cellulase system. This results into the formation of an enzyme-substrate intermediate complex, (2) during second step called deglycosylation, an another conserved glutamate residue activates a water molecule present in the proximity by general acid/base catalyst reaction and now this activated water molecule acts on the intermediate complex to release the free glucose residue
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Fig2: Proposed “retaining” mechanism for hydrolysis of β-glycosidic bond by β-glucosidase: (1) during first glycosylation step, a conserved glutamate residue acts as nucleophile and attacks on the glycosidic bonds or cellobiose and other oligosaccharides formed by the hydrolytic action of other enzyme of cellulase system. This results into the formation of an enzyme-substrate intermediate complex, (2) during second step called deglycosylation, an another conserved glutamate residue activates a water molecule present in the proximity by general acid/base catalyst reaction and now this activated water molecule acts on the intermediate complex to release the free glucose residue

Mentions: For elucidating the catalytic mechanism of the enzyme and the active site topology, several techniques such as pH-dependence, inhibition, isotopic effect, and structure–reactivity studies (Kempton and Withers 1992), essential amino acid labeling with fluorosugars (Withers et al. 1992), reactions with deoxy substrate analogues (Street et al. 1992), and site-directed mutagenesis (Wang et al. 1995) have been used. β-Glucosidases cleave β-glycosidic bonds between two or more carbohydrates, or between a carbohydrate and a non-carbohydrate moiety (http://www.cazy.org/Glycoside-Hydrolases.html). Most β-glucosidases that have been characterized (i.e., GH1, GH3 and GH30 family enzymes) are retaining enzymes, and they perform catalysis in two steps, glycosylation and deglycosylation. Their catalytic mechanism is described diagrammatically in Fig. 2. Glutamate is the key active site residue and conserved among all reported β-glucosidases (Davies and Henrissat 1995; Wang et al. 1995). Two glutamate residues performed the overall catalytic reaction of β-glucosidase and one of them acts as a nucleophile (conserved as ‘I/VTENG’ motif) and the second residue works as a general acid/base catalyst (conserved as a ‘TFNEP’ motif) (Davies and Henrissat 1995). In the initial (glycosylation) step, glutamate, which acts as nucleophile, undergoes a nucleophilic attack on the anomeric carbon and results in a glucose–enzyme intermediate product. In the second (deglycosylation) step, a water molecule, which is activated by acid/base catalyst glutamate residue, acts as a nucleophile and breaks the glycosidic bond to release glucose (Litzinger et al. 2010). The formation of the covalent intermediate was first demonstrated with the GH1 β-glucosidase from Agrobacterium sp. by covalent labeling with 2, 4-dinitrophenyl-2-deoxy-2-fluoroglucoside (Withers et al. 1987, 1990). Tribolo et al. (2007) have reported X-ray crystallographic structure of human cytosolic β-glucosidase with the pocket shaped active site containing two glutamate residues and formed by large surface loops, surrounding the C termini of the barrel of β-strands. Out of two catalytic glutamate residues, the acid/base catalyzing residue was located on strand 4 while the nucleophilic residue was located on strand 7.Fig. 2


Catalytic properties, functional attributes and industrial applications of β -glucosidases
Proposed “retaining” mechanism for hydrolysis of β-glycosidic bond by β-glucosidase: (1) during first glycosylation step, a conserved glutamate residue acts as nucleophile and attacks on the glycosidic bonds or cellobiose and other oligosaccharides formed by the hydrolytic action of other enzyme of cellulase system. This results into the formation of an enzyme-substrate intermediate complex, (2) during second step called deglycosylation, an another conserved glutamate residue activates a water molecule present in the proximity by general acid/base catalyst reaction and now this activated water molecule acts on the intermediate complex to release the free glucose residue
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4697909&req=5

Fig2: Proposed “retaining” mechanism for hydrolysis of β-glycosidic bond by β-glucosidase: (1) during first glycosylation step, a conserved glutamate residue acts as nucleophile and attacks on the glycosidic bonds or cellobiose and other oligosaccharides formed by the hydrolytic action of other enzyme of cellulase system. This results into the formation of an enzyme-substrate intermediate complex, (2) during second step called deglycosylation, an another conserved glutamate residue activates a water molecule present in the proximity by general acid/base catalyst reaction and now this activated water molecule acts on the intermediate complex to release the free glucose residue
Mentions: For elucidating the catalytic mechanism of the enzyme and the active site topology, several techniques such as pH-dependence, inhibition, isotopic effect, and structure–reactivity studies (Kempton and Withers 1992), essential amino acid labeling with fluorosugars (Withers et al. 1992), reactions with deoxy substrate analogues (Street et al. 1992), and site-directed mutagenesis (Wang et al. 1995) have been used. β-Glucosidases cleave β-glycosidic bonds between two or more carbohydrates, or between a carbohydrate and a non-carbohydrate moiety (http://www.cazy.org/Glycoside-Hydrolases.html). Most β-glucosidases that have been characterized (i.e., GH1, GH3 and GH30 family enzymes) are retaining enzymes, and they perform catalysis in two steps, glycosylation and deglycosylation. Their catalytic mechanism is described diagrammatically in Fig. 2. Glutamate is the key active site residue and conserved among all reported β-glucosidases (Davies and Henrissat 1995; Wang et al. 1995). Two glutamate residues performed the overall catalytic reaction of β-glucosidase and one of them acts as a nucleophile (conserved as ‘I/VTENG’ motif) and the second residue works as a general acid/base catalyst (conserved as a ‘TFNEP’ motif) (Davies and Henrissat 1995). In the initial (glycosylation) step, glutamate, which acts as nucleophile, undergoes a nucleophilic attack on the anomeric carbon and results in a glucose–enzyme intermediate product. In the second (deglycosylation) step, a water molecule, which is activated by acid/base catalyst glutamate residue, acts as a nucleophile and breaks the glycosidic bond to release glucose (Litzinger et al. 2010). The formation of the covalent intermediate was first demonstrated with the GH1 β-glucosidase from Agrobacterium sp. by covalent labeling with 2, 4-dinitrophenyl-2-deoxy-2-fluoroglucoside (Withers et al. 1987, 1990). Tribolo et al. (2007) have reported X-ray crystallographic structure of human cytosolic β-glucosidase with the pocket shaped active site containing two glutamate residues and formed by large surface loops, surrounding the C termini of the barrel of β-strands. Out of two catalytic glutamate residues, the acid/base catalyzing residue was located on strand 4 while the nucleophilic residue was located on strand 7.Fig. 2

View Article: PubMed Central - PubMed

ABSTRACT

β-Glucosidases are diverse group of enzymes with great functional importance to biological systems. These are grouped in multiple glycoside hydrolase families based on their catalytic and sequence characteristics. Most studies carried out on β-glucosidases are focused on their industrial applications rather than their endogenous function in the target organisms. β-Glucosidases performed many functions in bacteria as they are components of large complexes called cellulosomes and are responsible for the hydrolysis of short chain oligosaccharides and cellobiose. In plants, β-glucosidases are involved in processes like formation of required intermediates for cell wall lignification, degradation of endosperm’s cell wall during germination and in plant defense against biotic stresses. Mammalian β-glucosidases are thought to play roles in metabolism of glycolipids and dietary glucosides, and signaling functions. These enzymes have diverse biotechnological applications in food, surfactant, biofuel, and agricultural industries. The search for novel and improved β-glucosidase is still continued to fulfills demand of an industrially suitable enzyme. In this review, a comprehensive overview on detailed functional roles of β-glucosidases in different organisms, their industrial applications, and recent cloning and expression studies with biochemical characterization of such enzymes is presented for the better understanding and efficient use of diverse β-glucosidases.

No MeSH data available.