Glucan 1,3-beta-glucosidase
Barley beta-D-glucan exohydrolasesbelong to family 3 of glucosyl hydrolases. ExoI is one of two beta-D-glucan exohydrolase isoenzymes that hydrolyse a broad range of substrates with (1-2)-, (1-3)-, (1-4)-, and (1-6)-beta-D-glucosidic linkages. The enzyme removes single glucose units from the non-reducing termini of polymeric and oligomeric substrates with the retention of anomeric configuration. The selective hydrolysis of glycosidic bonds is crucial for key processes of plant development and growth, such as cell wall expansion and degradation, and turnover of signalling molecules.
Reference Protein and Structure
- Sequence
-
Q9XEI3
(Sequence Homologues)
(PDB Homologues)
- Biological species
-
Hordeum vulgare subsp. vulgare (Domesticated barley)
- PDB
-
1ex1
- BETA-D-GLUCAN EXOHYDROLASE FROM BARLEY
(2.2 Å)
- Catalytic CATH Domains
-
3.40.50.1700
3.20.20.300
(see all for 1ex1)
Enzyme Reaction (EC:3.2.1.58)
Enzyme Mechanism
Introduction
The reaction follows a double displacement mechanism at the anomeric chiral centre. Hydrolysis of the glycosidic linkage is believed to be initiated by protonation of the glycosidic oxygen atom via proton transfer from an active site-located general acid catalyst (Glu491) to form an oxonium ion. Following protonation, the aglycone part of the substrate is released from the enzyme surface and the reducing terminal residue is converted to a positively charged oxycarbenium ion. The carbocation is stabilised by a nucleophilic amino acid residue (Asp285) acting as a base to form a covalent glucosyl-enzyme intermediate. Glu491, acting as a base catalyst by proton abstraction, activates a water molecule which hydrolyses the covalent intermediate, resulting in the regeneration of the enzyme and the formation of the glucose product. The glucose remains bound to the enzyme until a new substrate approaches the enzyme.
Catalytic Residues Roles
| UniProt | PDB* (1ex1) | ||
| Asp310 | Asp285A | Asp285 is called a catalytic nucleophile, but acts as a base to form a covalent glucosyl-enzyme intermediate.It may also be involved in stabilising the carbocation transition state. | proton shuttle (general acid/base), electrostatic stabiliser |
| Glu516 | Glu491A | Glu491 acts as a general acid/base catalyst. It protonates the substrate to initiate the hydrolysis mechanism. As a base, it activates water as a nucleophile to hydrolyse the covalent glycosyl-enzyme intermediate. | proton shuttle (general acid/base) |
Chemical Components
References
- Hrmova M et al. (2001), Structure, 9, 1005-1016. Catalytic Mechanisms and Reaction Intermediates along the Hydrolytic Pathway of a Plant β-D-glucan Glucohydrolase. DOI:10.1016/s0969-2126(01)00673-6. PMID:11709165.
- Thongpoo P et al. (2013), Biochim Biophys Acta, 1830, 2739-2749. Identification of the acid/base catalyst of a glycoside hydrolase family 3 (GH3) beta-glucosidase from Aspergillus niger ASKU28. DOI:10.1016/j.bbagen.2012.11.014. PMID:23201198.
- Varghese JN et al. (1999), Structure, 7, 179-190. Three-dimensional structure of a barley β-D-glucan exohydrolase, a family 3 glycosyl hydrolase. DOI:10.1016/s0969-2126(99)80024-0. PMID:10368285.
Catalytic Residues Roles
| Residue | Roles |
|---|---|
| Glu491A | proton shuttle (general acid/base) |
| Asp285A | electrostatic stabiliser, proton shuttle (general acid/base) |