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CSA LITERATURE entry for 1ex1

E.C. nameglucan 1,3-beta-glucosidase
SpeciesHordeum vulgare (Barley)
E.C. Number (IntEnz)
CSA Homologues of 1ex1
CSA Entries With UniProtID
CSA Entries With EC Number
PDBe Entry 1ex1
PDBSum Entry 1ex1
MACiE Entry 1ex1

Literature Report

IntroductionBarley 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.
MechansimThe 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 Sites for 1ex1

Annotated By Reference To The Literature - Site 1 (Perform Site Search)
ResidueChainNumberUniProtKB NumberFunctional PartFunctionTargetDescription
GluA491516macie:sideChainGlu491 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.
AspA285310macie:sideChainAsp285 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.

Literature References

Notes:pH dependence data is in another article which does not have a PMID and is not on pubmed.
Varghese JN
Three-dimensional structure of a barley beta-D-glucan exohydrolase, a family 3 glycosyl hydrolase.
Structure 1999 7 179-190
PubMed: 10368285