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Catalytic Site Atlas Version 2.2.12
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CSA entry for 1un1
Original Entry
Title:
Transferase
Compound:
Xyloglucan endotransglycosylase
Mutant:
No
UniProt/Swiss-Prot:
Q8GZD5-Q8GZD5
EC Class:
2.4.1.207
Other CSA Entries:
Overview of all sites for 1un1
Homologues of 1un1
Entries for UniProt/Swiss-Prot: Q8GZD5
Entries for EC: 2.4.1.207
Other Databases:
PDB entry: 1un1
PDBsum entry: 1un1
UniProt/Swiss-Prot: Q8GZD5
IntEnz entry: 2.4.1.207
Literature Report:
Introduction:
Xyloglucan Endotransglycosylase (XET) is sourced from Populus tremula. It cleaves and religates xyloglucan (a soluble hemicellulose with a backbone composed of beta(1->4)-linked glucose residues similar to cellulose) in plant cell walls via a transglycosylation mechanism. XET is therefore a key enzyme in plant cell processes requiring cell wall remodelling, such as germination, growth, fruit ripening, organ abscission and vascular differentiation. Specifically, XET breaks a beta-(1->4) bond in the backbone of a xyloglucan and transfers the xyloglucanyl segment onto the O-4 of the non-reducing terminal glucose residue of an acceptor. This acceptor can be a xyloglucan or an oligosaccharide of xyloglucan. During processes such as fruit ripening, hydrolytic enzymes can be produced to degrade the cell wall. After cell wall expansion and elongation, cell wall loosening is a temporary requirement that must be followed by reinforcement of the cell wall. In the first step XET catalyses endolytic cleavage of a crosslinking xyloglucan polymer, which permits cellulose microfibres to separate allowing cell expansion. In the second step XET transfers the newly generated end to another sugar polymer, restoring stable cell wall structure. XET requires sugar residues in both the donor and acceptor sites before it is able to catalyse the reaction.

Mechanism:
1. Sugar residues bind to the acceptor and donor sites, aiding orientation of the glycosidic bond relative to the catalytic residues. Glu 85 performs nucleophilic attack upon the anomeric carbon of the sugar ring. Asp 87 stabilises Glu 85 through hydrogen bonding. Glu 89 facilitates fission of the glycosidic bond by general acid catalysis. The nascent nonreducing end diffuses away from the hydrolytically stable covalent glycosyl-enzyme intermediate.
2. The acceptor carbohydrate is deprotonated by Glu 89. This activates the acceptor carbohydrate, causing it to perform nucleophilic attack upon the anomeric carbon. This gives retention of configuration.

Sites:

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Found by:
Literature reference 
PsiBLAST alignment on 2ayh

ResidueChainNumberUniProt numberFunctional part FunctionTargetDescription
GLUA 85 107Sidechain
NucleophileSubstrate
Glu 85 performs nucleophilic attack upon the anomeric carbon of the sugar ring.
Evidence from paper Evidence concerns Evidence type
PubMed ID 15020748 Current protein Structural similarity to homologue of known mechanism
PubMed ID 15020748 Current protein Residue is positioned appropriately (ligand position known)
PubMed ID 15020748 Current protein Conservation of residue
PubMed ID 15020748 Current protein Mutagenesis of residue

ResidueChainNumberUniProt numberFunctional part FunctionTargetDescription
ASPA 87 109Sidechain
ElectrostaticResidue
Asp 87 stabilises Glu 85 through hydrogen bonding.
Evidence from paper Evidence concerns Evidence type
PubMed ID 15020748 Current protein Structural similarity to homologue of known mechanism
PubMed ID 15020748 Current protein Residue is positioned appropriately (ligand position known)
PubMed ID 15020748 Current protein Conservation of residue

ResidueChainNumberUniProt numberFunctional part FunctionTargetDescription
GLUA 89 111Sidechain
Acid/baseSubstrate
1. Glu 89 facilitates fission of the glycosidic bond by general acid catalysis. 2. Glu 89 activates the acceptor carbohydrate, causing it to perform nucleophilic attack upon the anomeric carbon.
Evidence from paper Evidence concerns Evidence type
PubMed ID 15020748 Current protein Structural similarity to homologue of known mechanism
PubMed ID 15020748 Current protein Residue is positioned appropriately (ligand position known)
PubMed ID 15020748 Current protein Conservation of residue
Notes:
It is suggested that His 83 may play a role in the XET mechanism, as it is conserved in some XET subfamilies. His 83 is thought to affect the nucleophilicity of Glu 85.

References:
1
Crystal structures of a poplar xyloglucan endotransglycosylase reveal details of transglycosylation acceptor binding.
P. Johansson and H. Brumer and M. J. Baumann and A. M. Kallas and H. Henriksson and S. E. Denman and T. T. Teeri and T. A. Jones
Plant Cell 16, (4) 874-86, (2004).
15020748
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