Cellulase (GH12)

 

The glycosyl hydrolase family 12 member CelB is able to catalyse the breakdown of cellulose into glucose with retention of anomeric configuration. As a result, it is part of the endoglucanase clan GH-C which also includes the family 11 Xylanases, with a common fold and catalytic mechanism defining the clan.

 

Reference Protein and Structure

Sequence
Q54331 UniProt IPR002594 (Sequence Homologues) (PDB Homologues)
Biological species
Streptomyces lividans (Bacteria) Uniprot
PDB
2nlr - STREPTOMYCES LIVIDANS ENDOGLUCANASE (EC: 3.2.1.4) COMPLEX WITH MODIFIED GLUCOSE TRIMER (1.2 Å) PDBe PDBsum 2nlr
Catalytic CATH Domains
2.60.120.180 CATHdb (see all for 2nlr)
Click To Show Structure

Enzyme Reaction (EC:3.2.1.4)

water
CHEBI:15377ChEBI
+
cellotetraose
CHEBI:62974ChEBI
→
cellobiose
CHEBI:17057ChEBI
Alternative enzyme names: 9.5 cellulase, Beta-1,4-endoglucan hydrolase, Beta-1,4-glucanase, Alkali cellulase, Avicelase, Celluase A, Celludextrinase, Cellulase A 3, Cellulosin AP, Endo-1,4-beta-D-glucanase, Endoglucanase D, Pancellase SS, Endo-1,4-beta-glucanase, Carboxymethyl cellulase, Endoglucanase, Endo-1,4-beta-D-glucanohydrolase, 1,4-(1,3;1,4)-beta-D-glucan 4-glucanohydrolase,

Enzyme Mechanism

Introduction

The reaction proceeds via a double displacement mechanism. Initial attack of the nucleophilic residue Glu 120 on the anomeric carbon of the beta-1-4-glycosidic bond results in cleavage of the bond, assisted by protonation of the leaving group at the 1C OH by Glu 203. This leads to a covalently bound enzyme intermediate, hydrolysis of which is achieved by a water molecule activated by deprotonation by Glu 203, releasing the products and regenerating the catalytically active forms of the nucleophilic and acid-base glutamates.

Catalytic Residues Roles

UniProt PDB* (2nlr)
Glu243 Glu203A Protonates the leaving group in the first stage and activates the water for hydrolysis in the second stage. proton acceptor, proton donor, activator, increase nucleophilicity, promote heterolysis
Glu160 Glu120A Acts as nucleophile to attack the anomeric carbon resulting in the formation of a covalently bound enzyme intermediate. covalently attached, nucleofuge, nucleophile
*PDB label guide - RESx(y)B(C) - RES: Residue Name; x: Residue ID in PDB file; y: Residue ID in PDB sequence if different from PDB file; B: PDB Chain; C: Biological Assembly Chain if different from PDB. If label is "Not Found" it means this residue is not found in the reference PDB.

Chemical Components

overall product formed, overall reactant used, bimolecular nucleophilic substitution, proton transfer, intermediate formation, hydrolysis, intermediate terminated

References

  1. Sulzenbacher G et al. (1999), Biochemistry, 38, 4826-4833. The Crystal Structure of a 2-Fluorocellotriosyl Complex of theStreptomyceslividansEndoglucanase CelB2 at 1.2 Å Resolution†,‡. DOI:10.1021/bi982648i. PMID:10200171.
  2. Crennell SJ et al. (2006), J Mol Biol, 356, 57-71. Dimerisation and an Increase in Active Site Aromatic Groups as Adaptations to High Temperatures: X-ray Solution Scattering and Substrate-bound Crystal Structures of Rhodothermus marinus Endoglucanase Cel12A. DOI:doi:10.1016/j.jmb.2005.11.004.

Step 1. Glu120 performs a nucleophilic attack on the anomeric carbon this causes the cleavage of the glycosidic bond assisted by Glu203 protonating the leaving group.

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Catalytic Residues Roles

Residue Roles
Glu120A covalently attached
Glu203A promote heterolysis, proton donor
Glu120A nucleophile

Chemical Components

overall product formed, overall reactant used, ingold: bimolecular nucleophilic substitution, proton transfer, intermediate formation

Step 2. Glu203 activates a water molecule which hydrolyses the intermediate.

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Glu203A activator, increase nucleophilicity, proton acceptor
Glu120A nucleofuge

Chemical Components

proton transfer, hydrolysis, overall product formed, ingold: bimolecular nucleophilic substitution, intermediate terminated
Download: Image, Marvin File

Step 1. Glu120 performs a nucleophilic attack on the anomeric carbon this causes the cleavage of the glycosidic bond assisted by Glu203 protonating the leaving group.

Step 2. Glu203 activates a water molecule which hydrolyses the intermediate.

Products.

Contributors

Peter Sarkies, Gemma L. Holliday, James Willey