Chitosanase

 

The enzyme chitosanase is an inverting glycoside hydrolase belonging to family 46. It catalyses the hydrolysis of chitosan, a linear polysaccharide of beta-(1,4)-linked D-glucosamine residues. Many species of soil bacteria, fungi and certain plants express chitosanases capable of degrading chitosan. The enzyme has been implicated in the resistance of E.Coli against the toxic effects of chitosan by breaking the high molecular weight polymer into short chain forms, which are non toxic to the bacteria.

 

Reference Protein and Structure

Sequence
P33665 UniProt (3.2.1.132) IPR000400 (Sequence Homologues) (PDB Homologues)
Biological species
Streptomyces sp. N174 (Bacteria) Uniprot
PDB
1chk - STREPTOMYCES N174 CHITOSANASE PH5.5 298K (2.4 Å) PDBe PDBsum 1chk
Catalytic CATH Domains
3.30.386.10 CATHdb 1.20.141.10 CATHdb (see all for 1chk)
Click To Show Structure

Enzyme Reaction (EC:3.2.1.132)

water
CHEBI:15377ChEBI
+
beta-D-glucosaminyl-(1->4)-beta-D-glucosamine
CHEBI:50677ChEBI
→
beta-D-glucosamine
CHEBI:28393ChEBI
+
beta-D-glucosamine
CHEBI:28393ChEBI

Enzyme Mechanism

Introduction

The reaction is thought to proceed through a single displacement mechanism involving an oxocarbenium ion transition state. Asp 40 acts as a general base to a structurally conserved water molecule which then attacks the substrate's anomeric carbon in an SN2 manner, with inversion of stereochemistry. Simultaneously, Glu22 acts as a general acid to the displaced polysaccharide.

Catalytic Residues Roles

UniProt PDB* (1chk)
Glu62 Glu22A The residue acts as a general acid towards the departing polysaccaride during the displacement reaction, and is thought to promote the attack by the hydrolytic water molecule through its electrostatic interaction with the bridging oxygen. proton shuttle (general acid/base), electrostatic stabiliser
Asp80 Asp40A The residue acts as a general base to the hydrolytic water, enhancing its nucleophilic character. proton shuttle (general acid/base)
Thr85 Thr45A The residue has been shown, by mutagenesis experiments and sequence homology with related enzymes to be important in directing the hydrolytic water towards the substrate anomeric carbon through hydrogen bond interactions. electrostatic stabiliser
*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

References

  1. Viens P et al. (2015), Mar Drugs, 13, 6566-6587. Chitosanases from Family 46 of Glycoside Hydrolases: From Proteins to Phenotypes. DOI:10.3390/md13116566. PMID:26516868.
  2. Lyu Q et al. (2015), Biochim Biophys Acta, 1850, 1953-1961. Structural and biochemical insights into the degradation mechanism of chitosan by chitosanase OU01. DOI:10.1016/j.bbagen.2015.06.011. PMID:26145578.
  3. Lacombe-Harvey ME et al. (2009), FEBS J, 276, 857-869. Accessory active site residues of Streptomyces sp. N174 chitosanase. DOI:10.1111/j.1742-4658.2008.06830.x. PMID:19143844.
  4. Fukamizo T et al. (1997), Biochem Cell Biol, 75, 687-696. Chitosanase from Streptomyces sp. strain N174: a comparative review of its structure and function. PMID:9599657.
  5. Marcotte EM et al. (1996), Nat Struct Biol, 3, 155-162. X-ray structure of an anti-fungal chitosanase from streptomyces N174. DOI:10.1038/nsb0296-155. PMID:8564542.

Step 1.

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Catalytic Residues Roles

Residue Roles
Asp40A proton shuttle (general acid/base)
Glu22A electrostatic stabiliser, proton shuttle (general acid/base)
Thr45A electrostatic stabiliser

Chemical Components

Step 1.

Contributors

Gemma L. Holliday, Nozomi Nagano, Craig Porter