Acetylxylan esterase

 

Enzymes which deacetylate the main component of plane cell wall hemicelluloses (xylan). Xylan is composed of a linear chain of beta(1-4)glycosidic-linked xylopyranoses, having various substitutions at carbon 2 and 3 positions (including acetyl groups). Biodegradation of xylan is a complex process catalysed by several fungal and bacterial enzymes. Although the linear chain is cleaved by endoxylanases and beta-xylosidases, acetylxylan esterases (AXE)1 hydrolyse O-acetyl substitutions of-xylopyranose moieties.

 

Reference Protein and Structure

Sequence
O59893 UniProt (3.1.1.72) IPR000675 (Sequence Homologues) (PDB Homologues)
Biological species
Talaromyces purpureogenus (Soft rot fungus) Uniprot
PDB
1bs9 - ACETYLXYLAN ESTERASE FROM P. PURPUROGENUM REFINED AT 1.10 ANGSTROMS (1.1 Å) PDBe PDBsum 1bs9
Catalytic CATH Domains
3.40.50.1820 CATHdb (see all for 1bs9)
Click To Show Structure

Enzyme Reaction (EC:3.1.1.72)

acetyl xylan
CHEBI:134431ChEBI
+
water
CHEBI:15377ChEBI
→
(1->4)-beta-D-xylan
CHEBI:15447ChEBI
+
acetic acid
CHEBI:15366ChEBI

Enzyme Mechanism

Introduction

It is proposed that the deacetylation reaction occurs via the classical Ser-His-Asp nucleophilic substitution reaction mechanism in which the catalytic serine (Ser90) forms a covalent aduct with the substrate. The oxyanion hole is composed of the main chain amide and side chain of Thr13.

Catalytic Residues Roles

UniProt PDB* (1bs9)
Ser117 Ser90A The catalytic nucleophile of the Ser-His-Asp triad. covalent catalysis, proton shuttle (general acid/base)
Thr40, Thr40 (main-N) Thr13A, Thr13A (main-N) Forms part of the oxyanion hole. electrostatic stabiliser
Asp202 Asp175A Activates the catalytic histidine of the Ser-His-Asp triad. modifies pKa
Gln118 Gln91A Helps orientate the oxyanion hole forming threonine (Thr13) residue. steric role
His214 His187A Acts as the general acid/base that activates the serine in the Ser-His-Asp catalytic triad. proton shuttle (general acid/base)
*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. Hakulinen N et al. (2000), J Struct Biol, 132, 180-190. Three-Dimensional Structure of the Catalytic Core of Acetylxylan Esterase from Trichoderma reesei: Insights into the Deacetylation Mechanism. DOI:10.1006/jsbi.2000.4318. PMID:11243887.
  2. Ghosh D et al. (2001), J Biol Chem, 276, 11159-11166. Multiple Conformations of Catalytic Serine and Histidine in Acetylxylan Esterase at 0.90 A. DOI:10.1074/jbc.m008831200. PMID:11134051.
  3. Ghosh D et al. (1999), Acta Crystallogr D Biol Crystallogr, 55, 779-784. Determination of a protein structure by iodination: the structure of iodinated acetylxylan esterase. DOI:10.1107/s0907444999000244. PMID:10089308.

Step 1.

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Catalytic Residues Roles

Residue Roles
Ser90A proton shuttle (general acid/base), covalent catalysis
His187A proton shuttle (general acid/base)
Asp175A modifies pKa
Thr13A electrostatic stabiliser
Thr13A (main-N) electrostatic stabiliser
Gln91A steric role

Chemical Components

Step 1.

Contributors

Gemma L. Holliday, Atlanta Cook, Craig Porter