Galactosylgalactosylxylosylprotein 3-beta-glucuronosyltransferase

 

Beta-1,3-glucuronyltransferase (GlcAT-I) is involved in the biosynthesis of heparin sulphate and chondroitin sulphate. It catalyses the transfer of glucuronic acid (GlcUA) from UDP-GlcUA onto the terminal galactose of the linker Gal-Gal-Xyl- that is attached to a serine side chain of a core protein. GlcAT-I is an inverting glycosyltransferse, converting the alpha linkage in the UDP-GlcUA molecule to a beta linkage in the product.

 

Reference Protein and Structure

Sequence
O94766 UniProt (2.4.1.135) IPR005027 (Sequence Homologues) (PDB Homologues)
Biological species
Homo sapiens (Human) Uniprot
PDB
1kws - CRYSTAL STRUCTURE OF BETA1,3-GLUCURONYLTRANSFERASE I IN COMPLEX WITH THE ACTIVE UDP-GLCUA DONOR (2.1 Å) PDBe PDBsum 1kws
Catalytic CATH Domains
3.90.550.10 CATHdb (see all for 1kws)
Cofactors
Manganese(2+) (1)
Click To Show Structure

Enzyme Reaction (EC:2.4.1.135)

O(3)-(beta-D-galactosyl-(1->3)-beta-D-galactosyl-(1->4)-beta-D-xylosyl)-L-serine residue
CHEBI:132090ChEBI
+
UDP-alpha-D-glucuronate(3-)
CHEBI:58052ChEBI
→
O(3)-(beta-D-glucuronosyl-(1->3)-beta-D-galactosyl-(1->3)-beta-D-galactosyl-(1->4)-beta-D-xylosyl)-L-serine(1-) residue
CHEBI:132093ChEBI
+
hydron
CHEBI:15378ChEBI
+
UDP(3-)
CHEBI:58223ChEBI
Alternative enzyme names: UDP-glucuronate:3-beta-D-galactosyl-4-beta-D-galactosyl-O-beta-D-xylosyl-protein D-glucuronosyltransferase, Glucuronosyltransferase I, Uridine diphosphate glucuronic acid:acceptor glucuronosyltransferase, UDP-glucuronate:3-beta-D-galactosyl-4-beta-D-galactosyl-O-beta-D-xylosylprotein D-glucuronosyltransferase,

Enzyme Mechanism

Introduction

The reaction is thought to proceed in an SN2 mechanism via an oxo-carbenium ion-like transition state. Accumulation of negative charge on the departing pyrophosphate moiety of the UDP leaving group is stabilised by a divalent metal ion, while the attacking C3 OH of the terminal galactose is deprotonated by Glu 281. The resulting products formed are UDP and GlcUA now attached to Gal-Gal-Xyl-Ser.

Catalytic Residues Roles

UniProt PDB* (1kws)
Asp196 Asp196(122)A Forms bidentate coordination with the Mn2+ ion, which in turn stabilises the phosphate leaving group (apart of UDP). metal ligand
Glu281 Glu281(207)A Deprotonates the C3 hydroxyl of the terminal galactose moiety that attacks C1 of the UDP-GlcUA molecule. proton acceptor, proton donor
*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

proton transfer, bimolecular nucleophilic substitution, overall reactant used, overall product formed, native state of enzyme regenerated, inferred reaction step

References

  1. Pedersen LC et al. (2002), J Biol Chem, 277, 21869-21873. Crystal Structure of beta 1,3-Glucuronyltransferase I in Complex with Active Donor Substrate UDP-GlcUA. DOI:10.1074/jbc.m112343200. PMID:11950836.
  2. Lairson LL et al. (2008), Annu Rev Biochem, 77, 521-555. Glycosyltransferases: structures, functions, and mechanisms. DOI:10.1146/annurev.biochem.76.061005.092322. PMID:18518825.
  3. Kozmon S et al. (2006), J Am Chem Soc, 128, 16921-16927. Catalytic mechanism of glycosyltransferases: hybrid quantum mechanical/molecular mechanical study of the inverting N-acetylglucosaminyltransferase I. DOI:10.1021/ja065944o. PMID:17177443.
  4. Pedersen LC et al. (2000), J Biol Chem, 275, 34580-34585. Heparan/chondroitin sulfate biosynthesis. Structure and mechanism of human glucuronyltransferase I. DOI:10.1074/jbc.M007399200. PMID:10946001.

Step 1. Glu281 deprotonates 3'OH of galactose group, increasing its nucleophilicity to then attack the C1 of GlcUA apart of UDP-GlcUA group.

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Asp196(122)A metal ligand
Glu281(207)A proton acceptor

Chemical Components

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

Step 2. Deprotonation of Glu281 by a water molecule.

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Asp196(122)A metal ligand
Glu281(207)A proton donor

Chemical Components

proton transfer, native state of enzyme regenerated, inferred reaction step
Download: Image, Marvin File

Step 1. Glu281 deprotonates 3'OH of galactose group, increasing its nucleophilicity to then attack the C1 of GlcUA apart of UDP-GlcUA group.

Step 2. Deprotonation of Glu281 by a water molecule.

Products.

Contributors

Steven Smith, Gemma L. Holliday, Morwenna Hall