Isomaltulose synthase

 

Isomaltulose synthase (PalI), also known as sucrose isomerase, catalyses the isomerisation of sucrose to produce isomaltulose (alpha-D-glucosylpyranosyl-1,6-D-fructofuranose) and trehalulose (alpha-D-glucosylpyranosyl-1,1-D-fructofuranose) as the main products with residual amounts of glucose and fructose isomaltulose and trehalulose, two functional isomers of sucrose, have been suggested as non-cariogenic alternatives to sucrose and are widely used in health products and the food industry.

 

Reference Protein and Structure

Sequence
Q8KR84 UniProt IPR006047 (Sequence Homologues) (PDB Homologues)
Biological species
Klebsiella sp. LX3 (Bacteria) Uniprot
PDB
1m53 - CRYSTAL STRUCTURE OF ISOMALTULOSE SYNTHASE (PALI) FROM KLEBSIELLA SP. LX3 (2.2 Å) PDBe PDBsum 1m53
Catalytic CATH Domains
3.20.20.80 CATHdb (see all for 1m53)
Click To Show Structure

Enzyme Reaction (EC:5.4.99.11)

sucrose
CHEBI:17992ChEBI
→
6-O-alpha-D-glucopyranosyl-D-fructofuranose
CHEBI:18394ChEBI
Alternative enzyme names: Isomaltulose synthetase, Sucrose alpha-glucosyltransferase, Trehalulose synthase,

Enzyme Mechanism

Introduction

Glu295 protonates the oxygen atom of the glycosidic link, in concert with the nucleophile Asp241 attacking at C1, yielding a molecule of fructose and a glucose-enzyme complex. Sucrose then attacks at C1 with the C6 hydroxyl group, displacing Asp241 to form Isomatulose.

Catalytic Residues Roles

UniProt PDB* (1m53)
Glu295 Glu295(267)A Acts as a general acid/base. proton donor
Asp241 Asp241(213)A Forms a covalent linkage with the sugar. 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

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

References

  1. Zhang D et al. (2003), J Biol Chem, 278, 35428-35434. Isomaltulose Synthase (PalI) of Klebsiella sp. LX3: CRYSTAL STRUCTURE AND IMPLICATION OF MECHANISM. DOI:10.1074/jbc.m302616200. PMID:12819210.

Step 1. There is nucleophilic attack from Asp 241 on the C1 atom of the glucose component, this is in concert with oxygen of the glycosidic link being protonated by Glu 295. This results in the link being broken in an SN2 reaction and an enzyme bound intermediate being formed.

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Glu295(267)A proton donor
Asp241(213)A nucleophile

Chemical Components

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

Step 2. There is a second SN2 reaction where C6 hydroxyl of the fructose attacks C1 of the Asp-glucose intermediate. This results in a new glycosidic linkage and the formation of Isomaltulose.

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Asp241(213)A nucleofuge

Chemical Components

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

Step 1. There is nucleophilic attack from Asp 241 on the C1 atom of the glucose component, this is in concert with oxygen of the glycosidic link being protonated by Glu 295. This results in the link being broken in an SN2 reaction and an enzyme bound intermediate being formed.

Step 2. There is a second SN2 reaction where C6 hydroxyl of the fructose attacks C1 of the Asp-glucose intermediate. This results in a new glycosidic linkage and the formation of Isomaltulose.

Products.

Contributors

Anna Waters, Craig Porter, Gemma L. Holliday, James Willey