M-CSA Mechanism and Catalytic Site Atlas

Glucansucrases (GTF-SI)

Glucotransferases are multidomain enzymes that catalyze the polymerization of glucans. They are found in Streptococcus mutants and are an essential agent in dental caries pathogenesis for which they have gained importance in the search for new treatments against infection.

Reference Protein and Structure

Sequence: P13470 (2.4.1.5) (Sequence Homologues) (PDB Homologues)
Biological species: Streptococcus mutans UA159 (Bacteria)
PDB: 3aie - Crystal Structure of glucansucrase from Streptococcus mutans (2.1 Å)
Catalytic CATH Domains: 3.20.20.470 3.20.20.80 (see all for 3aie)

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Enzyme Reaction (EC:2.4.1.125)

(1->6)-alpha-D-glucan

CHEBI:18269

sucrose

CHEBI:17992

→

D-fructofuranose

CHEBI:37721

(1->6)-alpha-D-glucan (n+1)

CHEBI:X00693X00693

Enzyme reaction links: IntEnz ENZYME ExplorEnz

Alternative enzyme names: GTF-S, Sucrose-1,6-alpha-glucan 3(6)-alpha-glucosyltransferase, Sucrose:1,6-, 1,3-alpha-D-glucan 3-alpha- and 6-alpha-D-glucosyltransferase, Sucrose:1,6-alpha-D-glucan 3-alpha- and 6-alpha-glucosyltransferase, Water-soluble-glucan synthase, GTF-I, GTF-SI, Sucrose:1,6-alpha-D-glucan 3(6)-alpha-D-glucosyltransferase, GtfB (gene name), GtfC (gene name), GtfD (gene name),

Enzyme Mechanism

Mechanism Proposal 1 ☆☆☆

Summary
Step 1
Step 2
Products
All Steps

Introduction

A concerted mechanism for sucrose hydrolysis in which the first event corresponds to the glyosidic bond breakage assisted by Glu515, followed by the nucleophilic attack of Asp477, leading to the formation of the Covalent Glycosyl Enzyme (CGE) intermediate which breaks down to give the products.

Catalytic Residues Roles

UniProt	PDB* (3aie)
Asp477	Asp477(234)A	Asp477 catalytic residue is the nucleophile of the reaction.	nucleofuge, nucleophile
Glu515	Glu515(272)A	Glu515 acts as a base catalyst.	proton acceptor, proton donor
Asp588	Asp588(345)A	Asp588 catalytic residue stabilizes the two oxocarbenium ion-like transition state (TS) of the reaction.	transition state 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

bimolecular nucleophilic substitution, rate-determining step, proton transfer, overall reactant used, overall product formed

References

Jaña GA et al. (2018), Org Biomol Chem, 16, 2438-2447. A QM/MM approach on the structural and stereoelectronic factors governing glycosylation by GTF-SI from Streptococcus mutans. DOI:10.1039/C8OB00284C. PMID:29557467.

Step 1. Asp477 mounts a nucleophilic attack to the anomeric carbon of the glucosyl moiety of sucrose. Glu515 protonates the glycosidic oxygen which leads to fructosyl leaving group.

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Catalytic Residues Roles

Residue	Roles
Asp477(234)A	nucleophile
Glu515(272)A	proton donor
Asp588(345)A	transition state stabiliser

Chemical Components

ingold: bimolecular nucleophilic substitution, rate-determining step, proton transfer, overall reactant used

Step 2. The leaving fructose from the previous step is exchanged with another sucrose. Glu515 deprotonates this sucrose which inturn attacks the anomeric carbon of the Asp-sucrose molecule, Asp477 departs.

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Catalytic Residues Roles

Residue	Roles
Glu515(272)A	proton acceptor
Asp588(345)A	transition state stabiliser
Asp477(234)A	nucleofuge

Chemical Components

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

Download: Image, Marvin File

Step 1. Asp477 mounts a nucleophilic attack to the anomeric carbon of the glucosyl moiety of sucrose. Glu515 protonates the glycosidic oxygen which leads to fructosyl leaving group.

Products.

Contributors

Yordanos Abeje, Antonio Ribeiro