Glycoside Hydrolase Family 68

This page is currently under construction. This means that the Responsible Curator has deemed that the page's content is not quite up to CAZypedia's standards for full public consumption. All information should be considered to be under revision and may be subject to major changes.

• Author: ^^^Tirso Pons^^^ and ^^^Wim Van den Ende^^^
• Responsible Curator: ^^^Wim Van den Ende^^^

---

Glycoside Hydrolase Family GH68
Clan	GH-J
Mechanism	retaining
Active site residues	known
CAZy DB link
http://www.cazy.org/fam/GH68.html

Substrate specificities

Glycoside hydrolase family GH68 contains enzymes that hydrolyze fructose containing polysaccharides such as levansucrase (sucrose:2,6-β-D-fructan 6-β-D-fructosyltransferase; EC 2.4.1.10); β-fructofuranosidase (EC 3.2.1.26); and inulosucrase (EC 2.4.1.9)

Kinetics and Mechanism

Family GH68 enzymes are retaining enzymes, as first shown by Koshland and Stein by performing the reaction in ¹⁸O-labeled water and determining the ¹⁸O content of the products [1]. The levansucrases from Bacillus subtilis, Gluconacetobacter diazotrophicus, and Streptococcus salivarius follows a ping-pong mechanism [2, 3, 4, 5]. At low sucrose concentrations levansucrase functions as a hydrolase with water as acceptor, whereas at higher substrate concentrations it adds fructosyl units to a growing levan chain [2].

Catalytic Residues

Retaining glycosidases catalyze hydrolysis in two steps involving a covalent glycosyl enzyme intermediate. The two invariant residues, responsible for the catalytic reaction in family GH68 enzymes, have first been identified experimentally in bacterial levansucrases as an aspartate located close to the N-terminus acting as the catalytic nucleophile and a glutamate acting as the general acid/base [6, 7]. In addition, a conserved aspartate residue in the "Arg-Asp-Pro (RDP) motif" stabilize the transition state [5, 7, 8].

Three-dimensional structures

Currently, only two different three dimensional structures of family GH68 enzymes have been solved so far. The first crystal structure was reported for the bacterial levansucrase (SacB) from Bacillus subtilis subsp. subtilis str. 168 [6]. The second one corresponds to levansucrase (LdsA) from Gluconacetobacter diazotrophicus SRT4 [9].

Family Firsts

First stereochemistry determination

Bacillus subtilis levansucrase [2].

First catalytic nucleophile identification

Bacillus subtilis levansucrase [6].

First general acid/base residue identification

Zymomonas mobilis levansucrase [7].

First stabilizing transition-state residue identification

Gluconacetobacter diazotrophicus levansucrase [8].

First prediction of a common beta-propeller catalytic domain in GH68 / clan GH-J

Gluconacetobacter diazotrophicus levansucrase [10, 11].

First 3-D structure

Bacillus subtilis levansucrase [6].

References

Error fetching PMID 13174523:
Error fetching PMID 4206083:
Error fetching PMID 814002:
Error fetching PMID 7619044:
Error fetching PMID 10393084:
Error fetching PMID 14517548:
Error fetching PMID 12359071:
Error fetching PMID 9895294:
Error fetching PMID 15869470:
Error fetching PMID 9829697:
Error fetching PMID 11305239: