Glycoside Hydrolase Family 123

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: ^^^Tomomi Sumida^^^
• Responsible Curator: ^^^Tomomi Sumida^^^

---

Glycoside Hydrolase Family GH123
Clan	none
Mechanism	probably retaining
Active site residues	known
CAZy DB link
https://www.cazy.org/GH123.html

Substrate specificities

The glycoside hydrolases family 123 contains β-N-acetylgalactosaminidases (EC 3.2.1.53) that degrade glycosphingolipids. These enzymes specifically hydrolyze the non-reducing terminal β-GalNAc linkage, but not β-GlcNAc linkage. The β-N-acetylgalactosaminidase (EC 3.2.1.53) is distinguished from β-hexosaminidase (EC 3.2.1.52) or β-N-acetylglucosaminidase (EC 3.2.1.52) because the β-N-acetylgalactosaminidase is specific to β-GalNAc linkage while β-N-acetylglucosaminidase is specific to β-GlcNAc linkage. β-Hexosaminidase hydrolyzes both β-GlcNAc and β-GalNAc linkages at non-reducing terminus. NgaP, N-acetylgalactosaminidase from Paenibacillus sp., is the first cloned β-N-acetylgalactosaminidase and its primary structure is not similar to any glycoside hydrolases reported so far [1], and, thus, this family is created. The recombinant NgaP hydrolyzes pNP-β-GalNAc but not pNP-β-GlcNAc, pNP-β-Gal, pNP-α-GalNAc or other pNP-glycosides, indicating that NgaP is a typical β-N-acetylgalactosaminidase.

Kinetics and Mechanism

Glycoside hydrolases belonging to GH18, GH20 and GH85 cleave the sugar residues containing C2-acetamide group such as β-GlcNAc and β-GalNAc through substrate-assisted catalysis involving neighboring group participation. Since NgaP hydrolyzes the β-GalNAc linkage, NgaP is proposed to use substrate-assisted catalysis. A comparison of secondary structure of NgaP with that of other enzymes that utilize substrate-assisted catalysis suggested that Glu608 and Asp607 of NgaP functions as a proton donor and a stabilizer of the 2-acetamide group of the β-GalNAc at the active site. Point mutation analysis confirmed that Glu608 and Asp607 are integral for the activity of NgaP. GalNAc-thiazoline, a structural analog of the oxazolinium intermediate of neighboring group participation, was found to competitively inhibit the activity of NgaP. These results indicate that NgaP hydrolyzes the terminal β-GalNAc linkage through substrate-assisted catalysis.

Catalytic Residues

Point mutation analysis suggested that Glu608 and Asp607 functions as a proton donor and a stabilizer of the 2-acetamide group of the substrate in NgaP.

Three-dimensional structures

Unknown

Family Firsts

First stereochemistry determination

First catalytic nucleophile identification

The carbonyl oxygen of the C-2 acetamide group of the substrate behaves as a catalytic nucleophile.

First general acid/base residue identification

Site-directed mutagenesis indicated that Glu608 is an essential amino acid for the catalytic reaction in NgaP.

First 3-D structure

Not known

References

1. Sumida T, Fujimoto K, and Ito M. (2011). Molecular cloning and catalytic mechanism of a novel glycosphingolipid-degrading beta-N-acetylgalactosaminidase from Paenibacillus sp. TS12. J Biol Chem. 2011;286(16):14065-72. DOI:10.1074/jbc.M110.182592 |