CAZypedia needs your help!
We have many unassigned pages in need of Authors and Responsible Curators. See a page that's out-of-date and just needs a touch-up? - You are also welcome to become a CAZypedian. Here's how.
Scientists at all career stages, including students, are welcome to contribute.
Learn more about CAZypedia's misson here and in this article.
Totally new to the CAZy classification? Read this first.
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 | GH-x |
| Mechanism | retaining/inverting |
| Active site residues | known/not 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). 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 terminal. NgaP, N-acetylgalactosaminidase from Paenibacillus sp., is the first cloned β-N-acetylgalactosaminidase and its primary structure is not similar to any glycohydrolases 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 of NgaP functions as a proton donor. Point mutation analysis confirmed that Glu608 is 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 functions as a proton donor 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