Glycoside Hydrolase Family 25

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: ^^^Ed Taylor^^^
• Responsible Curator: ^^^Gideon Davies^^^

---

Glycoside Hydrolase Family GHnn
Clan	GH-K
Mechanism	likely to be via an oxazoline intermediate
Active site residues	Asp/Gul
CAZy DB link
http://www.cazy.org/fam/GHnn.html

Substrate specificities

Family GH25 lysozymes otherwise known as Chalaropsis (CH) type of lysozymes (from is initial characterisation from Chalaropsis species of fungus[1]) cleaves the β-1,4-glycosidic bond between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in the carbohydrate backbone of bacterial peptidoglycan. The characterized lysozymes from this family exhibit both β-1,4-N-acetyl- and β-1,4- N ,6-O-diacetylmuramidase activities and are able to degrade O-acetylated peptidoglycan present in Staphylococcus aureus and other pathogens[2]. The activity of GH25 enzymes appears to fulfil two main biological roles in bacteria. These roles are the re-modelling of peptidoglycan in cellular process such as division and promoting the dissemination of phage progeny toward the end of the phage lytic cycle, which is achieved by bacterial cell lysis. For this reason many GH25 proteins are found to be either chromosomal, phage or prophage encoded. The majority of the GH25 family is comprised of bacterial or prokaryotic viral (phage) members. There are however a few eukaryotic representatives, but these are so far restricted to the fungal kingdom. The roles of these fungal enzymes are less clear, many possess signal secretion peptides indicating a likely extracellular location, possibly for the purpose of obtaining or gaining assess to nutrients or even as a selective agent against bacteria.

Kinetics and Mechanism

The lack of activity on chitooligosaccharides and complexity in producing defined synthetic peptidoglycan substrates, has inevitably hampered the determination of kinetic parameters. Structural studies have shown the active-centre is extremely similar to those from GH18, 20, 56, 84 and 85 implying that, in the absence of evidence to the contrary, GH25 enzymes act with net retention of anomeric configuration using a neighboring-group catalytic mechanism common to this “super-family” of enzymes[3].

Catalytic Residues

This super-family has been shown to use an active site DE or DXE sequence motif ( or exceptionally in the case of GH85 a DXN moitif[4],[5] ). This carboxylate pair promotes a double displacement mechanism in which the nucleophile is not enzyme-derived but is provided by the substrate in an intramolecular “neighboring group” attack of the N-acetyl carbonyl group[6]. Thus catalysis occurs via the formation, and subsequent breakdown of a covalent oxazoline intermediate.

Three-dimensional structures

So far four members of this family of enzymes have been structural characterized, that of the Streptomyces coelicolor enzyme “cellosyl”[7], the bacteriophage lysine PlyB[8], and Clp-1lysozyme from a Streptococcus pneumoniae phage [9],[10], whose structure was also obtained in complex with fragments of peptidoglycan analogues 10 and BaGh25c from Bacillus anthracis str. Ames[3]. The GH25 lysozymes, are structurally unrelated to the GH22-24, 73 and 108 lysozyme folds and instead these enzymes display a modified α -barrel-like fold that, like the classical “TIM-barrel” is composed of a eight-stranded β-barrel, but which is flanked by just three (as opposed to the normal eight) α-helices[7].There is a prominent, long groove, very likely the substrate binding site, located on the C-terminal face. This groove culminates in a deep hole of a highly negative electrostatic potential forming the catalytic site[11].

Family Firsts

First sterochemistry determination

Has yet to be experimentally determined.

First catalytic nucleophile identification

This remains to be experimentally proven, however it is likely that the nucleophile is not enzyme-derived, but is provided by the substrate in a “neighboring group" participation reaction[3].

First general acid/base residue identification

Has yet to be fully experimentally determined.

First 3-D structure

The Streptomyces coelicolor enzyme “cellosyl" [7] was the first GH25 structure to be described in 2001.

References

Error fetching PMID 12325376:
Error fetching PMID 18070068:
Error fetching PMID 19595298:
Error fetching PMID 19327363:
Error fetching PMID 19181667:
Error fetching PMID 18558099:
Error fetching PMID 11427528:
Error fetching PMID 17182056:
Error fetching PMID 14527392:
Error fetching PMID 17581815:
Error fetching PMID 18266855: