D
IPR019799

Glycoside hydrolase family 22 domain

InterPro entry
Short nameGlyco_hydro_22_CS
Overlapping
homologous
superfamilies
 

Description

O-Glycosyl hydrolases (
3.2.1.
) are a widespread group of enzymes that hydrolyse the glycosidic bond between two or more carbohydrates, or between a carbohydrate and a non-carbohydrate moiety. A classification system for glycosyl hydrolases, based on sequence similarity, has led to the definition of 85 different families
[9, 3]
. This classification is available on the CAZy (CArbohydrate-Active EnZymes) website.

Glycoside hydrolase family 22
GH22
comprises enzymes with two known activities; lysozyme type C (
3.2.1.17
) and alpha-lactalbumins. Asp and/or the carbonyl oxygen of the C-2 acetamido group of the substrate acts as the catalytic nucleophile/base.

Alpha-lactalbumin
[7, 6]
is a milk protein that acts as the regulatory subunit of lactose synthetase, acting to promote the conversion of galactosyltransferase to lactose synthase, which is essential for milk production. In the mammary gland, alpha-lactalbumin changes the substrate specificity of galactosyltransferase from N-acetylglucosamine to glucose.

Lysozymes (
3.2.1.17
) act as bacteriolytic enzymes by hydrolyzing the beta(1->4) bonds between N-acetylglucosamine and N-acetylmuramic acid in the peptidoglycan of prokaryotic cell walls. It has also been recruited for a digestive role in certain ruminants and colobine monkeys
[4]
. There are at least five different classes of lysozymes
[1]
: C (chicken type), G (goose type), phage-type (T4), fungi (Chalaropsis), and bacterial (Bacillus subtilis). There are few similarities in the sequences of the different types of lysozymes.

Lysozyme type C and alpha-lactalbumin are similar both in terms of primary sequence and structure, and probably evolved from a common ancestral protein
[5]
. Around 35 to 40% of the residues are conserved in both proteins as well as the positions of the four disulphide bonds. There is, however, no similarity in function. Another significant difference between the two enzymes is that all lactalbumins have the ability to bind calcium
[2]
, while this property is restricted to only a few lysozymes
[8]
.

The binding site was deduced using high resolution X-ray structure analysis and was shown to consist of three aspartic acid residues. It was first suggested that calcium bound to lactalbumin stabilised the structure, but recently it has been claimed that calcium controls the release of lactalbumin from the golgi membrane and that the pattern of ion binding may also affect the catalytic properties of the lactose synthetase complex.

This domain includes three cysteines which are involved in two of the disulphide bonds found in these proteins.

References

1.Amino acid sequence of a lysozyme (B-enzyme) from Bacillus subtilis YT-25. Kamei K, Hara S, Ikenaka T, Murao S. J. Biochem. 104, 832-6, (1988). View articlePMID: 3148618

2.Alpha-lactalbumin possesses a novel calcium binding loop. Stuart DI, Acharya KR, Walker NP, Smith SG, Lewis M, Phillips DC. Nature 324, 84-7, (1986). View articlePMID: 3785375

3.Structures and mechanisms of glycosyl hydrolases. Davies G, Henrissat B. Structure 3, 853-9, (1995). View articlePMID: 8535779

4.Multiple cDNA sequences and the evolution of bovine stomach lysozyme. Irwin DM, Wilson AC. J. Biol. Chem. 264, 11387-93, (1989). View articlePMID: 2738070

5.The evolution of lysozyme and alpha-lactalbumin. Nitta K, Sugai S. Eur. J. Biochem. 182, 111-8, (1989). View articlePMID: 2731545

6.Alpha-lactalbumin and related proteins: a versatile gene family with an interesting parentage. Hall L, Campbell PN. Essays Biochem. 22, 1-26, (1986). PMID: 3104032

7.Evolution of alpha-lactalbumins. The complete amino acid sequence of the alpha-lactalbumin from a marsupial (Macropus rufogriseus) and corrections to regions of sequence in bovine and goat alpha-lactalbumins. Shewale JG, Sinha SK, Brew K. J. Biol. Chem. 259, 4947-56, (1984). View articlePMID: 6715332

8.The calcium-binding property of equine lysozyme. Nitta K, Tsuge H, Sugai S, Shimazaki K. FEBS Lett. 223, 405-8, (1987). View articlePMID: 3666156

9.Conserved catalytic machinery and the prediction of a common fold for several families of glycosyl hydrolases. Henrissat B, Callebaut I, Fabrega S, Lehn P, Mornon JP, Davies G. Proc. Natl. Acad. Sci. U.S.A. 92, 7090-4, (1995). View articlePMID: 7624375

Cross References

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