Glycoside hydrolase family 22

In molecular biology, glycoside hydrolase family 22 is a family of glycoside hydrolases.

C-type lysozyme/alpha-lactalbumin family
bobwhite quail lysozyme
Identifiers
Symbol	Lys
Pfam	PF00062
Pfam clan	CL0037
InterPro	IPR001916
PROSITE	PDOC00716
SCOP	1e0g
SUPERFAMILY	1e0g
TCDB	9.B.41
CAZy	GH22
CDD	cd00119
Available protein structures: Pfam structures PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Glycoside hydrolases EC 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 glycoside hydrolases, based on sequence similarity, has led to the definition of >100 different families.^[1][2][3] This classification is available on the CAZy(http://www.cazy.org/GH1.html) web site,^[4] and also discussed at CAZypedia, an online encyclopedia of carbohydrate active enzymes.^[5]

Glycoside hydrolase family 22 CAZY GH_22 comprises lysozyme type C (EC 3.2.1.17) lysozyme type i (EC 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,^[6][7] 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 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.^[8] There are at least five different classes of lysozymes:^[9] 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.^[10] 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,^[11] while this property is restricted to only a few lysozymes.^[12]

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.

References

1. Henrissat B, Callebaut I, Mornon JP, Fabrega S, Lehn P, Davies G (1995). "Conserved catalytic machinery and the prediction of a common fold for several families of glycosyl hydrolases". Proc. Natl. Acad. Sci. U.S.A. 92 (15): 7090–7094. doi:10.1073/pnas.92.15.7090. PMC 41477. PMID 7624375.
2. Henrissat B, Davies G (1995). "Structures and mechanisms of glycosyl hydrolases". Structure 3 (9): 853–859. doi:10.1016/S0969-2126(01)00220-9. PMID 8535779.
3. Bairoch, A. "Classification of glycosyl hydrolase families and index of glycosyl hydrolase entries in SWISS-PROT". 1999.
4. Henrissat, B. and Coutinho P.M. "Carbohydrate-Active Enzymes server". 1999.
5. CAZypedia, an online encyclopedia of carbohydrate-active enzymes.
6. Shewale JG, Sinha SK, Brew K (April 1984). "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". J. Biol. Chem. 259 (8): 4947–56. PMID 6715332.
7. Hall L, Campbell PN (1986). "Alpha-lactalbumin and related proteins: a versatile gene family with an interesting parentage". Essays Biochem. 22: 1–26. PMID 3104032.
8. Irwin DM, Wilson AC (July 1989). "Multiple cDNA sequences and the evolution of bovine stomach lysozyme". J. Biol. Chem. 264 (19): 11387–93. PMID 2738070.
9. Kamei K, Hara S, Ikenaka T, Murao S (November 1988). "Amino acid sequence of a lysozyme (B-enzyme) from Bacillus subtilis YT-25". J. Biochem. 104 (5): 832–6. PMID 3148618.
10. Nitta K, Sugai S (June 1989). "The evolution of lysozyme and alpha-lactalbumin". Eur. J. Biochem. 182 (1): 111–8. doi:10.1111/j.1432-1033.1989.tb14806.x. PMID 2731545.
11. Stuart DI, Acharya KR, Walker NP, Smith SG, Lewis M, Phillips DC (1986). "Alpha-lactalbumin possesses a novel calcium binding loop". Nature 324 (6092): 84–7. doi:10.1038/324084a0. PMID 3785375.
12. Nitta K, Tsuge H, Sugai S, Shimazaki K (November 1987). "The calcium-binding property of equine lysozyme". FEBS Lett. 223 (2): 405–8. doi:10.1016/0014-5793(87)80328-9. PMID 3666156.