Glycoside hydrolase family 22

In molecular biology, glycoside hydrolase family 22 is a family of glycoside hydrolases. EC 3.2.1., which 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 web site,[4][5] and also discussed at CAZypedia, an online encyclopedia of carbohydrate active enzymes.[6][7]

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,[8][9] 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 bacterial cell walls. It has also been recruited for a digestive role in certain ruminants and colobine monkeys.[10] There are at least five different classes of lysozymes:[11] 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.[12] 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,[13] while this property is restricted to only a few lysozymes.[14]

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, Fabrega S, Lehn P, Mornon JP, Davies G (July 1995). "Conserved catalytic machinery and the prediction of a common fold for several families of glycosyl hydrolases". Proceedings of the National Academy of Sciences of the United States of America. 92 (15): 7090–4. Bibcode:1995PNAS...92.7090H. doi:10.1073/pnas.92.15.7090. PMC 41477. PMID 7624375.
2. Davies G, Henrissat B (September 1995). "Structures and mechanisms of glycosyl hydrolases". Structure. 3 (9): 853–9. doi:10.1016/S0969-2126(01)00220-9. PMID 8535779.
3. Henrissat B, Bairoch A (June 1996). "Updating the sequence-based classification of glycosyl hydrolases". The Biochemical Journal. 316 (Pt 2): 695–6. doi:10.1042/bj3160695. PMC 1217404. PMID 8687420.
4. "Home". CAZy.org. Retrieved 2018-03-06.
5. Lombard V, Golaconda Ramulu H, Drula E, Coutinho PM, Henrissat B (January 2014). "The carbohydrate-active enzymes database (CAZy) in 2013". Nucleic Acids Research. 42 (Database issue): D490-5. doi:10.1093/nar/gkt1178. PMC 3965031. PMID 24270786.
6. "Glycoside Hydrolase Family 22". CAZypedia.org. Retrieved 2018-03-06.
7. CAZypedia Consortium (December 2018). "Ten years of CAZypedia: a living encyclopedia of carbohydrate-active enzymes". Glycobiology. 28 (1): 3–8. doi:10.1093/glycob/cwx089. hdl:21.11116/0000-0003-B7EB-6. PMID 29040563.
8. 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". The Journal of Biological Chemistry. 259 (8): 4947–56. doi:10.1016/S0021-9258(17)42938-3. PMID 6715332.
9. Hall L, Campbell PN (1986). "Alpha-lactalbumin and related proteins: a versatile gene family with an interesting parentage". Essays in Biochemistry. 22: 1–26. PMID 3104032.
10. Irwin DM, Wilson AC (July 1989). "Multiple cDNA sequences and the evolution of bovine stomach lysozyme". The Journal of Biological Chemistry. 264 (19): 11387–93. doi:10.1016/S0021-9258(18)60476-4. PMID 2738070.
11. Kamei K, Hara S, Ikenaka T, Murao S (November 1988). "Amino acid sequence of a lysozyme (B-enzyme) from Bacillus subtilis YT-25". Journal of Biochemistry. 104 (5): 832–6. doi:10.1093/oxfordjournals.jbchem.a122558. PMID 3148618.
12. Nitta K, Sugai S (June 1989). "The evolution of lysozyme and alpha-lactalbumin". European Journal of Biochemistry. 182 (1): 111–8. doi:10.1111/j.1432-1033.1989.tb14806.x. PMID 2731545.
13. 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. Bibcode:1986Natur.324...84S. doi:10.1038/324084a0. PMID 3785375. S2CID 4349973.
14. Nitta K, Tsuge H, Sugai S, Shimazaki K (November 1987). "The calcium-binding property of equine lysozyme". FEBS Letters. 223 (2): 405–8. doi:10.1016/0014-5793(87)80328-9. PMID 3666156. S2CID 12630658.