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Glycoside Hydrolase Family 22
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- Author: ^^^David Vocadlo^^^
- Responsible Curator: ^^^David Vocadlo^^^
| Glycoside Hydrolase Family GH22 | |
| Clan | none, lysozyme-type fold |
| Mechanism | retaining |
| Active site residues | known |
| CAZy DB link | |
| https://www.cazy.org/GH22.html | |
Substrate specificities
Glycoside hydrolase family 22 contains proteins with two main functions: lysozymes and α-lactalbumin.
Lysozymes catalyse the hydrolysis of (1→4)-β-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in peptidoglycan and between ‘’N’’-acetyl-D-glucosamine residues in chitooligosaccharides. Lysozymes are also referred to as muramidase. Lysozymes from family GH22 are classified as c-type lysozymes (c = chicken), to distinguish them from lysozymes of family GH23, which are sometimes referred to as g-type (g = goose) lysozymes. Lysozymes are antibacterial lytic proteins, protecting against bacterial infection through their ability to degrade the bacterial cell wall. Human lysozyme is abundant in secretions including tears, saliva, milk and in mucus. Human lysozyme defects can result in a rare hereditary condition, amyloidosis VIII, in which lysozyme deposits as amyloid.
α-Lactalbumins are auxiliary proteins that modify the substrate specificity of galactosyltransferase, converting it to lactose synthase. It is believed that α-lactalbumins evolved at the outset of mammalian evolution, after divergence of mammalian and avian lineages.
Kinetics and Mechanism
Content is to be added here.
Catalytic Residues
HEWL operates through a Koshland retaining mechanism involving a covalent glycosyl enzyme intermediate. Asp52 functions as the catalytic nucleophile, as shown by X-ray crystallographic observation of a covalent bond for the 2-fluoroglycosyl enzyme formed on the E35Q mutant of HEWL using ‘’N’’-acetylglucosaminyl-1,4-2-deoxy-2-fluoroglycosyl fluoride, and by mass spectrometric observation of a covalent adduct of the same complex [1].
Three-dimensional structures
The first structure of a GH22 member was that of hen egg white lysozyme (HEWL) [2]. In fact, HEWL was the first enzyme to have its structure determined and attracted great interest as it provided a molecular view of enzyme catalysis.
Family Firsts
- First sterochemistry determination
- Cite some reference here, with a short (1-2 sentence) explanation [3].
- First catalytic nucleophile identification
- Asp52 of hen egg white lysozyme (HEWL), by X-ray crystallography of covalent complex formed with a 2-fluorosugar [1].
- First general acid/base residue identification
- Cite some reference here, with a short (1-2 sentence) explanation [4].
- First 3-D structure
- Hen egg-white lysozyme (HEWL) was the first glycosidase, and the first enzyme, to have its three-dimensional structure determined by X-ray diffraction techniques [2].