Glycoside Hydrolase Family 22

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• Author: ^^^Spencer Williams^^^
• Responsible Curator: ^^^David Vocadlo^^^

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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 classical 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].

References

1. Vocadlo DJ, Davies GJ, Laine R, and Withers SG. (2001). Catalysis by hen egg-white lysozyme proceeds via a covalent intermediate. Nature. 2001;412(6849):835-8. DOI:10.1038/35090602 |
2. Blake CC, Koenig DF, Mair GA, North AC, Phillips DC, and Sarma VR. (1965). Structure of hen egg-white lysozyme. A three-dimensional Fourier synthesis at 2 Angstrom resolution. Nature. 1965;206(4986):757-61. DOI:10.1038/206757a0 |