Glycoside hydrolase family 22 comprises enzymes with two known activities; lysozyme type C ( EC:3.2.1.17 ) (also known as 1, 4-beta-N-acetylmuramidase or LYZ) and alpha-lactalbumins (also known as lactose synthase B protein or LA). Asp and/or the carbonyl oxygen of the C-2 acetamido group of the substrate acts as the catalytic … expand

Glycoside hydrolase family 22 comprises enzymes with two known activities; lysozyme type C ( EC:3.2.1.17 ) (also known as 1, 4-beta-N-acetylmuramidase or LYZ) and alpha-lactalbumins (also known as lactose synthase B protein or LA). Asp and/or the carbonyl oxygen of the C-2 acetamido group of the substrate acts as the catalytic nucleophile/base.

Alpha-lactalbumin [ PUBMED:6715332 PUBMED:3104032 ] 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 ( EC: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 [ PUBMED:2738070 ]. There are at least five different classes of lysozymes [ PUBMED:3148618 ]: 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 [ PUBMED:2731545 ]. 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 [ PUBMED:3785375 ], while this property is restricted to only a few lysozymes [ PUBMED:3666156 ].

O-Glycosyl 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 glycosyl hydrolases, based on sequence similarity, has led to the definition of 85 different families [ PUBMED:7624375 PUBMED:8535779 ]. This classification is available on the CAZy (CArbohydrate-Active EnZymes) website.