PROSITE documentation PDOC00119

Glycosyl hydrolases family 22 (GH22) domain signature and profile

α-lactalbumin [1], a milk protein, is the regulatory subunit of lactose synthase. In the mammary gland, α-lactalbumin changes the substrate specificity of galactosyltransferase from N-acetylglucosamine to glucose.

Lysozymes (EC 3.2.1.17) [2] act as bacteriolytic enzymes by hydrolyzing the β(1->4) bonds between N-acetylglucosamine and N-acetylmuramic acid in the peptidoglycan of prokaryotic cell walls. There are at least five different classes of lysozymes [3,4]: C (chicken type), G (goose type), phage-type (T4), fungi (Chalaropsis), and bacterial (Bacillus subtilis) but there are few similarities in the sequences of the different types of lysozymes.

α-lactalbumin and lysozyme C are evolutionary related [5] and form family 22 in the classification of glycosyl hydrolases [6,E1]. Around 35 to 40% of the residues are conserved in both proteins as well as the positions of the four disulfide bonds (see the schematic representation). The pattern for this family of proteins includes three cysteines involved in two of these disulfide bonds (the first cysteine is linked to the third one).

                                  +-------+
                                  |     **|*******
     xxCxxxxxxxxxxCxxxxxxxxxxxxxxxCxxxxxCxCxxxxxxCxxxxxxxxxCxxxCxx
       |          |                     +--------+         |    |
       |          +----------------------------------------+    |
       +--------------------------------------------------------+

'C': conserved cysteine involved in a disulfide bond.
'*': position of the pattern.

We also developed a profile that covers the entire glycosyl hydrolases family 22 domain.