Lysozymes (EC 188.8.131.52) catalyze the hydrolysis of bacterial cell wall polysaccharides into tetrasaccharides, as illustrated in fig. 12-14. Bacterial cell walls are constructed of cross-linked polysaccharides composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues, which are cleaved into tetrasaccharides at N-acetylglucosamine sites by the action of lysozymes. Lysozymes are small enzymes, 129 amino acids in the case of hen egg white (HEW) lysozyme, the first enzyme structurally characterized by x-ray crystallography (Blake et al., 1965). Hen egg white lysozyme is in family 22 (retaining) and bacteriophage T4 lysozyme is in family 24 (inverting). The two act by different mechanisms, and lysozymes are found in other families as well. We focus on HEW lysozyme because of the intensity of research on its mechanism of action.
The first structures of HEW lysozyme documented the interactions of NAM-NAG oligomers with the active site and led to the concept of subsites A to F for binding a hexa-meric substrate, in which hydrolysis occurred between glycosides in the D and E sites
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