Lysozyme is a protein enzyme that attacks the peptidoglycan of bacterial cell walls. Its active site is a cleft in the folded protein structure, where a suitable substrate can interact with positioned chemical groups. This illustrates why possessing the right amino acid residues is insufficient if their spatial arrangement is lost. See Alberts et al.: lysozyme as a worked example of protein binding and enzyme function.
A concrete structural example is Cheetham, Artymiuk and Phillips's 1992 study of hen egg-white lysozyme bound to a short carbohydrate that inhibits its action. The bound carbohydrate occupies part of the active site; it is not a reacting substrate in this complex. The study describes specific interactions and changes of shape accompanying binding. Thus a real binding site has chemical interactions and flexibility as well as a geometric outline. See Cheetham, Artymiuk and Phillips, 1992: structure of a lysozyme complex with bound inhibitor and RCSB PDB entry 1HEW: the authors' structure and bound three-unit carbohydrate. The diagrams above illustrate the idea; their twelve-residue strings are not lysozyme sequences or coordinates.
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The second protein to have its structure determined, after myoglobin, by X-ray crystallography, in 1965.
Breaks up peptidoglycan present in the bacterial cell wall, which is thicker in Gram-positive bacteria, which is what this enzyme seems to target.
Part of the inate immune system.