Lysozyme: a model enzyme in protein crystallography

The review concentrates on the crystal structure results from several protein crystallography laboratories on three different lysozymes, the type-c lysozymes such as hen egg-white lysozyme (HEWL), the type-g lysozyme, such as goose egg-white lysozyme (GEWL), and the lysozyme from T4 bacteriophage (T4L). The crystallographic studies on HEWL in several different crystal forms have shown that the lysozyme molecule is relatively rigid with the residues of the active site Glu35 and Asp52 adopting almost identical conformations in all structures and species variants. The NMR results also confirm the presence of a similar conformation of HEWL in solution. All three enzymes, HEWL, GEWL and T4L are composed of two domains, one that is predominantly alpha-helical and a smaller domain that is mainly beta-sheet in nature. The general acid/general base residue in each lysozyme (Glu35 in HEWL, Glu73 in GEWL and Glu11 in T4L) is contributed by the larger alpha-helical domain. The beta-sheet domains of HEWL and T4L contribute an aspartate to their respective active sites, which is likely involved in electrostatic stabilization of the oxycarbonium ion intermediate of the site D sugar on the hydrolytic pathway of oligosaccharides. There is no analogous aspartate carboxylate group in GEWL although minor conformational changes could position one or other of Asp86 or Asp97 for such a stabilization role. The binding of substrate analogues, transition state mimics and oligosaccharide products of hydrolysis to HEWL, GEWL and T4L have contributed greatly to our understanding of sugar binding to proteins. The observed subtle conformational differences of the free vs bound forms of these enzymes are best described by a narrowing of the active site clefts in the presence of the inhibitors. Details of the binding interactions of those residues lining the oligosaccharide binding clefts of the three-enzymes HEWL, GEWL and T4L with the sugar residues in sites A, B, C and D are presented and discussed. Oligosaccharides of (GlcNAc)n and alternating MurNAc-GlcNAc-MurNAc have been bound to these three enzymes and the structures determined at high resolution. These binding studies have contributed greatly to our understanding of the catalytic mechanism of the lysozyme glycosidase activity. The currently accepted view of this mechanism is presented and discussed in this review.