Structural studies on the binding of 4-methylumbelliferone glycosides of chitin to rainbow trout lysozyme

Two complexes between rainbow trout lysozyme (RBTL) and 4-methylumbelliferyl chitobioside, 4MeU-(GlcNAc)₂, and chitotrioside, 4MeU-(GlcNAc)₃, were produced by co-crystallization and soaking, respectively, and the crystal structures were solved at 2.0 Å resolution. The results show that 4-MeU-(GlcNAc)₃ binds in subsites A-D and that 4-MeU-(GlcNAc)₂ binds in subsites B-D in the active-site cleft of RBTL. This agrees well with earlier crystallographic studies on the binding of oligosaccharides of chitin to RBTL, which showed that (GlcNAc)₃ binds to sites B-D in RBTL and not to A-C as seen in the human and turkey egg-white lysozymes. For both complexes the 4-MeU moiety in site D has diffuse electron density and is flexible, as it is only bound to water molecules and not to the protein. Since no electron density was observed in site E, the solved structures give views of nonproductive enzyme-substrate complexes.