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

Two complexes between rainbow trout lysozyme (RBTL) and 4-methylumbellifery l 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 Angstrom resolution. The results show that 4- MeU(GlcNAc)(3) binds in subsites A-D and that 4-MeU-(GlcNAc)(2) binds in su bsites B-D in the active-site cleft of RBTL. This agrees well with earlier crystallographic studies on the binding of oligosaccharides of chitin to RB TL, 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 th e 4-MeU moiety in site D has diffuse electron density and is flexible, as i t is only bound to water molecules and not to the protein. Since no electro n density was observed in site E, the solved structures give views of nonpr oductive enzyme-substrate complexes.