Catalytic Reaction Mechanism of Goose Egg-white Lysozyme by Molecular Modelling of EnzymeSubstrate Complex

Despite the low similarity between their amino acid sequences, the core structures of the fold between chicken-type and goose-type lysozymes are conserved. However, their enzymatic activities are quite different. Both of them exhibit hydrolytic activities, but the goose-type lysozyme does not exhibit transglycosylation activity. The chicken-type lysozyme has a retaining-type reaction mechanism, while the reaction mechanism of the goose-type lysozyme has not been clarified. To clarify the latter mechanism, goose egg-white lysozyme (GEL)N-acetyl-D-glucosamine (GlcNAc)6 complexes were modelled and compared with hen egg-white lysozyme (HEL)(GlcNAc)6 complexes. By systematic conformational search, 48 GEL(GlcNAc)6 complexes were modelled. The right and left side, and the amino acid residues in subsites EG were identified in GEL. The GlcNAc residue D could bind towards the right side without distortion and there was enough room for a water molecule to attack the C1 carbon of GlcNAc residue D from -side in the right side and not for acceptor molecule. The result of molecular dynamics simulation suggests that GEL would be an inverting enzyme, and Asp97 would act as a second carboxylate and that the narrow space of the binding cleft at subsites EG in GEL may prohibit the sugar chain to bind alternative site that might be essential for transglycosylation.