A goose-type lysozyme from ostrich (Struthio camelus) egg white: multiple roles of His101 in its enzymatic reaction

A goose-type lysozyme from ostrich egg white (OEL) was produced by Escherichia coli expression system, and the role of His101 of OEL in the enzymatic reaction was investigated by NMR spectroscopy, thermal unfolding, and theoretical modeling of the enzymatic hydrolysis of hexa-N-acetylchitohexaose, (GlcNAc)6. Although the binding of tri-N-acetylchitotriose, (GlcNAc)3, to OEL perturbed several backbone resonances in the (1)H-(15)N HSQC spectrum, the chemical shift of the backbone resonance of His101 was not significantly affected. However, apparent pKa values of His101 and Lys102 determined from the pH titration curves of the backbone chemical shifts were markedly shifted by (GlcNAc)3 binding. Thermal unfolding experiments and modeling study of (GlcNAc)6 hydrolysis using a His101-mutated OEL (H101A-OEL) revealed that the His101 mutation affected not only sugar residue affinities at subsites -3 and -2 but also the rate constant for bond cleavage. His101 appears to play multiple roles in the substrate binding and the catalytic reaction.