Interactions of α- and β-N-Acetyl-D-glucosamines with Hen and Turkey Lysozymes1

Abstract

The binding constants of α- and β-GlcNAc to hen and turkey lysozymes [EC 3.2.1.17] were determined at various pH's using the method proposed by Ikeda and Hama-guchi ((1975) J. Biochem. 77, 1–16). The pH dependence of the binding of β-GlcNAc to hen lysozyme was essentially the same as that for turkey lysozyme. The pH dependence curves of the binding constants of β-GlcNAc to hen and turkey lysozymes were interpreted in terms of the participation of Glu 35 (pK 6.0), Asp 52 (pK 3.5), Asp 48 (pK 4.5), and Asp 66 (pK 1.5). The binding constants of α-GlcNAc to hen and turkey lysozymes were the same below pH 3.5 but were different above this pH. The main participant residues in the binding of α-GlcNAc were Glu 35, Asp 48, and Asp 66 for hen lysozyme and Glu 35 and Asp 66 for turkey lysozyme. The results obtained here were well explained by the following assumptions: (1) above about pH 4, α-GlcNAc binds to hen lysozyme in both α- and β-modes, which correspond to the binding orientation of α-GIcNAc and that of β-GlcNAc, respectively, as determined by X-ray crystallographic studies, but it binds predominantly in the β-mode below about pH 4, (2) β-GlcNAc binds to hen and turkey lysozymes predominantly in the β-mode above about pH 4 and in both α- and β-modes below pH 4, and (3) α-GlcNAc binds to turkey lysozymes predominantly in the β-mode over the whole pH range studied.