Quantitative studies of the non-productive binding of lysozyme to partially N-acetylated chitosans : Binding of large ligands to a one-dimensional binary lattice studied by a modified McGhee and von Hippel model

Abstract

This paper considers the non-productive (inhibitory) binding of chitosans to lysozyme from chicken egg white. Chitosans are linear, binary heteropolysaccharides consisting of 2-acetamido-2-deoxy-β- d-glucose (GlcNAc; A-unit) and 2-amino-2-deoxy-β- d-glucose (GlcN, D-unit). The active site cleft of lysozyme can bind six consecutive sugar residues in subsites named A–F, and specific binding of chitosan sequences to lysozyme occurs with A-units in subsite C. Chitosans with different fractions of A-units (F A) induced nearly identical changes in the 1H NMR spectrum of lysozyme upon binding, and the concentration of bound lysozyme could be determined. The data were analysed using a modified version of the McGhee and von Hippel model for binding of large ligands to one-dimensional homogeneous lattices. The average value of the dissociation constant for different sequences that may bind to lysozyme ( K ave D) was estimated, as well as the number of chitosan units covered by lysozyme upon binding. K ave D decreased with increasing F A-values at pH* 3 and 4.5, while the opposite was true at pH* 5.5. Contributions from different hexamer sequences to K ave D of the chitosans were considered, and the data revealed interesting features with respect to binding of lysozyme to partially N-acetylated chitosans. The relevance of the present data with respect to understanding lysozyme degradation kinetics of chitosans is discussed.