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.

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-beta-D-glucose (GlcNAc; A-unit) and 2-amino-2-deoxy-beta-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 (FA) 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 FA-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.