Sequence specificities for lysozyme depolymerization of partially N-acetylated chitosans

The influence of sugar residue sequence in partially N-acetylated chitosans on relative hydrolysis rate catalyzed by lysozyme was studied. The relative rates were modelled assuming an Arrhenius-type relation for the relative rate constants. The apparent activation energy was assumed to consist of additive contributions from GlcN or GlcNAc residues within the polymer chain interacting with sites A-F of the active cleft of lysozyme. This model accounted well for the relative hydrolysis rates reported for well-defined oligomers. Calculated and experimental data for the dependence of the initial relative hydrolysis rates on fraction of acetylated units, F-A, showed an F-A(3,6) dependence. A fully water-soluble highly N-acetylated chitosan with F-A = 0.68 was depolymerized using lysozyme for further testing of the model. Analyses of the C-13 nuclear magnetic resonance spectra of the diad sequences at the new reducing and nonreducing ends formed by lysozyme showed that this enzymatic depolymerization was dominated by chitosan sequences presenting GlcNAc residues to sites C, D, and E of the active cleft. In contrast, there was no selectivity between GlcNAc and GlcN residues interacting with site E These selectivities were confirmed by the calculated contributions to the apparent activation energy of these sites. The experimentally determined depletion in the diad and triad frequencies of GlcNAc during the course of lysozyme hydrolysis was in good agreement with the model calculations.