Abstract
Chitosans, prepared by homogeneous N-deacetylation of chitin, with
degrees of N-acetylation ranging from 4 to 60% (F(A) = 0.04 to 0.60)
exhibiting full water solubility and known random distribution of
acetyl groups, were degraded with lysozyme. Initial degradation rates
(r) were determined from plots of the viscosity decrease (DELAT1/eta)
against time of degradation. The time course of degradation of chitosans
with lysozyme were non-linear, while the time course of degradation
of chitosans with an oxidative-reductive depolymerization reaction
(using H2O2) showed the expected linear relationship for a first-order,
random depolymerization reaction, independent of the chemical composition
of the chitosan. The effect of lysozyme concentration and substrate
concentration on the initial degradation rates were determined, showing
that this lysozyme-chitosan system obeys Michaelis-Menten kinetics.
The initial degradation rates of chitosan with lysozyme increased
strongly with increasing fraction of acetylated units (F(A)). From
a Michaelis-Menten analysis of the degradation data that assumes
different catalytic activities of lysozyme for the different hexameric
substrates in the polysaccharide chain, it is concluded that the
hexameric substrates that contain three-four or more acetylated units
contribute mostly to the initial degradation rate when lysozyme degrades
partially N-acetylated chitosans. A chitosan with a very low fraction
of acetylated units (F(A) = 0.010) was studied as an enzyme inhibitor.
Initial degradation rates of chitosan (with different F(A) values)
decreased as the inhibitor concentration increased, while the relative
rates stayed constant, indicating that the ratio between initial
reaction rates for productive sites (hexamers containing three-four
or more N-acetylated units) are unaffected by non-productive sites,
as deduced from the theory of competing substrates.
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