Effect of Overall Charge and Local Charge Density of Pectin on the Structure and Thermal Stability of Lysozyme

In this paper, effects of pectin varying in overall charge (OCH) and local charge density (LCD) on the structure and thermal stability of Lysozyme (lys) are studied. The secondary and local tertiary structures of lys undergo at pH 5.1 a two-step change upon binding with increasing amounts of pectin, in which the peak of fluorescence first increases and shows a blue shift and the amount of helix conformation drops at a pectin/lys mass ratio (q) below or equal to that corresponding to maximal complexation qmax. With increasing amount of pectin, the native like state is recovered at q > qmax. The decrease in helix content depends mainly on the OCH of pectin. Analysis of the thermodenaturation behavior of lys reveals that affinity of pectin with denatured lys is higher than for the native state of lys. At the same OCH of pectin, unfolding of lys is more significant in case of a high LCD (blockwise distribution of charges) of the former. At q = qmax (first stage of binding) the key factors stabilizing lys within its complexes with pectin against thermo-aggregation are a high LCH of pectin and, to a lesser degree, a low OCH. At q > > qmax (second stage of binding), a high LCH of pectin is a key factor inhibiting thermo-aggregation of lys