Temperature-dependent behavior of lysozyme within the reverse hexagonal mesophases (H II)

Abstract

This manuscript is the second part of a study on the structural behavior of lysozyme-loaded reverse hexagonal mesophases. In the current paper we focused mainly on the mutual temperature-dependency relationship between the protein and the mesophase. The conformational stability of the enzyme and the structural effects on the host system were characterized using small-angle X-ray scattering (SAXS), ATR-FTIR spectroscopy, fluorescence, and rheological measurements. It was found that the mesophase does not change the solubilized lysozyme (LSZ) active site conformation. The obtained data suggested that LSZ embedment within the H II mesophase improved its thermal stability by hampering its helical structure destruction, apparently due to hydrogen bonding of the protein with monoolein polar heads. Examination of the structural parameters of the hexagonal carrier revealed a strong thermal dependency. The lattice parameter of both empty and LSZ-loaded systems had a similar temperature-dependent behavior. However, comparing the domain size of the LSZ-loaded system to the empty system showed different trends. LSZ incorporation induced a decrease in crystal size and lower order at room temperature. Nevertheless, an increase in domain size was triggered by the enzyme at elevated temperatures, in contrast to its decrease in the empty carriers. Rheological measurements showed concentration-dependent elasticity in the presence of LSZ compared to the empty system, which took place in a concentration-dependent manner at all examined temperatures. Up to 60 °C, the elasticity of the LSZ-loaded hexagonal systems decreased with temperature increase. This finding was interpreted in the context of weakening and/or cleaving of the monoolein hydroxyls’ interactions with the protein, leading to partial reconstitution of the initially low domain size and elasticity decrease. However, in the range of 60–75 °C (in most systems), the prevailing effect was thermally induced dehydration of the monoglyceride hydrophilic heads, which imposed elasticity increase, owing to enhanced flow ability of the liquid crystalline structure.