Stability of equine lysozyme. I. Thermal unfolding behaviour.
The thermal denaturation of Ca(2+)- and apo-forms of equine lysozyme was followed by using far and near UV circular dichroism and intrinsic fluorescence methods. The difference found between the temperature dependence of the ellipticity at 222 nm and 287 nm, which show two stages in the thermal transition, and those at 228 nm and 294 nm, which indicate only one stage over a wide range of temperatures reflects that different subdivisions of the protein molecule are characterized by a different stability, cooperativity and pathway of denaturation. The first transition, reflected in the increase of the ellipticity at 222 nm and 287 nm, coincides with the transition detected by fluorescence and occurs at 30-50 degrees C for the apo-form and at 50-60 degrees C for the Ca(2+)-form of lysozyme. It seems to correlate with the transfer of some tryptophan residues to a more hydrophobic environment and with a local rearrangement of the tertiary and secondary structures. The unfolding transition detected by the decrease of the ellipticity at all wavelengths occurs nearly in the same temperature region for the apo- and Ca(2+)-forms, i.e. 50-80 degrees C and 55-80 degrees C, respectively. The presence of a Ca(2+)-binding loop in equine lysozyme may be partly responsible for the drastic destabilization of its structure as a whole both in the presence but especially in the absence of Ca2+ in comparison with hen and human lysozymes.
Morozova
L
L
Interdisciplinair Research Center, K.U. Leuven Campus Kortrijk, Belgium.
Haezebrouck
P
P
Van Cauwelaert
F
F
D000818:Animals; D002942:Circular Dichroism; D005454:Fluorescence Polarization; D006736:Horses; D006358:Hot Temperature; D008892:Milk / Q000201:enzymology; D009113:Muramidase / Q000737:chemistry*; D011487:Protein Conformation; D011489:Protein Denaturation; D013056:Spectrophotometry, Ultraviolet
D009113:Muramidase