Effect of W62G mutation of hen lysozyme on the folding in vivo

Aggregates with lysozyme and ovalbumin show features of amyloid-like fibrils

The interaction of egg-white lysozyme with N-ovalbumin, the native form of egg-white ovalbumin with the denaturation temperature, Tm, of 78 °C, was investigated by the inhibition of lysozyme muramidase activity, differential scanning calorimetry, and circular dichroism assay as indicators. Signals for the interaction were the most prominent when the mixture of lysozyme and N-ovalbumin was co-heated at 72 °C, slightly lower than the Tm of N-ovalbumin. The interaction was also marked when unheated lysozyme was mixed with N-ovalbumin preheated at 72 °C. Moreover, the mixture rapidly formed fibrous precipitates, which were positive for thioflavin T fluorescent emission, a marker for the amyloid fibril formation. Also electron microscopic observation exhibited features of fibrils. The interaction potency of ovalbumin was ascribed to the tryptic fragment ILELPFASGT MSMLVLLPDE VSGLEQLESIINFEK (residues 229–263), derived from the 2B strands 2 and 3 of ovalbumin. From lysozyme, on the other hand, the chymotryptic peptide RNRCKGTDVQAW (residues 112–123), including cluster 6, and the chymotryptic/tryptic peptide GILQINSRW (residues 54–62), including cluster 3, were responsible for the interaction with N-ovalbumin. Interestingly, this nonamer peptide was found to have the ability to self-aggregate. To the authors knowledge, this may be the first report to document the possible involvement of dual proteins in the formation of amyloid-like fibrils.

ATP-induced noncooperative thermal unfolding of hen lysozyme

To understand the role of ATP underlying the enhanced amyloidosis of hen egg white lysozyme (HEWL), the synchrotron radiation circular dichroism, combined with tryptophan fluorescence, dynamic light-scattering, and differential scanning calorimetry, is used to examine the alterations of the conformation and thermal unfolding pathway of the HEWL in the presence of ATP, Mg(2+)-ATP, ADP, AMP, etc. It is revealed that the binding of ATP to HEWL through strong electrostatic interaction changes the secondary structures of HEWL and makes the exposed residue W62 move into hydrophobic environments. This alteration of W62 decreases the beta-domain stability of HEWL, induces a noncooperative unfolding of the secondary structures, and produces a partially unfolded intermediate. This intermediate containing relatively rich alpha-helix and less beta-sheet structures has a great tendency to aggregate. The results imply that the ease of aggregating of HEWL is related to the extent of denaturation of the amyloidogenic region, rather than the electrostatic neutralizing effect or monomeric beta-sheet enriched intermediate.