Native-like secondary structure in a peptide from the alpha-domain of hen lysozyme

To gain insight into the local and nonlocal interactions that contribute to the stability of hen lysozyme, we have synthesized two peptides that together comprise the entire alpha-domain of the protein. One peptide (peptide 1-40) corresponds to the sequence that forms two alpha-helices, a loop region, and a small beta-sheet in the N-terminal region of the native protein. The other (peptide 84-129) makes up the C-terminal part of the alpha-domain and encompasses two alpha-helices and a 3(10) helix in the native protein.

As judged by CD and a range of NMR parameters, peptide 1-40 has little secondary structure in aqueous solution and only a small number of local hydrophobic interactions, largely in the loop region. Peptide 84-129, by contrast, contains significant helical structure and is partially hydrophobically collapsed. More specifically, the region corresponding to helix C in native lysozyme is disordered, whereas regions corresponding to the D and 3(10) helices in the native protein are helical in this peptide. The structure in peptide 84-129 is at least partly stabilized by interactions between residues in the two helical regions, as suggested by further NMR analysis of three short peptides corresponding to the individual helices in this region of the native protein.

Stabilization of structure in the sequence 1-40 appears to be facilitated predominantly by long-range interactions between this region and the sequence 84-129. In native lysozyme, the existence of two disulphide bonds between the N- and C-terminal halves of the alpha-domain is likely to be a major factor in their stabilization. The data show, however, that native-like secondary structure can be generated in the C-terminal portion of the alpha-domain by nonspecific and nonnative interactions within a partially collapsed state.