Structural and thermodynamic characterization of a bioactive peptide model of apolipoprotein E: side-chain lactam bridges to constrain the conformation.

Apolipoprotein E plays a critical role in plasma lipoprotein clearance. A peptide model of a highly conserved domain of this protein has been shown to increase low-density lipoprotein binding to fibroblast cell surface receptors. To distinguish between two potential structures--one essentially alpha-helical and nonamphiphilic, the other an amphiphilic pi-helix--synthetic side-chain lactam constraints have been incorporated into model peptides in order to restrict conformational flexibility favoring either the alpha- or pi-helix. Here we provide CD and 1H NMR data suggesting that the more biologically active, putatively alpha-helical peptide indeed contains two alpha-helical domains separated by a central bend. Whereas previous studies (Osapay & Taylor, 1992; Felix et al., 1988) indicated stabilization of alpha-helices by cross-links between the i and i + 4 residues, the current paper demonstrates that cross-links between the i and i + 3 residues also stabilize the helix. Indeed, the stabilization afforded by these cross-links is approximately 1 kcal/mol, similar to that reported for peptides cross-linked between the i and i + 4 residues, and derives exclusively from a loss of entropy of the unfolded state. The presence of the alpha-helical structure appears to correlate well with biological activity. This study provides initial insight into the bioactive structure of this domain of apo E and suggests strategies as to how peptides can be conformationally constrained to enhance their stability and biological function.