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

Biochemistry. 1994 Oct 18;33(41):12367-77. doi: 10.1021/bi00207a003.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Apolipoproteins E / chemistry*
  • Circular Dichroism
  • Computer Simulation
  • Hydrogen-Ion Concentration
  • Lactams / chemistry*
  • Magnetic Resonance Spectroscopy
  • Models, Molecular*
  • Molecular Sequence Data
  • Peptides / chemistry*
  • Protein Conformation
  • Protein Structure, Secondary
  • Thermodynamics

Substances

  • Apolipoproteins E
  • Lactams
  • Peptides