DESIGN AND SYNTHESIS OF A HELIX HEPARIN-BINDING PEPTIDE

Elaboration of heparin-protein-binding interactions is necessary to understand how heparin modulates protein function. Thc heparin-binding domain of some proteins is postulated to be a helix structure which presents a surface of high positive charge density. Thus, a synthetic 19-residue peptide designed to be alpha-helical in character was synthesized, and its interaction with heparin was studied. The peptide was shown to be 75% helix by circular dichroism (CD) spectrometry in neutral pH buffer (at 2-degrees-C); helicity increased to nearly 85% under high ionic strength conditions or to nearly 100% in 75% ethanol. Increasing the temperature of the solution caused a change in the spectral envelope consistent with a coil transition of the peptide. The midpoint of the transition (i.e., the temperature at which the helix content was determined to be 50%) was 25-degrees-C, and the determined van't Hoff enthalpy change (DELTA-H(vH)) was 3.2 kcal/mol of peptide. By CD, heparin increases the helix content of the peptide to 100% and increases the apparent thermal stability of the peptide by about 1 kcal/mol. The melting point for the helix/coil transition of the heparin-peptide complex was 50-degrees-C. The thermal coefficient of the transition (almost-equal-yo 300 deg.cm2.dmol-1.degrees-C-1) was essentially the same for the peptide alone or the peptide-heparin complex. Dissociation of the complex under high ionic strength conditions was also observed in the CD experiment. Biological assays showed less heparin-binding activity than expected (micromolar K(D) values), but this was attributed to the absence of critical lysyl residues in the peptide. Nonetheless, this type of helix peptide can serve as a framework for the design of other peptides which might bind heparin(s) with unique, specific biological properties.