CONFORMATIONAL DIFFERENCES OF OVINE AND HUMAN CORTICOTROPIN-RELEASINGHORMONE - A CD, IR, NMR AND DYNAMIC LIGHT-SCATTERING STUDY

The differences in the conformational properties of ovine to) and huma n (h) CRH in aqueous solution, structure-inducing TFE and in the prese nce of detergent micelles and lipid vesicles have been investigated by circular dichroism, Fourier transform infrared spectroscopy, NMR and dynamic light scattering. o-CRH was found to exist as a monomer with l ittle regular structure in dilute aqueous solution. Association at con centrations higher than 10(-3) mol/L results predominantly in dimers. The induction of a substantial amount of intermolecular beta-structure seems to be the result of interactions of the C-terminal hexapeptide and the N-terminal region 6-12 of o-CRH chains in antiparallel orienta tion. In contrast, h-CRH exhibits a high tendency of association which is highly sensitive to the pH. The formation of tetramers at millimol ar peptide concentration is related to a helical content of ca. 50%. T he potentially helical, highly hydrophobic region 6-20 enlarged by mor e hydrophobic residues in position 23 and 25 is proposed to stabilize the h-CRH associates. In the presence of structure inducing TFE (>40%v ) both CRH peptides exist as monomers. o-CRH reveals about 72% helicit y, in h-CRH the formation of about 85% helix is observed. The differen ces in helicity of the two CRH molecules are located in the C-terminal heptapeptide, as concluded on the basis of NMR studies. Both peptides bind to detergent micelles at pH 4 as well as 7.4 associated with an increase in the alpha-helical content. Interaction of the two peptides with DMPC vesicles was found exclusively at pH 4. Above the phase tra nsition temperature of DMPC the alpha-helical content in h-CRH increas es slightly; however, o-CRH reveals a substantial amount of beta-type structure. The intramolecular type of beta-structure is associated wit h a deeper insertion of the o-CRH region 6-12 into the hydrophobic reg ion of the lipid bilayer, whereas the corresponding region of h-CRH is kept in the bilayer surface. The higher helicity of h-CRH might expla in to some extent its higher affinity to the CRH receptor, CRH antibod ies and the CRH binding protein. (C) Munksgaard 1996.