Conformational analysis of the hydrophobic peptide alphas1-casein(136-196)

Hydrophobic interactions are important in the self-association of milk proteins, including alphas1-casein. The extent to which casein interaction sites are influenced by local secondary structure is not widely known. Both primary amino acid sequence and local secondary structure are shown to affect the self-association of the hydrophobic peptide alphas1-casein(136-196). The peptide is aggregated at low concentrations (7 microM and above), as determined by 1H nuclear magnetic resonance (NMR) measurements at pH 6.0 in phosphate buffer. Increase in temperature is shown to induce side chain mobility (melting) as indicated by both 1H NMR and near-UV circular dichroism (CD) measurements. As determined by far-UV CD, there is also a loss in the global amount of extended structure with increasing temperature, while beta-turn structures and some aromatic dichroism are conserved at temperatures as high as 70 degrees C. Similar retention of structure occurs at pH 2 and in 6 M guanidine HCl. The observed stability of beta-turns and some side chains in alphas1-casein(136-196) supports previous assumptions that hydrophobic, proline-based turns are important interaction sites in the self-association of alphas1-casein, and possibly in the formation of the calcium transport complexes, the casein micelles. It may be speculated that these areas of the peptide represent a 'molten globule-like', heat stable, core structure for alphas1-casein.