Cation-dependent structural features of β-casein-(1–25)

Complete sequence-specific, proton-resonance assignments have been determined for the calcium phosphate-stabilizing tryptic peptide β-casein-(1–25) containing the phosphorylated sequence motif Ser(P)¹⁷-Ser(P)-Ser(P)-Glu-Glu²¹. Spectra of the peptide have been recorded, in separate experiments, in the presence of excess ammonium ions, sodium ions and calcium ions, and of the dephosphorylated peptide in the presence of excess sodium ions. We observed significant changes to chemical shifts for backbone and side-chain resonances that were dependent upon the nature of the cation present. Medium-range nuclear Overhauser effect (nOe) enhancements, characteristic of small structured regions in the peptide, were observed and also found to be cation dependent. The secondary structure of the peptide was characterized by sequential and medium-range (i, i+2/3/4, which denotes an interaction between residue i and residue i+2, i+3 or i+4 in the peptide) nOe connectivities, and Hα chemical shifts. Four structured regions were identified in the calcium-bound peptide: residues Arg¹ to Glu⁴ were involved in a loop-type structure, and residues Val⁸ to Glu¹¹, Ser(P)¹⁷ to Glu²⁰ and Glu²¹ to Thr²⁴ were implicated in β-turn conformations. Comparison of the patterns of medium-range nOe connectivities in β-casein-(1–25) with those in α_S1-casein-(59–79) suggest that the two peptides have distinctly different conformations in the presence of calcium ions, despite having a high degree of sequential and functional similarity.