title: Cation-dependent structural features of beta-casein-(1-25).
abstract: Complete sequence-specific, proton-resonance assignments have been determined for the calcium phosphate-stabilizing tryptic peptide beta-casein-(1-25) containing the phosphorylated sequence motif Ser(P)(17)-Ser(P)-Ser(P)-Glu-Glu(21). 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 Halpha chemical shifts. Four structured regions were identified in the calcium-bound peptide: residues Arg(1) to Glu(4) were involved in a loop-type structure, and residues Val(8) to Glu(11), Ser(P)(17) to Glu(20) and Glu(21) to Thr(24) were implicated in beta-turn conformations. Comparison of the patterns of medium-range nOe connectivities in beta-casein-(1-25) with those in alpha(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.
authors.affiliation: School of Dental Science, The University of Melbourne, 711 Elizabeth Street, Melbourne, Victoria 3000, Australia.
authors.forename: K J
authors.initials: KJ
authors.lastname: Cross
authors.forename: N L
authors.initials: NL
authors.lastname: Huq
authors.forename: W
authors.initials: W
authors.lastname: Bicknell
authors.forename: E C
authors.initials: EC
authors.lastname: Reynolds
mesh_terms: D000595:Amino Acid Sequence; D002118:Calcium / Q000737:chemistry*; D002364:Caseins / Q000737:chemistry*; D002413:Cations, Divalent / Q000737:chemistry; D003435:Crotalid Venoms; D009498:Neurotoxins; D019906:Nuclear Magnetic Resonance, Biomolecular; D010446:Peptide Fragments / Q000737:chemistry*; D010748:Phosphopeptides / Q000737:chemistry*
chemical_list: D002364:Caseins; D002413:Cations, Divalent; D003435:Crotalid Venoms; C018107:Mojave toxin; D009498:Neurotoxins; D010446:Peptide Fragments; D010748:Phosphopeptides; C412562:caseinophosphopeptide 1-25; D002118:Calcium