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Interaction of oxytocin with Ca 2+ : II. Proton magnetic resonance and molecular modeling studies of conformations of the hormone and its Ca 2+ complex
Author(s) -
Ananthanarayanan V. S.,
Belciug M.P.,
Zhorov B. S.
Publication year - 1996
Publication title -
peptide science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/(sici)1097-0282(1996)40:5<445::aid-bip3>3.0.co;2-w
Subject(s) - chemistry , oxytocin , proton magnetic resonance , hormone , nuclear magnetic resonance , biophysics , biochemistry , neuroscience , psychology , physics , biology
Drastic changes in the CD and fluorescence spectra of oxytocin [cyclo(Cys 1 ‐Tyr 2 ‐Ile 3 ‐Gln 4 ‐Asn 5 ‐Cys 6 )‐Pro 7 ‐Leu 8 ‐Gly 9 ‐NH 2 ] occur on binding Ca 2+ in trifluoroethanol (Ananthanarayanan and Brimble, preceding paper). To further characterize the conformation of the Ca 2+ ‐bound hormone, we carried out 1 H‐nmr measurements in deuterated trifluoroethanol of oxytocin and its 1 : 1 Ca 2+ complex. The one‐dimensional nmr data identified residues involved in Ca 2+ binding and the extent of their perturbation on Ca 2+ addition. The 3 J NH‐CH coupling constants and two‐dimensional nuclear Overhauser effect (NOE) spectral cross peaks confirmed the helical nature of the Ca 2+ complex deduced from CD data. Interproton distances in the free hormone and its Ca 2+ complex were estimated from the respective NOE data. Apparent global minimum‐energy conformations of free and Ca 2+ bound oxytocin were computed using the Monte Carlo with energy minimization protocol, with and without incorporating the NOE‐derived distance constraints. Taken together, our results show Ca 2+ binding to oxytocin to be a two‐step process. The binding of the first Ca 2+ brings the otherwise extended tail segment of oxytocin closer to the ring moiety so that it wraps around the cation. This causes the maximal extent of change in all the spectral parameters. The subsequent formation of the 2 : 1 Ca‐oxytocin complex results in the tail detaching itself away from the ring so as to bind the second Ca 2+ ion. This leads to further spectral changes in the hormone molecule. The tail segment plays a major role in both steps. These observations may be useful in understanding the structural basis of oxytocin action. © 1997 John Wiley & Sons, Inc. Biopoly 40: 445–464, 1996