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Binding of Mg2+ and Ca2+ to β-casein A1: a multi-nuclear magnetic resonance study

Published online by Cambridge University Press:  01 June 2009

N. Magnus Wahlgren ,
Petr Dejmek  and
Torbjörn Drakenberg
N. Magnus Wahlgren
Affiliation:
Department of Food Technology, University of Lund, PO Box 124, S-221 00 Lund, Sweden
Petr Dejmek
Affiliation:
Department of Food Engineering, University of Lund, PO Box 124, S-221 00 Lund, Sweden
Torbjörn Drakenberg
Affiliation:
Department of Physical Chemistry 2, Chemical Center, University of Lund, PO Box 124, S-221 00 Lund, Sweden Chemical Laboratory, The Technical Research Center of Finland (VTT), PO Box 204, SF-0 2151 Espoo, Finland
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Summary

25Mg, 43Ca and 31P NMR have been used to study the binding of Mg2+ and Ca2+ ions to β-casein A1. The concentration dependence of the line width of the 25Mg NMR signal shows that β-casein contains at least two different types of binding sites for Mg2+ ions, one with strongly bound, slowly exchanging ions and one with more weakly bound ions which undergo fast exchange. The strong Mg2+ binding site has an unexpectedly high binding constant, Kbstrong 104 M–1, which has not been reported earlier. Mg2+ and Ca2+ compete for the Ca2+ binding sites of β-casein, while Na+ does not compete for these binding sites under physiological conditions. The dependence of the 43Ca NMR chemical shifts on total concentration of Mg2+ and Ca2+, in the presence of β-casein, could be equally well fitted with a model assuming up to five identical and independent sites as with a model assuming five or more sites with negative cooperativity. The proton dissociation constant, pka, for the strongest Ca2+ binding site was found to be 7·1.

Type
Original Articles
Information
Journal of Dairy Research , Volume 60 , Issue 1 , February 1993 , pp. 65 - 78
Copyright
Copyright © Proprietors of Journal of Dairy Research 1993

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