Binding of Mg2+ to beta-casein A1: A multinuclear magnetic resonance study

N. Wahlgren, P. Dejmek, Torbjörn Drakenberg

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Abstract

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.
Original languageEnglish
Pages (from-to)65-78
JournalJournal of Dairy Research
Volume60
Issue number1
DOIs
Publication statusPublished - 1993
MoE publication typeA1 Journal article-refereed

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