The Effect of Mg2+ on the Ca2+-Binding Properties of Non-activated Phosphorylase Kinase

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Abstract
The calcium binding properties of non-activated phosphorylase kinase at pH 6.8 have been studied by the gel filtration technique at calcium concentrations from 50 nM to 50 μM. Taking into account the subunit structure α4β4γ4 the enzyme binds 12 mol Ca2+ per mol with an association constant of 6.0 × 107 M−1, 4 mol with an association constant of 1.7 × 106 M−1 and 36 mol with a binding constant of 3.9 × 104 M−1 at low ionic strength. In buffer of high ionic strength, i.e. 180 mM NH4Cl or 60 mM (NH4)2SO4, only a single set of eight binding sites with a binding constant of 5.5 × 107 M−1 is left. In a buffer containing 155 mM NH4Cl and 10 mM MgCl2, the calcium affinity of these sites is reduced to a KCa of 3.0 × 106 M−1, indicating competition between Ca2+ and Mg2+. From these measurements, the binding constant of Mg2+ for these sites is calculated to be 1.7 × 103 M−1. Additionally, 10 mM Mg2+ induces a set of four new Ca2+ binding sites which show positive cooperativity. Their half-saturation constant under the conditions described is 3.5 × 105 M−1, and they, too, exhibit competition between Ca2+ and Mg2+. Since this set of sites is induced by Mg2+ a third group of binding sites for the latter metal must be postulated.