Troponin (Tn) isolated as the complex of TnC, TnI and TnT binds 4 moles of Ca2+ per mole in the presence of 2 mM MgCl2, 150 mM KCl, pH 7.0 with a single affinity constant of 5 x 10'M-1. In the absence of Mg2+ Tn contains 2 classes of sites with n1 = 2, K1 = 5 x 108--1; and n2 = 2, K2 = 5 x 106--1. Purified TnC, the calcium binding component of Tn, contains two classes of binding sites with n1 = 2, K1 = 2 x 106M-1; and n2 = 2, K2 = 2 x 105--1 (100 mM KCl, 2 mM MgCl2, pH 7.0). In the absence of Mg2+ the higher affinity constant increases to 2x107M-1 with no effect on the sites of lower affinity. The complex of TnC and the inhibitory component TnI has the same binding characteristics as Tn. Mg2+ binding studies are consistent with there being competition between Mg2+(K=103M-1) and Ca2+. Studies of myofibrillar ATPase activity as a function of pCa at several pMg levels show that full activation occurs only when all 4 sites of TnC are occupied.
|Original language||English (US)|
|Pages (from-to)||No. 1367|
|Issue number||5 II|
|State||Published - Jan 1 1974|
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