The calcium and magnesium binding sites on cardiac troponin and their role in the regulation of myofibrillar adenosine triphosphatase

J Biol Chem. 1980 Dec 25;255(24):11688-93.


The cardiac troponin (Tn) complex, consisting of a Ca2+-binding subunit (TnC), an inhibitory subunit (TnI), and a tropomyosin-binding subunit (TnT), has been reconstituted from purified troponin subunits isolated from bovine heart muscle. The Ca2+-binding properties of cardiac Tn were determined by equilibrium dialysis using either EGTA or EDTA to regulate the free Ca2+ concentration. Cardiac Tn binds 3 mol Ca2+/mol and contains two Ca2+-binding sites with a binding constant of 3 X 10(8) M-1 and one binding site with a binding constant of 2 X 10(6) M-1. In the presence of 4 mM MgC12, the binding constant of the sites of higher affinity is reduced to 3 X 10(7) M-1, while Ca2+ binding to the site at the lower affinity is unaffected. The two high affinity Ca2+-binding sites of cardiac Tn are analogous to the two Ca2+-Mg2+ sites of skeletal Tn, while the single low affinity site is similar to the two Ca2+-specific sites of skeletal Tn (Potter, J. D., and Gergely, J. (1975) J. Biol. Chem. 250, 4625-5633). The Ca2+-binding properties of the complex of TnC and TnI (1:1 molar ratio) were similar to those of Tn. Cardiac TnC also binds 3 mol of Ca2+/mol and contains two sites with a binding constant of 1 X 10(7) M-1 and a single site with a binding constant of 2 X 10(5) M-1. Assuming competition between Mg2+ and Ca2+ for the high affinity sites of TnC and Tn, the binding constants for Mg2+ were 0.7 and 3.0 X 10(3) M-1, respectively. The Ca2+ dependence of cardiac myofibrillar ATPase activity was similar to that of an actomyosin preparation regulated by the reconstituted troponin complex. Comparison by the Ca2+-binding properties of cardiac Tn and the cardiac myofibrillar ATPase activity as a function of [Ca2+] and at millimolar [Mg2+] suggests that activation of the ATPase occurs over the same range of [Ca2+] where the Ca2+-specific site of cardiac Tn binds Ca2+.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adenosine Triphosphatases / metabolism*
  • Animals
  • Calcium* / pharmacology
  • Cattle
  • Kinetics
  • Macromolecular Substances
  • Magnesium* / pharmacology
  • Muscle Proteins / metabolism*
  • Myocardium / enzymology*
  • Myofibrils / enzymology*
  • Protein Binding
  • Troponin / isolation & purification
  • Troponin / metabolism*


  • Macromolecular Substances
  • Muscle Proteins
  • Troponin
  • Adenosine Triphosphatases
  • Magnesium
  • Calcium