Calcium binding to troponin C. II. A Ca2+ ion titration study with a Ca2+ ion sensitive electrode.
The effects of pH,Mg2+, and ionic strength on Ca2+ binding to rabbit skeletal troponin C were studied by using a Ca2+ sensitive electrode. Troponin C has two high affinity and two low affinity sites and the Ca2+ affinity of both sites was increased by increasing pH in a pH range from pH 5.6 to 10.4. The affinity was decreased by increasing ionic strength. The change of the Ca2+ affinity can be explained by the electrostatic interaction between Ca2+ and the protein. At alkaline pH, the four Ca2+ binding sites bind Ca2+ with the same affinity and the distinction between the high and the low affinity sites vanished. This result shows that the difference of the Ca2+ affinity is owing to differences of the secondary or the tertiary structure of the Ca2+ binding sites, not owing to a difference of the primary structures of the Ca2+ binding sites. The two high affinity sites bound two Ca2+ ions cooperatively in neutral pH. The cooperativity was diminished at both acidic and alkaline pH. Mg2+ ion decreased the affinity of the low affinity sites.
Iida
S
S
Department of Physics, Faculty of Science, Nagoya University, Aichi.
D000818:Animals; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism*; D004566:Electrodes; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008274:Magnesium / Q000494:pharmacology; D009132:Muscles / Q000378:metabolism; D011817:Rabbits; D014336:Troponin / Q000378:metabolism*; D019209:Troponin C
D007477:Ions; D014336:Troponin; D019209:Troponin C; D008274:Magnesium; D002118:Calcium