Comparison of the Mg2+ and Ca2+ Binding Properties of Troponin Complexes P1‐TI2C and TI2C

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Abstract
The phosphoserine present in troponin T of freshly isolated [rabbit] skeletal muscle troponin P1-TI2C was dephosphorylated by alkaline phosphatase and the resulting troponin Tl2C characterized by phosphorous content and gel electrophoresis in presence of sodium dodecyl sulfate. Both complexes bind Ca2+ in an identical manner with a K0.5 of 5.3 .times. 10-9 M for the Ca2+/Mg2+ binding sites and of 1.1 .times. 10-6 M for the Ca2+-specific sites. Mg2+ (3.5 mM) lowers the K0.5 value at the Ca2+/Mg2+ binding sites to 1.3 .times. 10-7 M in the phospho-troponin P1-TI2C and leaves nearly unchanged the value of the dephosphorylated troponin TI2C at 1.2 .times. 10-8 M. At 10 mM Mg2+ only 1 Kd of about 1.0 .times. 10-6 M is determined with both complexes. In analogy dephosphorylation of troponin P1-TI2C reduces the affinity for Mg2+ at the Ca2+/Mg2+ binding sites from 6.7 .times. 10-5 M to 2.0 .times. 10-3 M. Again the Mg2+-specific sites are uninfluenced. The possibility is discussed that removal of the phosphate group from troponin T allows the interaction of the N-terminal domain of troponin T with other amino acid side chains of troponin.