Comparison of the Mg2+ and Ca2+ binding properties of troponin complexes P1-TI2C and TI2C

The phosphoserine present in troponin T of freshly isolated skeletal muscle troponin P1-TI2C was dephosphorylated by alkaline phosphatase and the resulting troponin TI2C characterized by phosphorous content and gel electrophoresis in presence of sodium dodecylsulfate. Both complexes bind Ca2+ in an identical manner with a K0.5 of 5.3 X 10(-9) M for the Ca2+/Mg2+ binding sites and of 1.1 X 10(-6) M for the Ca2+-specific sites. 3.5 mM Mg2+ lowers the K0.5 value at the Ca2+/Mg2+ binding sites of 1.3 X 10(-7) M in the phospho-troponin P1-TI2C and leaves nearly unchanged the value of the dephosphorylated troponin TI2C at 1.2 X 10(-8) M. At 10 mM Mg2+ only one dissociation constant of about 1.0 X 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 X 10(-5) M to 2.0 X 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.