Abstract
The increase observed in alpha helix from 20 to 38%, the spectral red shift of the absorption bands of the side chain chromophores, the increase of phenylalanine side chain optical activity and a decrease of cysteinyl side chain reactivity with 5,5'-dithiobis(2-nitrobenzoic acid) are consistent with a coil to helix change in the segment containing Cys-98 and Phe-99 and 102 when the CB9 fragment of troponin C (TnC) with an intact binding site III binds Ca2+. Since similar spectrophotometric observations have been made on the whole molecule the data with CB9 confirm our previous suggestion that binding site III of TnC is one of the strong binding sites for Ca2+. The estimate of the binding constant of Ca2+ for CB9 is about 2 orders of magnitude lower than in whole TnC, indicating further stabilization in the intact molecule.
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