Calcium and cadmium binding to troponin C. Evidence for cooperativity.

Proton NMR is used to compare the structural changes induced in bovine cardiac troponin C on binding of cadmium and calcium ions. The same spectral changes are observed for both ion species. The rate of the conformational changes associated with cadmium binding to the two high-affinity sites is slow, that associated with cadmium ions binding to the low-affinity site is high. 113Cd-NMR spectra of cardiac troponin C feature two signals interpreted as due to cadmium ions bound to the strong sites. Strong arguments are given in favour of cooperativity in binding of the first two cadmium or calcium ions to cardiac and skeletal muscle troponin C.