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Cardiac muscle contraction is activated via the single Ca(2+)-binding site (site II) in the N-domain of troponin C (cTnC). The two Ca(2+)/Mg(2+) binding sites in the C-domain of cTnC (sites III and IV) have been considered to play a purely structural role in anchoring cTnC to the thin filament. However, several recent discoveries suggest a possible role of this domain in contractile regulation. The green tea polyphenol (-)-epigallocatechin 3-gallate (EGCg), which binds specifically to the C-domain of cTnC, reduces cardiac myofilament Ca(2+) sensitivity along with maximum force and acto-myosin ATPase activity. We have determined the effect of EGCg on Ca(2+) and Mg(2+) binding to the C-domain of cTnC. In the absence of Mg(2+) there was no significant effect of EGCg on the Ca(2+)-cTnC affinity. Surprisingly, in the presence of Mg(2+) EGCg caused an increase in Ca(2+) affinity for sites III and IV of cTnC. However, in the absence of Ca(2+) the addition of EGCg caused a significant reduction in Mg(2+)-cTnC affinity. This reduction is presumably responsible for the increase in Ca(2+)-cTnC affinity produced by EGCg in the presence of Mg(2+). We propose that the inhibitory effect of EGCg on myofilament Ca(2+) activation may be related to an enhanced Ca(2+)-Mg(2+)exchange at sites III and IV of cTnC, which might reduce the myosin crossbridge dependent component of thin filament activation.

The green tea polyphenol (−)-epigallocatechin-3-gallate inhibits magnesium binding to the C-domain of cardiac troponin C