Helix variants of troponin C with tailored calcium affinities.

Muscle fiber contraction is regulated through calcium-induced changes in the conformation of troponin C. In this study, we explored the relationship between the stability of a specific helix in the protein and the metal ion affinity of associated binding sites. Serial replacement of the amino acid at position 130 caused the calcium affinity of the paired Ca2+/Mg2+ sites to be attenuated. In the crystal structures of chicken and turkey troponin C, position 130 is the N-cap residue of the G-helix. The ion affinities of variant proteins were shifted in the order Ile < Gly < Asp < Asn < Thr < Ser. Although differing in ion affinities, the variant proteins all exhibited high cooperativity. The results of this study point to a specific relationship between alpha-helix stability and ion affinity in troponin C and suggest that troponin C may be a paradigm for protein folding problems.