Muscle fiber contraction is regulated through calcium-induced changes
in the conformation of troponin C. In this study, we explored the rela
tionship 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 th
e G-helix. The ion affinities of variant proteins were shifted in the
order Ile < Gly < Asp < Asn < Thr < Ser. Although differing in ion aff
inities, the variant proteins all exhibited high cooperativity. The re
sults of this study point to a specific relationship between alpha-hel
ix stability and ion affinity in troponin C and suggest that troponin
C may be a paradigm for protein folding problems.