The three-dimensional structure of troponin-C: Supersecondary structural homology of calcium binding folds

Abstract

The X-ray structure of chicken troponin-C at 3 A resolution was recently reported [1]. The structure exhibits an unusual dumbbell shape with the–COOH and—NH2 domains connected by a long α-helix of about nine turns. Only the two Ca2+-binding sites of the—COOH domain are occupied by metal ions. The individual helix-loop-helix (HLH) calcium-binding motifs as well as the combined calcium-binding sites in the—COOH domain exhibit a supersecondary structure similar to that of parvalbumin. However, the helix–loop–helix supersecondary structure of the two putative calcium-binding folds (metal free) in the—NH2 domain, while similar to each other, are markedly different from those of the—COOH domain. The conformational differences between the four Ca2+ (wherein the HLH regions of the—NH2 domain also display the same conformation as the—COOH domain) and two Ca2+ states of TnC would provide a better insight into the conformational changes in TnC and the molecular interactions of TnC with TnI and TnT in the troponin complex.