Common structural framework of the two Ca2+/Mg2+ binding loops of troponin C and other Ca2+ binding proteins.
The refinement of the crystal structure of turkey skeletal muscle troponin C at 2.2-A resolution reveals that the two calcium binding loops that are occupied by Ca2+ ions adopt conformations very similar to those of the two homologous loops of parvalbumin and to that of loop III-IV of the intestinal calcium binding protein. This specific fold assures suitable spatial positioning of the Ca2+ ligands. It consists of two reverse turns, one located at each end of the loop, and four Asx turns (a cyclic hydrogen-bonded structure involving an oxygen of the side chain of residue n and the main-chain amide nitrogen of residue n + 2) whenever such a side chain coordinates to the metal ion. The fifth Ca2+ coordination position in both loops of troponin C is occupied by a water molecule that is within hydrogen-bonding distance of an aspartic acid, thus mediating indirect interaction between the cation and the negatively charged carboxylate. The same loop framework is conserved in the two Ca2+ binding loops of parvalbumin and loop III-IV of the intestinal Ca2+ binding protein in spite of the variability in the nature of the side chains at equivalent positions. The disposition of the Ca2+ and of its coordinating water molecule relative to the protein main chain is conserved in all these cases.
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism*; D008958:Models, Molecular; D009132:Muscles / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D014336:Troponin / Q000378:metabolism*; D019209:Troponin C; D014422:Turkeys
D002135:Calcium-Binding Proteins; D014336:Troponin; D019209:Troponin C; D008274:Magnesium; D002118:Calcium