[The presence of cooperative structures in tryptic fragments of troponin C]

Scanning microcalorimetry has been used to study the ability of TR-1 and TR-2 tryptic fragments of troponin C to form an ordered compact structure in solution under different conditions. It has been shown that: (1) in the presence, as well as in the absence of bivalent ions both fragments have a structure which can melt with an intensive heat absorption at heating; (2) the structure of fragment TR-1 containing two Ca2+-specific domains (domains I and II) melts as a whole under all conditions studied and therefore the domains form one cooperative block. Binding of Ca2+ or Mg2+ ions stabilizes the block structure, however, significant conformational rearrangements which would lead to a change of denaturational enthalpy do not occur; (3) Ca2+Mg2+-domains of fragment TR-2 (domains III and IV) represent individual cooperative units, blocks. Stability of these cooperative blocks strongly depends on concentration of bivalent ions and in the presence of 2 mM EDTA the melting temperature of one of them is below 10 degrees. Thermodynamic melting temperature of one of them is below 10 degrees. Thermodynamic melting parameters of cooperative blocks within peptides and in the intact molecule of troponic C are compared.