Structure-Function Relations in Troponin C. Chemical Modification Studies
Troponin C (TnC) is the calcium binding component of thin filaments in skeletal and cardiac muscles. Calcium induced changes in the conformation of TnC transmitted to other components of the thin filament result in triggering of muscle contraction (for review see Leavis and Gergely 1984; Zot and Potter 1987). The 3-D crystal structure of TnC isolated from turkey and chicken skeletal muscle has been solved and refined recently at better than 0.2 nm resolution (Satyshur et al 1988; Herzberg and James 1988). The molecule appears to have two globular domains each containing a pair of Ca2+ -binding sites. Each site consists of a Ca2+ -binding loop flanked by two -helices, which are labeled A through H starting with the N-terminus. Sites I and II in the N-terminal domain are the “triggering”, low affinity, Ca2+-specific sites, and sites III and IV in the C-terminal domain are the high affinity Ca2+ -Mg2+ sites believed to be always occupied by a divalent metal in the living muscle. The two domains are linked by a 29-residue -helix (D + E or central helix) a considerable portion of which is exposed to the solvent. Since under the conditions of crystallization only sites III and IV are filled with Ca2+ the crystal structure represents, at best, the “inhibited” form of TnC. Herzberg et al. (1986) speculated that the conformational changes associated with Ca2+ -binding to sites I and II involve a change in the relative disposition of helices in the N-terminal domain leading to a structure similar to that found at the Ca2+-filled sites III and IV in the C-terminal domain of TnC and in other Ca2+ -binding proteins of known 3-D structure. Such transition would result in separation of helices B and C from the central helix. The Ca2+ induced increase in accessibility of Cys-84 in cardiac TnC (Ingraham and Hodges, 1988, Fuchs and Grabarek 1989) seems to be consistent with such a model.
KeywordsCircular Dichroism Spectrum Thin Filament Charged Side Chain Central Helix Chicken Skeletal Muscle
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