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Troponin T (blue) anchors the complex on tropomyosin. Troponin is found in both skeletal muscle and cardiac muscle, but the specific versions of troponin differ between types of muscle. The main difference is that the TnC subunit of troponin in skeletal muscle has four calcium ion-binding sites, whereas in cardiac muscle there are only three.
It contains four calcium-binding EF hands, although different isoforms may have fewer than four functional calcium-binding subdomains. It is a component of thin filaments, along with actin and tropomyosin. It contains an N lobe and a C lobe. The C lobe serves a structural purpose and binds to the N domain of troponin I (TnI).
The calcium sensitizer, levosimendan, is purported to bind to troponin C, but only weak or inconsistent binding has been detected, [57] [58] [59] precluding any structure determination. In contrast, levosimendan inhibits type 3 phosphodiesterase with nanomolar affinity, [ 60 ] so its biological target is controversial.
Troponin (Tn), is a key protein complex in the regulation of striated muscle contraction, composed of three subunits. The TnI subunit inhibits actomyosin ATPase , the TnT subunit binds tropomyosin and TnC, while the TnC subunit binds calcium and overcomes the inhibitory action of the troponin complex on actin thin filaments .
Calcium sensitizers function by binding to cardiac troponin C, thereby enhancing the sensitivity of heart muscle cells to naturally occurring calcium ions. [51] This heightened sensitivity fosters a more efficient interaction between calcium and the contractile apparatus of the heart muscle. [52]
Figure 1: Calcium binding to troponin, exposes sites on the actin filaments, to which myosin binds. Using ATP, myosin moves the actin along. The myosin releases the actin, resets itself and binds to another actin binding site. The increase in Ca 2+, produced by CICR, now does two things.
EF-hand Ca 2+ binding motif. The calcium ion is coordinated in a pentagonal bipyramidal configuration. The six residues involved in the binding are in positions 1, 3, 5, 7, 9 and 12; these residues are denoted by X, Y, Z, -Y, -X and -Z. The invariant Glu or Asp at position 12 provides two oxygens for liganding calcium (bidentate ligand).
Test Sensitivity and specificity Approximate peak Description Troponin test: The most sensitive and specific test for myocardial damage. Because it has increased specificity compared with CK-MB, troponin is composed of 3 proteins- Troponin C, Cardic troponin I, and Cardiac troponin T. Troponin I especially has a high affinity for myocardial injury.