<unk>  is a complex of three proteins that is integral to muscle contraction in skeletal and cardiac muscle, but not smooth muscle.<unk>  is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin crossbridge, thus preventing contraction. When the muscle cell is stimulated to contract by an action potential, calcium channels open in the sarcoplasmic reticulum and release calcium into the sarcoplasm. Some of this calcium attaches to<unk> , causing a conformational change that moves tropomyosin out of the way so that the cross bridges can attach to actin and produce muscle contraction.<unk>  is found in both skeletal muscle and cardiac muscle, but the specific versions of<unk>  differ between types of muscle. The main difference is that the<unk> C subunit of<unk>  in skeletal muscle has four calcium ion binding sites, whereas in cardiac muscle there are only three.
Discussions of<unk>  often pertain to its functional characteristics and/or to its usefulness as a diagnostic marker for various heart disorders. When cardiac injury occurs (such as in case of an acute MI), these intracellular proteins are then released into the bloodstream. Along with the patient's history and the electrocardiogram, the release of these enzymes forms the basis of the diagnosis of ST elevation myocardial infarction.
Until the 1980s, the enzymes SGOT and LDH were used to assess cardiac injury. In the early 1980s it was found that disproportional elevation of the MB subtype of the enzyme creatine kinase (CK) was very specific for myocardial injury. More recently,<unk>  sub-units I or T, have been used as an even more specific marker of myonecrosis. The 2007 Joint ESC/ACCF/AHA/WHF Task Force for the definition of myocardial infarction emphasized the importance of both elevated cardiac biomarkers and clinical evidence for MI.
Troponin
Creatine kinase
Tropomyosin
Myosin
Lipase
Amylase
D-dimer
Aspartate transaminase