Troponin

Troponin, or the troponin complex, is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that is integral to muscle contraction in skeletal muscle and cardiac muscle, but not smooth muscle. Blood troponin levels may be used as a diagnostic marker for stroke, although the sensitivity of this assay is low. Assays of cardiac-specific troponins I and T are extensively used as diagnostic and prognostic indicators in the management of myocardial infarction and acute coronary syndrome . Troponin, or the troponin complex, is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that is integral to muscle contraction in skeletal muscle and cardiac muscle, but not smooth muscle. Blood troponin levels may be used as a diagnostic marker for stroke, although the sensitivity of this assay is low. Assays of cardiac-specific troponins I and T are extensively used as diagnostic and prognostic indicators in the management of myocardial infarction and acute coronary syndrome . Troponin 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 membrane and release calcium into the sarcoplasm. Some of this calcium attaches to troponin, which causes it to change shape, exposing binding sites for myosin (active sites) on the actin filaments. Myosin's binding to actin causes crossbridge formation, and contraction of the muscle begins. 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. Views on the actual amount of calcium that binds to troponin vary from expert to expert and source to source. In both cardiac and skeletal muscles, muscular force production is controlled primarily by changes in the intracellular calcium concentration. In general, when calcium rises, the muscles contract and, when calcium falls, the muscles relax. Troponin is a component of thin filaments (along with actin and tropomyosin), and is the protein complex to which calcium binds to trigger the production of muscular force. Troponin itself has three subunits, TnC, TnI, and TnT, each playing a role in force regulation. Under resting intracellular levels of calcium, tropomyosin covers the active sites on actin to which myosin (a molecular motor organized in muscle thick filaments) binds in order to generate force. When calcium becomes bound to specific sites in the N-domain of TnC, a series of protein structural changes occurs such that tropomyosin is rolled away from myosin-binding sites on actin, allowing myosin to attach to the thin filament and produce force and/or shorten the sarcomere.