Troponin

Troponin is a complex of three regulatory proteins (troponin C, troponin I and troponin T) that is integral to muscle contraction in skeletal and cardiac muscle, but not smooth muscle.

Discussions of troponin often pertain to its functional characteristics and/or to its usefulness as a diagnostic marker for various heart disorders.

Function
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 binding to actin forms cross bridges and contraction (cross bridge cycling) 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. The actual amount of calcium that binds to troponin varies from expert to expert and source to source.

Physiology
Both cardiac and skeletal muscles are controlled by changes in the intracellular calcium concentration. 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 to which calcium binds to accomplish this regulation. Troponin has three subunits, TnC, TnI, and TnT. When calcium is bound to specific sites on TnC, tropomyosin rolls out of the way of the actin filament active sites, so that myosin (a molecular motor organized in muscle thick filaments) can attach to the thin filament and produce force and/or movement. In the absence of calcium, tropomyosin interferes with this action of myosin, and therefore muscles remain relaxed.

Troponin I has also been shown to inhibit angiogenesis in vivo and in vitro.

Individual subunits serve different functions:
 * Troponin C binds to calcium ions to produce a conformational change in TnI
 * Troponin T binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex
 * Troponin I binds to actin in thin myofilaments to hold the troponin-tropomyosin complex in place

Smooth muscle does not have troponin.

Diagnostic use
Troponin levels can be used as a test of several different heart disorders, including myocardial infarction.

Relation with contractile function and heart failure
Mutations in the cardiac troponin subunits can result in cardiomyopathies, including familial hypertrophic cardiomyopathy.