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Troponin is a complex of three proteins that is integral to muscle contraction in skeletal and cardiac muscle, but not smooth muscle. 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 reticulum and release calcium into the sarcoplasm. Some of this calcium attaches to troponin, causing a conformational change that moves tropomyosin out of the way so that the cross bridges can attach to actin and produce muscle contraction.
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.
Discussions of troponin often pertain to its functional characteristics and/or to its usefulness as a diagnostic marker for various heart disorders.
Role of troponins
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:
Certain subtypes of troponin (cardiac troponin I and T) are very sensitive and specific indicators of damage to the heart muscle (myocardium). They are measured in the blood to differentiate between unstable angina and myocardial infarction (heart attack) in patients with chest pain. A patient who had suffered from a myocardial infarction would have an area of damaged heart muscle and so would have elevated cardiac troponin levels in the blood.
It is important to note that cardiac troponins are a marker of all heart muscle damage, not just myocardial infarction. Other conditions that directly or indirectly lead to heart muscle damage can also therefore increase troponin levels:
Detection of cardiac troponin
Cardiac troponin T and I are measured by immunoassay methods. Due to patent regulations a single manufacturer distributes cTnT. A host of diagnostic companies make cTnI immunoassay methods available on many different immunoassay platforms.
Detection of cardiotoxixity and cardioprotection
Drug-induced cardiotoxicity is common to all classes of therapeutic drugs. It is essential that cardiotoxicity is detected with a high degree of sensitivity and specificity. The newly developed troponins are especially useful in this context
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Troponin". A list of authors is available in Wikipedia.|