Anatomy & Organs

Troponin – structure, function & diseases

Troponin

Troponin is a complex of three globular protein subunits. As part of the muscle contractile apparatus, troponin regulates muscle contraction. It is of particular importance in the diagnosis of heart attacks .

What is troponin?

As a component of the actin filament , troponin is part of the contractile unit of skeletal and cardiac muscle . It is a complex of globular proteins that together with F-actin and tropomyosin form the actin filament.

The actin filament interacts with the myosin filaments to allow muscle contraction. Due to its ability to start or stop muscle contraction, troponin is also known as a muscle regulator protein along with tropomyosin . The protein complex troponin consists of three subunits, the inhibitory troponin I, the troponin T responsible for tropomyosin binding and the calcium-binding troponin C.

Cardiac troponin plays an important role in the emergency diagnosis of heart attacks. If the heart muscles are damaged, the subunit troponin I is released and can be detected in the blood by laboratory diagnostics. This results in a classic progression that allows differentiation from other diseases of the muscles.

Anatomy & Structure

Troponin is part of the actin filaments which, through their interaction with the myosin filaments, allow the muscle to contract. Both filaments form the smallest contractile unit of muscle, the sarcomere . Troponin is a complex of globular proteins made up of three subunits.

A distinction is made between inhibitory troponin (TnI), tropomyosin-binding troponin (TnT) and calcium-binding troponin (TnC). Three troponin peptides regularly follow seven F-actin molecules in the filament. They lie as a complex almost horizontally in the actin filament. Troponin T binds to tropomyosin, which is bound to F-actin, on one side and to troponin I on the other. Troponin I has a strong affinity for F-actin, so that it is bound to F-actin in the non-contracted state. Troponin C also binds to troponin I and is exposed to the outside.

Troponin C is the smallest of the subunits and has a calcium-binding domain. Depending on the musculature, there are three isoforms of troponin I and troponin T. Cardiac troponin (cTn) is found in the cardiac musculature and two different troponins (sTn) exist in the skeletal musculature for the fast and slow skeletal muscle fibers.

Function & Tasks

As a component of the contractile apparatus, troponin plays an important role in the regulation of muscle contraction. In the non-excited state, the position of the tropomyosin filaments prevents the binding of the actin filament to the myosin head. Only when the tropomyosin is pulled further into the interior of the filament helix by troponin T is the binding site for myosin exposed. This change in position is achieved by a change in conformation in the troponin complex as a result of an increase in the calcium concentration. Calcium is released into the muscle fibers by electrical excitation of the plasma membrane .

Troponin C is the calcium receptor in the actin filament because it has a calcium-binding domain. This in turn consists of two structures, each of which carries four calcium binding sites. Two of these binding sites each have high affinity for calcium, two have low affinity. Only the low-affinity binding sites are involved in the contraction. The conformational change of troponin C after calcium binding is directly transmitted by troponin T to the tropomyosin, which is further drawn into the groove between the actin strands and frees the binding site for the myosin head.

At the same time, the inhibitory effect of troponin I on the ATPase is reversed and ATP can be cleaved at the myosin, resulting in the myosin head breaking off. The actin filament is pulled along the myosin filament and the muscle contracts. The binding of myosin and actin is broken by the addition of new ATP to the myosin. The calcium level in the muscle fiber drops and the tropomyosin threads cover the myosin binding site again. The muscle is in relaxation .

Diseases

The troponin value is the most important laboratory diagnostic parameter for heart attacks. When the heart muscle is damaged, the cardiac troponin, in particular troponin T and troponin I, is released into the blood . The troponin level can be determined in serum, plasma or whole blood.

The concentration of troponin in the blood shows a typical course after a heart attack, so that it can be differentiated from other damage to the heart muscle. An increase in troponin occurs about 3-8 hours after the onset of the heart attack. The highest values ​​can be measured 12-96 hours after the start. It takes about two weeks for the troponin level in the blood to return to normal after a heart attack. If the troponin level falls, there is a high probability that it is not a heart attack but another cause such as overstretching, inflammation of the skeletal muscles or other injuries.

Elevated levels of troponin are also found in a variety of other disorders associated with muscle tissue destruction. For example, the troponin level increases in the event of functional disorders or inflammation of the heart, diseases of the blood vessels, inflammation or injuries to the skeletal muscles, stroke , functional disorders of the lungs or even burns and sepsis . The rise in troponin is considered a major risk factor for the mortality rate after surgery. Since there are several days between a rise in troponin levels and the death of the patient, medication can be used in good time.

The increase in troponin levels after heavy physical exertion such as endurance sports has no disease value. The values ​​usually normalize within a few hours, so that the normal value is available again after a maximum of 72 hours.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.