Troponin

Troponin 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. It is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. When calcium is present, it binds to troponin, causing a conformational change in the troponin-tropomyosin complex. This shift exposes the myosin-binding sites on the actin filaments, allowing cross-bridge formation and muscle contraction.

Structure and Function:

• Troponin C (TnC): The calcium-binding subunit. It binds calcium ions (Ca2+), triggering the conformational change that initiates muscle contraction. Different isoforms exist for cardiac and skeletal muscle.

• Troponin I (TnI): The inhibitory subunit. It binds to actin, inhibiting contraction by preventing myosin from binding. Different isoforms exist for cardiac and skeletal muscle. Cardiac Troponin I (cTnI) is highly specific to the heart.

• Troponin T (TnT): The tropomyosin-binding subunit. It binds to tropomyosin, which helps position the troponin complex on the actin filament. Different isoforms exist for cardiac and skeletal muscle. Cardiac Troponin T (cTnT) is highly specific to the heart.

Clinical Significance:

Cardiac troponins (cTnI and cTnT) are highly specific biomarkers for myocardial damage (heart muscle damage). Elevated levels of cardiac troponins in the blood are indicative of myocardial infarction (heart attack) or other conditions that cause damage to the heart muscle, such as myocarditis, heart failure, or pulmonary embolism. Cardiac troponin levels are measured in the diagnosis and management of acute coronary syndrome (ACS) and are essential for differentiating between unstable angina and myocardial infarction. Serial measurements of troponin levels are often performed to assess the extent and progression of myocardial damage. Because of their high specificity for cardiac tissue, cardiac troponin assays have largely replaced older markers of cardiac injury, such as creatine kinase-MB (CK-MB).