Troponiiniin

Troponiiniin, commonly referred to as troponin in Finnish texts, is a regulatory protein complex in skeletal and cardiac muscle that modulates muscle contraction. It consists of three subunits: troponin C (TN-C), which binds calcium; troponin I (TN-I), the inhibitory subunit that blocks actin-myosin interaction in the absence of calcium; and troponin T (TN-T), which anchors the complex to tropomyosin on the actin filament. In resting muscle, TN-I prevents cross-bridge formation by holding tropomyosin over myosin-binding sites on actin. When calcium concentrations rise during excitation, calcium ions bind to TN-C, triggering a conformational shift that moves tropomyosin away from the binding sites and relieves TN-I inhibition, enabling actin-myosin interactions and contraction.

Cardiac muscle expresses cardiac-specific isoforms, cTnI and cTnT, while skeletal muscle expresses their own isoforms. Because cardiac troponins I and T are released into the bloodstream when cardiac muscle is damaged, cTnI and cTnT are widely used as biomarkers for myocardial infarction and other forms of cardiac injury. High-sensitivity assays can detect even small injuries, and troponin levels are interpreted with clinical context and time since onset.

Troponin complexes are regulators of contraction and are conserved across vertebrates; alterations or mutations in troponin genes can lead to cardiomyopathies.