Troponi

Troponi (plural of troponin) refers to a family of regulatory proteins that form the troponin complex on the thin filaments of skeletal and cardiac muscle. The complex consists of three subunits: troponin C (TnC), which binds calcium; troponin I (TnI), which inhibits actin–myosin interaction; and troponin T (TnT), which anchors the complex to tropomyosin. When intracellular calcium rises during excitation-contraction coupling, calcium binds to TnC, triggering a conformational change that moves tropomyosin away from the myosin-binding sites on actin and enables cross-bridge cycling and contraction. Removal of calcium returns troponin to its inhibited state and promotes relaxation.

Troponin isoforms exhibit tissue specificity. Cardiac muscle expresses cardiac troponin I (cTnI) and cardiac troponin T (cTnT), which differ from their skeletal muscle counterparts. The troponin C isoforms are more conserved across tissues, so cardiac troponin C is not used clinically as a heart-specific biomarker.

Clinical relevance is most prominent for the cardiac-specific subunits. Cardiac troponins I and T are released into the bloodstream when cardiomyocytes are damaged, and their elevated levels are central to diagnosing myocardial infarction. High-sensitivity assays improve early detection but can be influenced by factors such as renal function, age, and comorbidities. Troponin testing is widely used to assess suspected acute coronary syndromes and to monitor cardiac injury over time.

Notes: in some languages or contexts, troponi may simply refer to troponin proteins in general. In English medical usage, troponin and its cardiac isoforms are the standard terms.