troponinatropomiosina

Troponinatropomiosina, commonly referred to as the troponin-tropomyosin complex, is a regulatory assembly on the thin filament of striated muscle that governs calcium-dependent contraction. It sits with tropomyosin along actin filaments in skeletal and cardiac muscle, where it modulates access of myosin to its binding sites.

The complex consists of three troponin subunits: troponin C, which binds calcium; troponin I, which inhibits actin–myosin interaction; and troponin T, which anchors the complex to tropomyosin. Tropomyosin itself is a long coiled-coil protein that runs in the groove of actin filaments, blocking myosin-binding sites when the muscle is relaxed.

In the absence of calcium, troponin I holds tropomyosin over the binding sites, preventing cross-bridge formation. When intracellular calcium rises, calcium binds to troponin C, triggering conformational changes that reduce the inhibitory effect of troponin I and shift tropomyosin away from the myosin-binding sites, enabling contraction. Relaxation occurs when calcium is removed and the regulatory complex returns to its blocking position.

Clinical relevance: cardiac troponin I and cardiac troponin T are widely used as biomarkers of myocardial injury; elevated levels in the blood indicate cardiac damage such as myocardial infarction. Mutations in the genes encoding troponin or tropomyosin subunits can cause inherited cardiomyopathies and skeletal myopathies, reflecting the central role of the complex in muscle regulation.

The troponin-tropomyosin system is a defining feature of striated muscle regulation and is subject to extensive study in physiology, medicine and structural biology.