troponintropomysinkomplekset

The troponin-tropomyosin complex, here called troponintropomysinkomplekset, is a regulatory protein assembly on the thin filament of striated muscle that controls contraction in response to intracellular calcium. It comprises the troponin complex (troponin C, I, T) and the coiled-coil tropomyosin molecule. Tropomyosin lies in the actin groove; troponin binds to tropomyosin and to actin at regular intervals along the filament.

The complex is positioned along the thin filament in a way that coordinates the exposure of myosin-binding sites on actin. In relaxed muscle, tropomyosin blocks these sites, preventing cross-bridge formation. When calcium levels rise, calcium binds to troponin C, inducing a conformational change that shifts tropomyosin away from the myosin-binding sites, allowing contraction. As intracellular calcium falls, the complex returns to its blocking position, leading to relaxation.

Functionally, the troponin-tropomyosin complex fine-tunes the calcium sensitivity of muscle contraction and integrates signaling pathways that regulate force production. Phosphorylation of troponin I by protein kinase A can decrease calcium affinity, modulating cardiac contractility in response to physiological states.

Clinically, cardiac troponin I and troponin T are released into the bloodstream after myocardial injury and serve as key diagnostic biomarkers. Genetic variants in related components (such as TNNC1, TNNI3, TNNT2, TPM1) can affect regulation and contribute to cardiomyopathies or congenital myopathies. The troponin-tropomyosin complex remains a central focus in studies of muscle physiology and pharmacologic modulation of calcium sensitivity.