Troponiinitropomysiinijärjestelmä

The troponin tropomyosin system is a crucial regulatory mechanism in muscle contraction, particularly in striated muscle (skeletal and cardiac muscle). It is composed of three distinct protein subunits: troponin T (TnT), troponin I (TnI), and troponin C (TnC). These troponin subunits are part of a larger complex that sits on the actin filaments.

Tropomyosin is a filamentous protein that also associates with actin. In a relaxed muscle state, tropomyosin physically covers the myosin-binding sites on the actin filaments. This prevents the myosin heads from interacting with actin, thus inhibiting muscle contraction.

The troponin complex acts as a switch that controls the position of tropomyosin. When calcium ions (Ca2+) are present in the muscle cell, they bind to troponin C. This binding causes a conformational change in the troponin complex. This change is transmitted to troponin T and troponin I, which in turn causes tropomyosin to shift its position on the actin filament.

As tropomyosin moves away from the myosin-binding sites, these sites become exposed. This allows the myosin heads to bind to actin, initiating the cross-bridge cycle that drives muscle contraction. When calcium levels decrease, calcium detaches from troponin C, the troponin complex returns to its original conformation, and tropomyosin once again covers the myosin-binding sites, leading to muscle relaxation. This precise regulation is essential for coordinated muscle movement.