What inhibits linkage between actin and myosin filaments
The linkage between actin and myosin filaments is inhibited by a molecule known as tropomyosin and a protein complex called troponin.
Tropomyosin is a long, filamentous molecule that lies within the grooves of the F-actin helix, which is the filamentous form of actin. In its resting state, tropomyosin covers the myosin-binding sites on the actin filaments, preventing myosin heads from binding to actin. This prevents the cross-bridge formation between actin and myosin, thus inhibiting muscle contraction.
Troponin is a protein complex that consists of three subunits: troponin I, troponin T, and troponin C. Troponin is attached to tropomyosin and is responsible for regulating the exposure of the myosin-binding sites on actin. Troponin I inhibits the interaction between actin and myosin by blocking the binding sites on actin when the muscle is at rest. Troponin C binds to calcium ions, and when calcium levels increase within the muscle cell, it triggers a conformational change in troponin complex. This change causes tropomyosin to shift position, exposing the myosin-binding sites on actin, which allows the actin and myosin filaments to interact and initiate muscle contraction.
Therefore, both tropomyosin and troponin play essential roles in inhibiting the linkage between actin and myosin filaments, preventing muscle contraction when it is not required.
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