Q:

What is the tropomyosin molecule held in place by when at rest?

A:

Quick Answer

At rest, tropomyosin is held in place by troponin molecules. Thin muscle filaments are composed of actin, tropomyosin and troponin. During a muscle contraction, myosin proteins in thick muscle filaments bind to the actin in thin filaments, explains Linda Costanzo. Tropomyosin blocks actin until a muscle contraction is to occur.

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Full Answer

If contraction is to occur, tropomyosin must be moved out of the way so that actin and myosin can interact. A tropinin molecule is composed of three globular proteins: troponin T, tropinin I and troponin C. The troponin T globule connects the entire troponin complex to tropomyosin. The troponin I globule works with tropomyosin to cover the myosin-binding site on actin. The troponin C molecule has a Ca2+ binding site. When a muscle contraction is to occur, a muscle cell's Ca2+ concentration increases, and those calcium ions bind to troponin C. The binding Ca2+ ion changes the structural form of troponin, which causes tropomyosin to shift its position. Actin is no longer blocked, so the myosin head of the thick filament binds to the thin filament and a muscle contraction occurs.

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