Regulation of Contraction

In the scheme summarized in Figure 10-7, there is no mechanism to control the interaction between actin and myosin. That is, as long as ATP is present, we would expect every muscle in the body to be in a perpetual state of maximum contraction. This section will examine the molecular mechanisms that prevent the interaction of actin with myosin except when a contraction is triggered by an action potential in the muscle cell membrane.

Recall that thin filaments also contain the proteins troponin and tropo-myosin. These proteins are responsible for regulating the interaction between individual myosin and actin molecules in the thick and thin filaments. The regulatory scheme is summarized by the diagrams in Figure 10-9. In the resting muscle, tropomyosin is in a position on the thin filament that allows it to effectively cover the myosin binding site on actin. Myosin's access to the binding site is blocked by the tropomyosin. The position of tropomyosin on the actin polymer is in turn regulated by troponin. In the resting state, troponin locks tropomyosin in the blocking position. Thus, tropomyosin acts like a trapdoor

Figure 10-9 The regulation of the interaction between actin and myosin by calcium ions, troponin, and tropomyosin. In the absence of calcium ions, tropomyosin blocks access to the myosin-binding site of actin (upper diagram). In the absence of calcium ions, troponin locks tropomyosin in the blocking position. When calcium binds to troponin, the positions of troponin and tropomyosin are altered on the thin filament, and myosin then has access to its binding site on actin. The cycle of filament sliding is then free to begin. (Animation available at www.blackwellscience.com)

Troponin Tropomyosin Ca2+ binding site

Troponin Tropomyosin Ca2+ binding site

Thin filament

Thin filament

Actin

Actin

Thick filament

Thick filament

Ca2+

Ca2+

covering the myosin binding site, and troponin acts like a lock to keep the door from opening.

What is the trigger that causes tropomyosin to reveal the myosin binding sites on actin? The signal that initiates contraction is the binding of calcium ions to troponin. Each troponin molecule contains a specific binding site for a single calcium ion. Normally, the concentration of calcium inside the cell is very low, and the binding site is not occupied. It is in this state that troponin locks tropomyosin in the blocking position. When an action potential occurs in the muscle cell plasma membrane, however, the concentration of calcium ions in the intracellular fluid rises dramatically, and calcium binds to troponin. When this happens, there is a structural change in the troponin molecule, and the interaction between troponin and tropomyosin is altered in such a way that tropomyosin uncovers the myosin binding site on actin. The cycle of events depicted in Figure 10-7 is then allowed to occur, and the filaments slide past each other.

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