ROLE OF ATP IN THE MYOSIN CROSS-BRIDGE CYCLE AND MUSCLE CONTRACTION
The muscle contraction and relaxation process involves coordinated energy changes that enable the myosin crossbridge heads to attach and detach with the mechanical alterations of the actin molecule.
Before the initiation of muscle contraction, the heads of the cross-bridges bind with ATP. As soon as the myosin head attaches to ATP, the ATPase enzyme-like activity of the myosin head immediately breaks down the ATP molecule. However, the cleavage products, ADP + Pi, remain bound to the head. The energy released from ATP hydrolysis “cocks” the heads, increasing their angle of attachment to the cross-bridge arm and making them perpendicular to the active sites of the actin myofilaments. The tilting of the cross-bridge heads results in the release of ATP and Pi and the exposure of sites on the heads to bind new ATP. The binding of new ATP causes the detachment of the myosin cross-bridge heads from the actin myofilaments. The ATPase of the myosin cross-bridge heads then hydrolyses ATP as before, “cocking” the heads, and the process is repeated when the subsequent neuromuscular transmission causes the depolarisation of the sarcotu- bular system. The cyclic process continues until the Z disk is pressed against the ends of the myosin filaments. If the muscle is overloaded, no further pulling can occur. When a muscle contracts, a significant amount of ATP is cleaved to form ADP, and the more work the muscle performs, the more ATP is cleaved; this is known as the Fenn effect.7.6
Source:
Rana Tanmoy (ed.). Principles of Veterinary Animal Physiology. CRC Press,2026. — 290 p.. 2026
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