Biochemical Interactions Of Actin And Myosin

The myosin molecule is an adenosine triphosphatase (ATPase), and adenosine triphosphate (ATP) is required for crossbridge cycling in intact muscle. Thus, muscle contraction requires energy expenditure. Actin and myosin can be extracted and purified. If purified striated muscle myosin is dissolved under the ionic conditions found in muscle cells, it exhibits a low level of ATPase activity. Upon addition of striated muscle actin,

Attachment

Attachment

Conformational changes
Conformational changes

FIGURE 4 The events during a crossbridge cycle. First, activated crossbridges attach to binding sites on the actin (attachment). Then, the crossbridge heads tilt due to conformational changes. This tilt causes movement of the thin filaments and the Z disks. The crossbridges then release. Finally, the crossbridges are reactivated to assume the position conducive for attaching to actin (conformational changes). If cystosolic calcium levels are still high, the cycle will repeat. ATP hydrolysis during a crossbridge cycle: The activated crossbridges that attach to actin (attachment) contain bound adenosine diphosphate-inorganic phosphate (ADP-Pi). Binding to actin results in release of the ADP-Pi products and conformational changes in the crossbridge heads. ATP binding to the attached crossbridge is necessary to disassociate the crossbridge from the actin (release). Once disassociated, ATP is hydrolyzed, but the products are not released. This induces conformational changes to produce activated crossbridges.

FIGURE 4 The events during a crossbridge cycle. First, activated crossbridges attach to binding sites on the actin (attachment). Then, the crossbridge heads tilt due to conformational changes. This tilt causes movement of the thin filaments and the Z disks. The crossbridges then release. Finally, the crossbridges are reactivated to assume the position conducive for attaching to actin (conformational changes). If cystosolic calcium levels are still high, the cycle will repeat. ATP hydrolysis during a crossbridge cycle: The activated crossbridges that attach to actin (attachment) contain bound adenosine diphosphate-inorganic phosphate (ADP-Pi). Binding to actin results in release of the ADP-Pi products and conformational changes in the crossbridge heads. ATP binding to the attached crossbridge is necessary to disassociate the crossbridge from the actin (release). Once disassociated, ATP is hydrolyzed, but the products are not released. This induces conformational changes to produce activated crossbridges.

however, the ATPase activity increases dramatically. In the cell, the cycling of the myosin crossbridge with actin also results in the splitting of ATP. As depicted in Fig. 4, this splitting takes place in several steps. Upon activation of the muscle, (1) the charged myosin crossbridges present in resting muscle bind to the actin molecules; (2) the crossbridges undergo a conformational change to slide the thick and thin filaments over one another, which also results in the loss of adenosine diphosphate (ADP) from the myosin heads; (3) ATP binds with the actin-myosin complexes to bring about a dissociation; and (4) ATP is hydrolyzed to bring the crossbridges to their original charged conformation. Because ATP is required for the dissociation of actin and myosin, a depletion of ATP results in muscle stiffness, not relaxation. This is the cause of the rigor that is seen shortly after death.

Because purified striated muscle actin and myosin are always ready to interact to hydrolyze the ever-present ATP and to cause contraction, regulatory mechanisms must exist to ensure that muscle will contract only when excited. Tropomyosin and troponin are two proteinprotein complexes located on the thin filament that mediate the regulation (see Fig. 2). Tropomyosin is a filamentous protein that lies close to the groove between the two strands of actin. Troponin is a complex of three globular proteins—troponin T, troponin I, and troponin C. This complex is bound to tropomyosin periodically along the thin filament. The tropomyosin-troponin complex interferes with the interaction of the myosin crossbridges with actin. In the resting muscle, the crossbridge binding sites on the actin molecules are protected. When the muscle is stimulated to contract, a conformational change in the thin filament uncovers the binding sites, and crossbridge cycling takes place. The conformational change comes about because of the interaction of calcium with troponin C of the tropomyosin-troponin complex.

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