Actin–myosin interactions cause filaments to slide

To understand the mechanism causing the actin and myosin filaments to slide past each other, we must first examine the structures of actin and myosin (Figure 47.3). A myosin molecule consists of two long polypeptide chains coiled together, each ending in a large globular head. A myosin filament is made up of many myosin molecules arranged in parallel, with their heads projecting sideways at each end of the filament like a bunch of golf clubs.

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Figure 47.3 Actin and Myosin Filaments Overlap to Form Myofibrils Myosin filaments are bundles of molecules with globular heads and polypeptide tails; the protein titin holds these filaments centered within the sarcomeres. Actin filaments consist of two chains of actin monomers twisted together. They are wrapped by chains of the polypeptide tropomyosin and are studded at intervals with another protein, troponin.

Animation 47.1 Molecular Mechanisms of Muscle Contraction

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An actin filament consists of actin monomers polymerized into long chains that look like two strands of beads twisted together. Twisting around the actin chains is another protein, tropomyosin, and attached to tropomyosin at intervals are molecules of troponin. We’ll discuss these two proteins in more detail later in this section.

The myosin heads can bind specific sites on actin, to form cross-bridges between the myosin and the actin filaments. Moreover, when a myosin head binds to an actin filament, the head’s conformation changes. As the head bends, it exerts a tiny force that causes the actin filament to move 5–10 nanometers relative to the myosin filament. When the myosin heads are bound to actin, they can bind and hydrolyze ATP. The energy released when this happens changes the conformation of the myosin head, causing it to release the actin and return to its extended position, from which it can bind to actin again.

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Together these details explain the cycle of events that cause the actin and myosin filaments to slide past each other and shorten the sarcomere. They also explain rigor mortis—the stiffening of muscles soon after death. ATP binding causes myosin to release from actin, so when ATP production stops with death, myosin cannot release and the muscles stay contracted. Eventually, however, the proteins lose their integrity and the muscles soften. The timing of these events helps a medical examiner estimate the time of death.

We have been discussing the cycle of contraction in terms of a single myosin head. Remember that each myosin filament has many myosin heads at both ends and is surrounded by six actin filaments; thus the contraction of the sarcomere involves a great many cycles of interaction between actin and myosin molecules. That is why when a single myosin head breaks its contact with actin, the actin filaments do not slip backward.