Myosin cross-bridges

Figure 14.12 The swinging cross-bridge model of muscle contraction driven by ATP hydrolysis, (a) A myosin cross-bridge (green) binds tightly in a 45 conformation to actin (red), (b) The myosin cross-bridge is released from the actin and undergoes a conformational change to a 90 conformation (c), which then rebinds to actin (d). The myosin cross-bridge then reverts back to its 45° conformation (a), causing the actin and myosin filaments to slide past each other. This whole cycle is then repeated.

ATP binds to the myosin cross-bridges, leading to release of the bond between actin and myosin. 

Figure 13.7 A diagram of the actin helix showing position of the tropomyosin. Both actin chains are flanked by tropomyosin molecules, which are long string-like molecules that span seven actin monomers. The troponin complex is attached to the tropomyosin but is not shown. From this diagram, it should be clear how the tropomyosin molecule can conceal the actin-binding sites for the myosin cross-bridges in the relaxed condition. A small conformational change in tropomyosin exposes the sites for attachment of the cross-bridges.

One head of the myosin cross-bridge attaches to the actin filament. 

Figure 13.16 A summary of the control of muscle contraction by the motor neurone. When an electrical impulse arrives at the junction between a nerve axon and a muscle fibre, a small amount of acetylcholine is released. This initiates an action potential which is transmitted throughout the fibre via the T-tubules. This causes the sarcoplasmic reticulum to release Ca ions which initiate contraction of the myofibrils via changes in troponin and tropomyosin. Thus sites on the actin for binding of the myosin cross-bridges are exposed.

Harford, J. J., and Squire, J. M. (1986). The crystalline myosin cross-bridge array in relaxed bony fish muscles. Biophys. J. 50, 145-155. 

Thin filament Myosin cross-bridge Thick filament backbone... 

These mixing curves can be analyzed by a crossbridge model to predict the relative force-producing capability of the two myosins (Harris et al., 1994). The results suggest that smooth muscle myosin exerts 2.1 times the force per myosin cross-bridge compared to skeletal muscle myosin (Harris et al., 1994). 

However, a nerve impulse to the muscle triggers a release of Ca2+ from the sarcoplasmic tubular system, where it is ordinarily bound, which increases the intracellular Ca2+ concentration to 10 5-10 6M. This level of Ca2+ allows the actin in the thin filament to accept the energized-ADP-myosin cross-bridge to initiate contraction. As each cross-bridge completes the swivel part of its cycle, it loses the bound ADP and immediately accepts a molecule of ATP that is always supplied to living muscle. The ATP immediately causes a dissociation of the actin-myosin complex, and the myosin catalyzes the hydrolysis of ATP to yield the myosin—ADP energized state again, ready to repeat the cycle. 

Warshaw DM, Desrosiers JM, Work SS, Trybus KM (1990) Smooth muscle myosin cross-bridge interactions modulate actin filament shding velocity in vitro. J Cell Biol 111 453463... 

Lowered maximum specific force, reflecting the lower concentration of myosin cross-bridges (MHC... 

R. Cooke, Stress Does Not Alter the Conformation of a Domain of the Myosin Cross Bridge in Rigor Muscle Fiber, Nature 294, 570 (1981). 

By the elastic consilient mechanism the extension of single flexible loops causes an increase in the elastic force. It appears that one such loop has been identified in Figure 8.55. Rayment et al. note a number of flexible loops in the myosin cross-bridge. Each of these becomes a candidate for elastic force... 

---