Swinging lever arm

Holmes, K. C. (1997). The swinging lever-arm hypothesis of muscle contraction. Curr. Biol. 7, R112-R118. 

Fig. 31. Implications about the contractile mechanism in insect flight muscle. Blue is insect flight muscle SI shape in pre-powerstroke state (Al-Khayat et al., 2003), and green is chicken skeletal muscle SI in the rigor state with no nucleotide bound (Rayment et al., 1993a). The actin filament (right) is shown with the Z-band at the bottom and M-band at the top. A transition from the pre-powerstroke/resting SI shape to the rigor/end of post-powerstroke shape would involve an axial swing of the lever arm by 100 A, resulting in the sliding of the actin filaments past the myosin filaments and toward the M-band.

For the outline in Scheme 3, two situations can be considered the isometric case where a large load attached to the myosin lever-arm prevents the lever-arm from completing its swing, and the unloaded situation as seen in solution for a single motor domain. 

It has long been surmised that switch-2 movement and the concomitant swinging of the lever arm must be controlled by binding to and detachment from the actin filament to avoid futile consumption of ATP. However, direct evidence was lacking because near-atomic resolution crystal structures are necessarily obtained in the absence of the filament. Now, crystal structures of Dictyostelium myosin II (Reubold et at, 2003) and chicken myosin-V (Coureux et al., 2003) have revealed that the switch-1 motif can also exist in open and closed conformations. It has been inferred that switch-1 opening may be coupled to cleft closure and tight binding of the myosin head to the actin filament. This conclusion is supported by electron microscopy (Holmes et al., 2003) and fluorescence spectroscopy (Conibear et al., 2003) studies of the acto-myosin complex, which show that the concepts derived from crystal structures of isolated myosin heads are indeed valid for the functional complex. 

Single-pan balances typically have damping devices that help to minimize the free swinging of the lever arm. This device speeds the weighing process immensely over the time spent counting swings for two-pan balances. 

To fire, the candle w as gras]>ed with lever held firmly against the handle body and the safety pin was withdrawn, kef ping a firm gra p around the candle and lever. The candle was tlirown with u full swing of the arm, like a grenade, or placed on the ground. As the candle wa rt leased from the hand, the lever dropped away, allowing the striker li> fire the primer. 

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