Top-of-powerstroke state

The strongly bound pre-powerstroke state or top-of-powerstroke state is the transitory state labeled 4 in Fig. 1. It is experimentally difficult to characterize this either kinetically or structurally. At present, the structure can only be guessed at by an extrapolation of the properties of the adjoining structures. It seems very likely that the actin-binding cleft closes on strong binding in the top-of-powerstroke state. Comparison of the structures of the pre-powerstroke and post-rigor states with the nucleotide-free... 

Fig. 7. The strongly bound top-of-powerstroke state. Shown is the truncated myosin crossbridge without the lever arm. The orientation is as in Fig. 5A. (A) Pre-powerstroke state with the upper 50K domain shown in yellow. (B) The rigor-like state with the upper 50K domain of the pre-powerstroke state (yellow) superimposed on the upper 50K domain of the rigor-like state (red). (C) The model produced by taking the superimposed orientation of the upper 50K domain and combining it with the original pre-powerstroke coordinates. This generates a pre-powerstroke state with a shut actin-binding cleft that serves as a model of the ephemeral strongly bound top-of-powerstroke state.

The modeled structure may also be used to generate the attached top-of-powerstroke state. This is shown in Fig. 9A compared with the rigor conformation (Fig. 9B). The same geometry for the attachment to actin has been used, as was found by electron microscopy for the binding of myosin V to actin. The lever arm from chicken skeletal myosin has been used to complete the model. 

The fourth actin-bound, top-of-powerstroke state is ephemeral. In the original Lymn—Taylor model it is not clear if this fourth state is strongly or weakly bound to actin, but P release, ADP release, and force generation all occur during the transition 4 to 1. Thus, in the following discussion, we attempt to break this transition down into a series of elementary events and to explore if it is possible to order the biochemical and mechanical events and to correlate them with structural changes. 

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See also in sourсe #XX -- [ ]

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