Actomyosin ATPase cycle

In summary, therefore, solution and fiber biochemistry have provided some idea about how ATP is used by actomyosin to generate force. Currently, it seems most likely that phosphate release, and also an isomerization between two AM.ADP.Pj states, are closely linked to force generation in muscle. ATP binds rapidly to actomyosin (A.M.) and is subsequently rapidly hydrolyzed by myosin/actomyosin. There is also a rapid equilibrium between M. ADP.Pj and A.M.ADP.Pj (this can also be seen in fibers from mechanical measurements at low ionic strength). The rate limiting step in the ATPase cycle is therefore likely to be release of Pj from A.M.ADP.Pj, in fibers as well as in solution, and this supports the idea that phosphate release is associated with force generation in muscle. 

Still, there is some controversy as to whether unphosphorylated myosin can interact with actin. Most studies suggest that myosin has to be phosphorylated for interaction with actin (Sobieszek and Small 1977, Sherry et al. 1978, Sellers et al. 1982, Chacko and Rosenfeld 1982, Sellers et al. 1985). However, under conditions where the unphosphorylated myosin is filamentous, Vmax of the MgATPase activity is about one-half of that of phosphorylated myosin as already mentioned (Wagner and Vu 1986,1987). This puzzle may be resolved on the basis of a recent study in which the minimal molecular requirement for myosin to be in the off-state was determined (Trybus et al. 1997). Mutants of myosin with different lengths of the rod showed that a length approximately equal to the myosin head was necessary to achieve a completely off-state . It was concluded that the myosin rod mediates specific interactions with the head that are required to obtain a completely inactive state of vertebrate smooth myosins. If this interaction could be prevented, e.g. by constraints imposed by the native thick filament structure or accessory proteins, then partial activation of the actomyosin ATPase and slowly cycling of unphosphorylated cross-bridges could occur. 

Brenner B (1986) The cross-bridge cycle in muscle. Mechanical, biochemical, and structural studies on single skinned rabbit psoas fibers to characterize cross-bridge kinetics in muscle for correlation with the actomyosin-ATPase in solution. Basic Res Cardiol 81 (Suppll) 1-15... 

However, release of ADP and P from myosin is much slower. Actin activates myosin ATPase activity by stimulating the release of P and then ADP. Product release is followed by the binding of a new ATP to the actomyosin complex, which causes actomyosin to dissociate into free actin and myosin. The cycle of ATP hydrolysis then repeats, as shown in Figure 17.23a. The crucial point of this model is that ATP hydrolysis and the association and dissociation of actin and myosin are coupled. It is this coupling that enables ATP hydrolysis to power muscle contraction. 

An insufficient rate of ATP resynthesis for optimal energy supply for actomyosin crossbridge formation and cycling, or for the additional ATPase reactions, Na" -K pumping and Ca reuptake and/or release by the SR. 

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