Cross-bridge cycling ATPase cycle

It simulates the activity of myosin ATPase, which results in contraction of the muscle (i.e. increased cross-bridge cycling). This increases the rate of utilisation of ATP and hence increases the concentration of ADP. 

The decrease in ATP/ADP concentration ratio, since it decreases the energy released on hydrolysis of ATP which could decrease two processes that would result directly in fatigue the cross bridge cycle and the Na+/K+ ion ATPase (Figure 13.24). 

Mackey, A. T., and Gilbert, S. P. (2003). The ATPase cross-bridge cycle of the Kar3 motor domain implications for single head motility./. Biol. Chem. 278, 3527-3535. 

One consequence of the simulations using the four state model (Hai and Murphy 1988a, Hai and Murphy 1992) was that the dependence of ATPase on force was non-linear and increased steeply at higher force levels, due to increased rates of both cross-bridge cycling and LC20 phosphorylation turnover. This result was found to be at variance with earlier data on ATP... 

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... 

When smooth muscle myosin is bound to F-actin in the absence of other muscle proteins such as tropomyosin, there is no detectable ATPase activity. This absence of activity is quite unlike the situation described for striated muscle myosin and F-actin, which has abundant ATPase activity. Smooth muscle myosin contains fight chains that prevent the binding of the myosin head to F-actin they must be phosphorylated before they allow F-actin to activate myosin ATPase. The ATPase activity then attained hydrolyzes ATP about tenfold more slowly than the corresponding activity in skeletal muscle. The phosphate on the myosin fight chains may form a chelate with the Ca bound to the tropomyosin-TpC-actin complex, leading to an increased rate of formation of cross-bridges between the myosin heads and actin. The phosphorylation of fight chains initiates the attachment-detachment contraction cycle of smooth 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. 

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