ATPase motor idling

Figure 1.6. The idling ATPase motor consumes the energy released on conversion of MgATP to MgADP and Pi without the performance of useful work. On binding MgATP to the globular apoprotein, a cleft, partially or entirely closed by association of oil-like groups, opens near the base of the...

The ATP molecule orients with its charged triphosphate tail at the base of the cleft such that it can access the water of the cleft. Because of this, water, which would normally form around the oil-like groups and effect reclosure, becomes oriented toward the phosphates and maintains the cleft in the open state. On hydrolysis of ATP and phosphate removal, the cleft partially or completely closes. Release of the ADP molecule recovers the original state to complete the cycle. Accordingly, energy has been consumed, but in this idling ATPase motor no work is accomplished. 

Figure 2.16. An initial step in converting the idling ATPase motor into a motor capable of performing work involves attachment and detachment as between components of actin and myosin involved in the sliding filaments of muscle contraction. A The cartoon depicts an equilibrium between states of oil-like association and dissociation with fluctuating cleft openings, but in the absence of ATP, the equilibrium markedly shifts toward association.

In fact, the ATPase of skeletal muscle exhibits a commonly recognizable structural feature for the ATP bound state. So in an initial illustration (Fig. 1.6), we approach ATPases with the most simplistic cycle, that of an idling motor. Then, in Chapter 2 we take a first step toward useful function by attachment to and detachment from a surface and progress from there to more complete depictions. Once the basic science is laid out in Chapter 5, a detailed molecular description is given in Chapter 8. 

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