PRIMARY EVENT IN MUSCLE ACTION 



largely reversed. From these observations we judge that 

 whereas the hydrolysis of adsorbed ATP is not what energetically 

 "drives" the deformation, it is necessary for reversing the de- 

 formation of this simplest model, and hence for cyclical de- 

 formation. 



Before testing this theoretical model against experiments or 

 before elaborating it to accommodate other facts, it is well to 

 consider certain theoretical questions. If the model is rigged to 

 do work, it becomes a simple transducer of "chemical" into 

 "mechanical" energy. One may inquire about the maximum 

 efficiency of this transduction or ask, as is often done in physiol- 

 ogy, whether chemical energy is supplied for contraction or for 

 relaxation. Thermodynamic analysis (38) of an idealized cyclic 

 isothermal process in which (7) the amount of ATP adsorbed on 

 a loaded myosin fiber is allowed to increase, thereby causing the 

 weight to be lifted; (2) the lifted weight is removed and the 

 bound ATP is (catalytically) decomposed to equilibrium with 

 ADP and P, thereby permitting the fiber to lengthen ; (J) the 

 weight is restored on the fiber; and (4) the decomposed ATP is 

 resynthesized, reveals that in steps (7) to {3) work is done at the 

 expense of converting ATP in solution into ADP + P in solu- 

 tion; ultimately this is "paid for" in the "recovery step" (4). 

 In the efficiency expression the "output" is therefore the net 

 work done in steps (7) to (J), and the "input" is the increment in 

 the number of moles of ATP bound, multiplied by the difference 

 in chemical potential of ATP before and during the binding 

 process. Therefore, although mechanistically the energy trans- 

 fer step does not involve hydrolysis of ATP, the over-all ener- 

 getics of the process can nonetheless be calculated from the net 

 changes in the system, i.e., work done, and a certain amount of 

 hydrolysis. The analysis is noncommittal as to "when" (con- 

 traction or relaxation) energy is supplied; it merely states that 

 in the cyclic process the system performs work at the expense of its 

 free energy. This result merely confirms what perhaps should be 

 (but sometimes is not) obvious to most of us, viz., that the ques- 

 tion of where in the cycle energetic coupling occurs is not a 



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