MANUEL F. MORALES AND JEAN BOTTS 



drop temporally prior to, or even without, actual ATP splitting. 

 What seems to us a clarification of this situation has come from 

 three types of research. First the experiments of Bowen (12,14) 

 and others (1) have shown that under various conditions ATP 

 splitting and myosin deformation can be dissociated, thereby dis- 

 proving any simple coupling between the two processes. Along 

 the same lines, the experiments of Koshland et al. (44) show that, 

 if it exists at all, simple phosphorylated myosin cannot be the con- 

 tracted form of myosin, for the steady-state concentration of such 

 myosin is negligibly small, whereas the percentage of contracted 

 myosin can exceed 80% (57). Second, recent investigation (60) 

 has shown that the thermodynamic constants of ATP hydrolysis 

 have probably been hitherto overrated, and that at 20 ° C. and 

 />H 7, —^F° is 6 to 8 kcal./mole (rather than 11), and —i^H 

 is 5 kcal./mole (rather than 12). Third, enzyme kinetic methods 

 give for — AF° of the reaction 



(undeformed myosin) + (nucleotide) > 



(deformed myosin nucleotide) 



values of the order of 6 to 7 kcal./mole when the nucleotide is 

 ATP^~ (59) and, under different conditions, over 5 kcal./mole 

 when the nucleotide is ADP^~ (29). The first type of research 

 has upset the idea that ATP splitting per se is coupled with 

 myosin deformation, but it is quite consistent with a coupling 

 between ATP adsorption and deformation ; the Koshland result 

 is likewise consistent with the hypothesis that the deformed 

 enzyme holds intact, noncovalently bonded ATP, which in a 

 subsequent step "explodes" into ADP and P through reaction 

 with water. The second and third types of research suggest 

 that when ATP is, as in our model, hydrolyzed through a route 

 involving an intermediate ATP complex with myosin (this being 

 deformed myosin) the free-energy drop between initial state 

 and complex is a large fraction of the free-energy drop in going 

 from initial to final state, i.e., to hydrolysis equilibrium.* 



* It has also seemed to us very significant that the newest values for the 

 — AF° of ATP hydrolysis exceed only slightly the difference between the 

 electrostatic free energy of ATP and ADP (36,37). 



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