THE MOLECULAR BASIS OF MUSCLE CONTRACTION 287 



The L-step is the splitting reaction proper. In the vicinity of pH = 7, the 

 values of the equilibrium constants, K\, K 2 and AT 3 , are such that most of the 

 adenosine is in the form of either ATP" 4 or ADP 2 ; and hence the meas- 

 ured values of AH and AF refer mainly to the hydrolysis itself — the hori- 

 zontal reaction. The reaction is both exothermic and exergonic, a source 

 of heat and a source of free energy for work. Respectable values (see com- 

 ments in Table 7-3) are: 



AF = -10.5kcal/mole 



AH = -9.2kcal/mole 



However, as is obvious from the reaction scheme, a shift in pH can shift the 

 position of equilibrium of reactions 1, 2, and 3, and therefore shift the free 

 energy of the splitting reaction. In a similar manner to the effect of hydro- 

 gen ions, metallic cations — principally Mg ++ and Ca + + — can and do form 

 complexes with the highly charged phosphate groups; each complex with its 

 own equilibrium to affect the reaction scheme, and thereby to affect the 

 values of A F and A H. 



TABLE 10-3. Sedimentation Constant (s), Diffusion Coefficient (D), Molecular Weight (M), 

 Intrinsic Viscosity ([r/ ]), Length (/) and Thickness (d) of the Muscle Proteins. 



The source of the free energy in the hydrolysis reaction is the breaking of 

 the intrinsically unstable, mutually repelling polyphosphates (as typified by 

 ATP) and the formation of products with strong electronic resonance. When 

 one remembers that during the splitting reaction both ATP and ADP are 

 bound more or less tightly to the protein, one can understand why with dif- 

 ferent proteins the energy available for doing useful work, AF, can vary. 



Although the free energy of the hydrolysis of ATP catalyzed by the en- 

 zyme myosin is certainly associated with the work done by the enzyme as it 

 shortens, there is evidence that this relationship is somewhat indirect. This 

 can be seen in the important facts which follow. 



To a fairly good first approximation, the Michaelis-Menten Law, which 

 relates the rate, v, of hydrolysis to catalyst (myosin) and substrate (ATP) 



