72 



A serious objection may be raised against this theory: the sta- 

 bility constant of the metal chelate of Mg and the adenine or 

 isonine, in water, is very low; the energy of this binding is not 

 great enough to hold the whole quadridentate chelate together 

 against the forces of heat agitation. The Mg-phosphate complex is 

 very stable so that we can expect the ATP to be present in muscle 

 as a Mg complex, there being twice as much Mg in muscle as there 

 is ATP, but links formed by Mg with the adenine or purine can 

 be expected to break up in water. Our model in Fig. 16 only says 

 that such a complex can be formed, not that it is formed. However, 

 we have to remember that the ATP, when undergoing splitting in 

 muscle, is not free but is linked to the protein, myosin, and is 

 "activated" by it. We do not know what "activation" means. 

 Probably it means a binding of the substrate with its consecutive 

 deformation. So it may be that the myosin holds the ATP molecule 

 in the position required for the formation of this bridge between 

 phosphate and purine and this is actually what we mean by activa- 

 tion. One could go even one step further with the argument and 

 say that if the stability constant of the Mg-purine complex were 

 high enough to bring the two ends of the molecule together and 

 make the energy transmission and the splitting of the ~P occur, 

 then this reaction would be of no use to the muscle, because then 

 the ^P would be split and its energy dissipated senselessly. It is 

 one of the basic principles of nature not to use spontaneous reac- 

 tions which occur by themselves and cannot be kept in hand. If 

 the energetics of the cell consisted of spontaneous reactions, the 

 whole mechanism would have to run down senselessly as a watch 

 does if released from its regulators. We have to demand from any 

 theory of the ATP-ase activity that it should make the splitting of 

 -^P possible only in ATP molecules, bound and activated by the 

 myosin, when the energy of the ^P can be transmitted to the 

 protein and applied usefully. It is thus just the low stability con- 

 stant of the hypothetic Mg-purine complex which makes the pre- 

 sented theory acceptable. 



A word may be said about the third member of the ATP mole- 



