416 BIOLOGICAL ENERGETICS 



exhaustion. Thus it appeared quite possible that the breakdown of 

 CrP into free creatine and phosphoric acid might furnish the energy for 

 contraction. However, it was demonstrated by Lohmann in 1934 that 

 muscle contains no enzyme capable of catalyzing the direct hydrolysis 

 of CrP, but that the substance is used up instead by reacting with adeno- 

 sine diphosphate (ADP) : 



CrP + ADP ^Ct + ATP 



The Lohmann reaction, as this is called, has a small AF and can there- 

 fore proceed readily in either direction, depending on changes in pH 

 and concentrations of reactants. It obviously will go to the right and 

 consume CrP only when ADP is available, ADP is present only in 

 very small amounts in resting muscle, but can be formed from ATP by 

 hydrolysis : 



ATP + H:0 — ^^^"'' > ADP4-H3P04 



This breakdown of ATP into ADP is catalyzed by adenosine triphospha- 

 tase (ATP-ase) and has a AF value of —11,500 cal. From the above 

 considerations it is evident that this breakdown of ATP must take place 

 before CrP can be used up by the Lohmann reaction. ATP, therefore, 

 is most probably the immediate energy source for muscle contraction. 



Just how the chemical energy in ATP is converted into the mechanical 

 energy of contraction is not well understood, but it is known that ATP-ase 

 is present in muscle fibers in large amounts. In fact, ATP-ase probably 

 makes up a part of the fiber, being itself a long, thread-like protein, 

 molecules of which are arranged lengthwise along the fiber. When ATP 

 breakdown occurs, some of the side-chain groups in these or other pro- 

 tein molecules in the muscle probably become altered in such a way 

 that they have an attraction for other groups in the same molecules. 

 This would cause a puckering and shortening of the molecules so affected, 

 and consequently a contraction of the whole fiber. 



High energy phosphate bonds 



The energy released when the terminal phosphate group of ATP is split 

 off (Ai^ = — 11,500 cal.) must have been contained in the particular 

 valence bond which held this group to the rest of the molecule. Numer- 

 ous other phosphate derivatives are also involved in metabolic reactions 







II 

 (see Chap. 13) . All are of the type (HO)oP— X, where X may be either 



an oxygen or nitrogen atom, which is attached in turn to another phos- 

 phate radical or to some organic structure. These phosphorus com- 

 pounds have been found to fall roughly into two main groups according 



