GENERAL PRINCIPLES OF BIOCHEMICAL ENERGETICS 137 



ently small), the two reactions proceed in the direction A -> B ^- C uip to 

 the time when the two AF values become equal to 0, which is the point of 

 final equilibrium. 



III. ENERGY-RICH BONDS 



It is to Lipmann (1941) that we owe the classification of the phosphory- 

 lated compounds which occur in Nature, into two types — those possessing 

 a bond whose hydrolysis is accompanied by a considerable release of free 

 energy, and those which yield much less energy. The bonds which are 

 "energy-rich" from the point of view of the release of free energy by 

 hydrolysis are, in fact, for the physical-chemist, weak bonds, being rapidly 

 and easily broken, whilst those bonds which release little free energy on 

 hydrolysis are stronger and more difficult to hydrolyse. When the different 

 types of phosphoric esters were listed in the first part of this book (p. 62), 

 we noted the existence of bonds of the acid anhydride type, resulting from 

 the union of two molecules of acid with elimination of a molecule of water. 

 These bonds are hydrolysed with the release of large amounts of energy 

 as can be seen, for example, when acetic anhydride is mixed with water : 

 the reaction takes place with a considerable increase in temperature. 



The most important type of acid anhydride in biochemistry is that 

 between two molecules of phosphoric acid, as exemplified by the two ter- 

 minal bonds of ATP (p. 67), but the pyrophosphate bond is not the only 

 type of energy-rich bond as can be seen from Table XI. Mixed anhydrides 

 of carboxylic and phosphoric acids (acyl-phosphates), phosphondated 

 enol groups (enolphosphates), phosphorylated guanidine groups (guanidine 

 phosphates), compounds of sulphhydryl groups with phosphoric or car- 

 boxylic acids (thioesters or thiophosphates), all these types of compound 

 are energy-rich. 



How does one explain this release of large amounts of free energy when 

 an energy-rich bond is hydrolysed ? As far as the pyrophosphate linkage is 

 concerned, one of the reasons is that in pyrophosphate the number of 

 resonating structures is much smaller than in inorganic phosphate. Also, 

 in the pyrophosphate molecule there are several Hke charges close to each 

 other and their reciprocal repulsion is balanced by a certain amount of 

 energy which is set free on hydrolysis. Moreover, the neutralization of the 

 acid groups liberated on hydrolysis is also productive of energy. Con- 

 siderations of a like character can explain the liberation of energy which 

 accompanies hydrolysis of the acyl-phosphate and guanidine-phosphate 

 bonds, but they cannot explain this in the case of the phosphoenolpyruvate 

 bond. In this case, one of the sources of the energy is the transformation 

 of the enol form of pyruvic acid into the keto form. 



The formation of glycogen from glucose is an example of an apparently 

 endergonic reaction which is, in fact, made exergonic by the mediation of a 



