136 UNITY AND DIVERSITY IN BIOCHEMISTRY 



II ENERGY COUPLING 



Exergonic reactions are the only reactions which can occur in the bio- 

 sphere. It is, therefore, necessary to explain by what mechanism cells per- 

 form numerous biosyntheses where the resulting molecules have an energy 

 content higher than that of their starting materials. 



In the cell, the energy of a chemical bond can be transferred to another 

 bond by only one possible mechanism, that in which two separate reactions 

 have one substance common to both. The transfer of energy is accom- 

 plished by utilizing part of the free energy of an exergonic reaction to bring 

 about a reaction which, by itself, would be endergonic and would not, there- 

 fore, otherwise take place. 



Consider the following endergonic reaction : 

 {K^ - 0-01, AF° = +2,470 cal.) Lt equilibrium ^qq^ ^ 



B\ 



We are given the value of the equilibrium constant and the reaction will 

 be at equilibrium when the concentration of ^ is a hundred times that of B, 

 and it will not take place from left to right except when the concentration 

 of B is less than one per cent of that oi A. If the concentration of B is 

 greater than one per cent of that of A, the reaction will take place from right 

 to left. 



Now, consider the exergonic reaction : 



(II) B-^C 



{K^ = 1000, AF° = -4110 cal.) /'at equilibrium ^ 



It will take place from left to right until C is a 1000 times more con- 

 centrated than B. 



Now let us suppose that the two reactions occur simultaneously. Re- 

 action II will continuously remove the product of reaction I, and this 

 reaction, endergonic when the ratio [5]/[^] is greater than 1/100, will become 

 exergonic and, consequently, will proceed from left to right, when the 

 concentration of B becomes sufficiently small for the ratio [5]/[^] to fall 

 below 1/100. For the combined reaction, AF = (2470 - 4110) = -1370 

 cal). The overall reaction is therefore exergonic. 



The exergonic reaction II causes B to disappear. As B disappears, the 

 AF of reaction II becomes progressively smaller in absolute value, whilst 

 the JF of reaction I approaches zero (the reaction becomes less and less 

 endergonic). When the AF of / becomes negative (because [B] is suffici- 



