420 BIOLOGICAL ENERGETICS 



tion the normal potential, Eq. If a substance of high Eo reacts with 

 one of lower £'0, the potential of the former (the oxidizing agent) drops 

 as more and more of it becomes changed to its reduced form, and the 

 potential of the latter (the reducing agent) rises as it is converted into 

 its oxidized form. Finally, the two potentials become equal and no 

 further reaction occurs. The free energy released depends on AE, the 

 difference between the two Eq values. This relationship is given by the 

 following equation: 



— AF = nF AE 



where n is the number of electrons involved in the reaction, and F is 

 the Faraday.^ 



These principles may now be used to explore the possibilities of '-'P 

 generation during the biological oxidation of metabolites. As explained 

 above, the energy released comes from hydrogen atoms split off at various 

 stages. Each pair of hydrogens passes through a system of coenzymes 

 or carriers before finally being united with oxygen (review cytochrome 

 system, p. 333). A series of oxidation-reduction systems is therefore 

 involved, each having its own characteristic £'0 value. The biological 

 oxidation, for example, of lactic to pyruvic acid probably involves the 

 various coenzymes and redox potentials shown in Fig. 16-1. 



The two hydrogens are split off at a potential of — 0.18 volt and, at 

 first, combine with DPN: 



CH3CHOHCOOH + DPN ^ CH3COCOOH + DPN • H2 



Since the AF of this reaction is +4600 cal., the equilibrium point lies 

 far to the left, and the lactic acid will not be oxidized unless the DPN • Ho 

 is removed. If the amount of DPN • H2 is in some manner kept very 

 low, the equilibrium point is displaced to the right in accordance with 

 the law of mass action. Of course, the hydrogens are, in fact, immediately 

 transferred from the DPN • Ho through the remaining steps shown in 

 Fig. 16-1, and for each of these steps AF has a large negative value. 

 This means that their equilibrium points lie far to the right, and the 

 hydrogens are therefore pulled along until they unite with oxygen. They 

 have then traveled over a potential span of 0.99 volt ( — 0.18 to +0.81), 

 an interval which corresponds to a free energy change of — 45,700 cal.^ 

 This energy, how^ever, is not released in a single burst, but in three suc- 

 cessive smaller portions, as shown in Fig. 16-1. One of these is near 

 10,000 cal., about the amount needed for a /^P bond, while the others are 



^ In any chemical process associated with electron transfer, a certain definite quantity 

 of electricity is always needed to bring about the transformation of one gram equivalent 

 weight of the reacting substance (examples, electrolysis of water, electroplating of 

 metals). This quantity, the Faraday, is 96,500 coulombs, which is equivalent to 

 23,060 calories per volt. 



2 Calculation : — AF = nF AE = 2x 23,060 x 0.99 = 45,700, 



