BIOLOGICAL ENERGETICS 



421 



considerably larger. Therefore, approximately the right amount of 

 energy is made available in about the right sized "packages" for the 

 generation of three high energy bonds, when one mole of lactic acid is 

 oxidized to pyruvic acid and water. 



Overall reaction 



CH3CHOHCOOH + iO, 7-^ CH3COCOOH + HjO 

 Lactic acid Pyruvic acid 



Corresponding redox potentials 

 and energy changes 

 Eo AE AF 



Inlermediate stages volts volts cal. 



1. Lactic acid < * pyruvic acid + 2H —0.18) ^ ,^ , . ^^« 



f —0.10 +4,600 

 2.DPN+2H ^ DPNH+H* -0.28 _ 



3. rAD+2H 5=^ FADH, -0.06 _u70n 



4. 2Cy,,. . Fe*- + 2H ^ 2Cyt. o Fe- + 2H* +0.26 ^^^ l^' 



5. iOj + 2H :i=^ HjO +0.81 ^ 



Fig. 16-1. Intermediate stages and energy relationships in the biological 

 oxidation of lactic to pyruvic acid. 



The five pairs of hydrog-ens that split out when pyruvic acid is com- 

 pletely oxidized by the citric acid cycle likewise pass through the hydro- 

 gen transport system. Each pair originates at a definite redox potential, 

 which is given in Table 16-3. Calculations of the energy released as each 

 pair becomes united with oxygen indicate that sufficient energy should 

 be available to generate the number of '~P bonds shown in the last 

 column. The total is 16 such bonds per mole of pyruvic acid. Since 

 three more could have been formed in the conversion of lactic into pyruvic 

 acid, the total for the aerobic phase of carbohydrate metabolism would 

 be 2 X (16 + 3) or 38 per mole of glucose. It must be emphasized that 

 these figures are only estimates based on the information now available. 



Table 16-3 



Redox potentials at which hydrogen is released during oxidation of pyruvic 



acid via fhe citric acid cycle 



* Voits. 



