PRIMING REACTIONS 225 



for each mole of triose which has undergone oxido-reduction two moles of 

 ATP have been formed, making four moles per mole of glucose. The net 

 gain is therefore two moles of ATP per mole of glucose. 



The free energy liberated when a mole of glucose is converted into two 

 moles of lactate is 49,700 calories. Two energy-rich bonds of ATP have 

 been recovered representing 24,000 calories, so the overall yield is one 

 of 47%. 



In the case of glycogen the formation of G — 1 — P in the presence of 

 phosphorylase and inorganic phosphate takes place without the interven- 

 tion of ATP (see Fig. 38). Consequently, only one molecule of ATP is used 

 up in the phosphorylations at the beginning of the chain. Hence, the net 

 gain per mole of glycogen monomer is three moles of ATP. 



Since the free energy liberated in the course of the passage of the 

 monomer to lactic acid is 56,700 calories and 36,000 calories is regained, 

 the yield is 63%. 



B. The Tricarboxylic Acid Cycle 



The changes in free energy which accompany the various stages of the 

 cycle are shown in Fig. 56. 



When we estimate the energy yield it must be noted that the molecules 

 of pyruvic acid which enter the cycle by way of acetyl- Co A come from the 

 glycolysis chain, but that the DPNH formed during the oxidation of 

 phosphoglyceraldehyde has not been taken into account. Since each 

 dehydrogenation involving DPN or TPN yields three moles of ATP 

 per mole of substrate, when glucose has provided two moles of pyruvate, 

 8 moles of ATP have been formed at the same time. In the tricarboxylic 

 acid cycle we have a series of dehydrogenations where the primary acceptor 

 is DPN. They are the malate-oxaloacetate and a-ketoglutarate-succinyl- 

 CoA reactions. Succinyl-CoA when it decomposes gives a further mole 

 of ATP. The succinate-fumarate reaction does not require the media- 

 tion of DPN and only two energy-rich bonds are formed. Thus during 

 one complete turn of the cycle, that is for all the various stages from 

 citrate to oxaloacetate, twelve energy-rich bonds of ATP are obtained : 



isocitrate -^ oxalosuccinate 3 



a-ketoglutarate -^ succinate 4 



succinate -> fumarate 2 



malate -> oxaloacetate 3 



12 



When pyruvate is condensed with oxaloacetate to form citrate, three 

 moles of ATP are formed per mole of pyruvate, or six per mole of glucose. 



