PRIMING REACTIONS 207 



two molecules of ATP. Reaction 10 is a reversible hydration of fumarate 

 to malate in the presence of fumarase. Although, in the liver, malate may- 

 give rise to pyruvate, this is not the case in most tissues. The malate is 

 dehydrogenated in the presence of malic dehydrogenase and DPN+ form- 

 ing oxaloacetate and DPNH. This reaction is reversible. The oxaloacetic 

 acid formed can, in its enolic form, react with acetyl-CoA to give citric 

 acid and traverse the cycle once more. 



IV. RESPIRATORY CHAINS 



This name is applied to the series of carriers along which pass the protons 

 and electrons liberated in the course of a dehydrogenation in the tricar- 

 boxylic acid cycle or other aerobic dehydrogenation before they reach 

 oxygen and unite with it to form water. As we have seen, it is along the 

 respiratory chain that by a series of phosphor}dations coupled to it by a still 

 unknown mechanism (see p. 144) the main quota of energy-rich bonds is 

 formed and placed at the disposition of the cells. 



In the course of the successive dehydrogenations of the cycle, the 

 greater part of the protons and electrons liberated pass to the same series 

 of carriers. The first acceptor of the series being most often DPN+, 

 DPNH appears as the principal "fuel" in cells. 



The successive transfers are the following : 



Substrate + DPN+ -> oxidized substrate + DPNH + H+ (A/^ variable) 



DPNH + H+ + Flavopr. -> DNN+ red Flavopr. 



{AF = — 10 kcal) 



red. Flavopr. + Ferricytochr. c -> Flavopr. + Ferrocytochr. c 



{AF = — 16 kcal) 



Ferrocytochr. c + O2 -> Ferricytochr. c (AF = — 25 kcal) 



The last reaction can be split up into the following stages : 



Ferrocytochr. c + Ferricytochr. a -> 



Ferricytochr. c + Ferrocytochr. a 



Ferrocytochr. a + Ferrocytochr. a^ -> 



Ferricytochr. a + Ferrocytochr. ^3 



Ferrocytochr. «3 + O -^ Ferricytochr. 03 + O 



REFERENCES 



Krebs, H. a. (1954). The tricarboxylic acid cycle. Chemical Pathzvays of Metabolism, 

 Greenberg, D. M. (Editor) vol. I, Academic Press, New York 109-171. 



OcHOA, S. (1954) Enzymic mechanisms in the citric acid cycle. Advance. EnzymoL, 

 15, 183-270. 



