208 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



succinate 



t 

 succinic dehydrogenase 



11 

 cy tochr. -b 



ot-ketoglutaric acid 

 (DPT) j (CoA) 



/3-hydroxybutyric acid 



(dehydrogenase) 



(Slater factor) 



(Slater factor) 



DPN 

 (or TPN) 



Flavoprotein 



I 

 y cy tochr. -c 



\ 

 cy tochr. -a 



\ 

 cytochr.-aj 



I 

 Fig. 46 (Slater) — Forms of the respiratory chain. 



Although DPN is the first hydrogen acceptor in the dehydrogenation 

 for example, of ^-hydroxybutyric acid, TPN is the primary acceptor in 

 other cases, and where succinic acid is concerned, the primary acceptor is 

 cytochrome-6. 



In addition, dehydrogenations exist, such as that of a-ketoglutaric acid, 

 where thioctic acid acts as an intermediate between the donor and DPN. 



All these paths converge at the level of cytochrome-^. 



The scheme in Fig. 46 summarizes the form of the respiratory chain. 



In Slater's scheme, a-ketoglutarate and ^-hydroxybutyrate are taken as 

 examples of hydrogen donors typical of the class to which each belongs. 

 The substances shown in parentheses are those which intervene at the stages 

 indicated, but it cannot be stated whether they give or receive protons or 

 electrons. This is especially the case for the Slater factor, coming between 

 cytochrome-6 and cytochrome-c or between flavoprotein and cytochrome-c. 



In the course of the transfers just described, a series of energy-rich bonds 

 is formed by phosphorylations associated with the respiratory chain. In 

 the scheme reproduced above, a single phosphorylation has been so far 

 identified and this is situated at the level of the reaction between a-keto- 

 glutarate and thioctic acid. However the sum of experimental observations 



«-ketoglutarate 



succinate 



succinic dehydrogenase 



I 

 1 or 2 ~ P cyt.-6 



P .. V I 



— *■ thioctic y DPN > Flavoprotein y cyt.-c 



acid or (Slater j 



TPN factor) cyt.-a 



; 



cyt.-a, 



I 

 O. 



Fig. 47 (Slater) — Phosphorylations tied to the respiratory chain. 



> -P 



