206 UNITY AND DIVERSITY IN BIOCHEMISTRY 



S 



COOH— CH2— CH2— CO— COOH + 



LTPP 



s 



COOH— CH2— CH2— CO - S 



\ 

 LTPP + CO 2 



/ 

 HS 



COOH— CH2— CH2— CO - S 



\ 



LTPP + CoA— SH -> 



/ 

 HS 



HS 



\ 

 LTPP + COOH— CH2— CH2— CO - S— CoA 



/ 

 HS 



HS S 



\ |\ 



LTPP + DPN+ -> I LTPP + DPNH + H+ 



/ 1/ 



H S 



Fig. 45 — Oxidative decarboxylation of a-ketoglutarate. 



Succinyl-CoA, in the presence of ADP and inorganic phosphate, under- 

 goes an internal oxido-reduction at substrate level with the formation of 

 succinic acid, CoA and a molecule of ATP whose energy-rich bond arises 

 from the transfer of the energy of the thioester bond (reaction 8). Thus 

 the route from a-ketoglutarate to succinate yields four molecules of ATP. 

 The reaction a-ketoglutarate-succinyl-CoA reaction is reversible, but the 

 pathway from a-ketoglutarate to succinate is not. Reaction 9, the reversible 

 passage of succinate to fumarate in the presence of succinic dehydrogenase, 

 involves the disposal of the resulting hydrogen by a respiratory chain 

 differing from those to be described presently in which DPN+ and TPN+ 

 are the initial receptors. In the case of the removal of two atoms of hydrogen 

 from succinic acid in the presence of succinic dehydrogenase, the electron 

 acceptor is cytochrome-6 followed by cytochrome-c, cytochrome-a, 

 cytochrome-flg and oxygen. This shortened respiratory chain gives only 



