114 



PROBLEMS OF PHOTOSYNTHESIS 



hH,0 



CH3-C0~S-A 



+ 



H0OC-CH,-C0^CO0H 

 oxaloacetic acid 



OH 



HOOC CHrC-CHrCOOH 



I 

 COOH 



Citric acid 



//- 



HO0C-CH=C-CHrCOOH 

 COOH 



cisaconitic acid 



-2H 



HOOC-CHOH-CH, COOH 



malic acid 



\\ 



H,0 



HOOC-CH=CH-C0OH 



tumanc aciO 



+ H,0 t 1 



..fl 



HOOC-CHOH-C-CHrCOOH 



HOOC-CHrCHrCOOH 



succinic acid 



COOH 



(so-citnc acid 



^" 



HOOC-CO-CH,-CHrCOOH 



u-keloglulacic acil 



H 



HOOC-CO-C-CH,-COOH 



COOH 



oxalosuccinic aci 



Fig. 44. The tricarboxylic acid cycle. 



the mitochondria and their various cofactors. There are four oxidation steps 

 hnked with the action of particular dehydrogenases. The substrates iso- 

 citric acid, a-ketoglutaric acid, succinic acid and mahc acid are dehydrogen- 

 ated, each giving 2 H atoms to appropriate hydrogen acceptors. The H 

 atoms are finally oxidized to H ions. 



As depicted in Figure 44, one acetyl group of acetylcoenzyme A is oxidized 

 in one cycle, 3 molecules H2O being used and 2 molecules COo and 8 H atoms 

 being produced. This corresponds to the over-all reaction 



CH3-CO~S— A + 3 HoO -^ 2 COo + 8 H + H— S— A 



The molecule of coenzyme A which is produced reacts with another mole- 

 cule of pyruvic acid to form a further molecule of acetylcoenzyme A. 



The first oxidation step is the conversion of /\yo-citric acid to oxalosuccinic 

 acid catalysed by the iso-citric dehydrogenase. This reaction is followed by 

 the decarboxylation to a-ketoglutaric acid. Thus, the over-all reaction rep- 

 resents an oxidative decarboxylation. 

 COOH • CHOH • CH • CHo • COOH + TPN+ Mn ions 



COOH 



COOH • CO CH.CHs- COOH + COo + TPNH + H + 



In the second oxidation step, a-ketoglutaric acid is converted to succinic 

 acid by oxidative decarboxylation. According to Green (7. 8), Kaufman 

 (14) and Sanadi (30) the following scheme with its three main reactions may 

 be accepted : 



1. Formation oj suainylcoenzyme A 

 a-ketoEflutaric acid + A— S— H + DPN + 



succinvl'-S— A + CO2 + DPNH + H + 



