THE CHEMISTRY OF PHOTOSYNTHESIS H5 



This reaction corresponds to the oxidative decarboxylation of pyruvic acid 

 (§ 45). The same 5 cofactors are indispensable. 



2. Formation of succinic acid 



succinyl^S — A + phosphate + GDP -^ succinic acid + A^S — H + GTP 



Inorganic phosphate is used. The enzyme succinylcoenzyme A-ADP-phos- 

 phorylase discovered by Hift et al. (10) needs guanosine diphosphate (GDP) 

 as an additional cofactor (30). 



3. Fort7iaiion of ATP 



Finally, a GTP-ADP-transphosphorylase transfers the phosphate group from 

 GTP to ADP 



GTP + ADP ^ GDP + ATP 



The third oxidation step is the dehydrogenation of succinic acid cataly.sed 

 by the succinic dehydrogenase. The H atoms are not accepted by either 

 DPN+ or TPN + ; they are probably oxidized to H ions by cytochrome b. 

 The enzyme fumarase catalyses the hydration of fumaric acid to malic acid. 



In the fourth and last oxidation step, malic acid is dehydrogenated to oxalo- 

 acetic acid by malic dehydrogenase and DPN+. This reaction terminates the 

 tricarboxylic acid cycle. Starting from one molecule acetylcoenzyme A, one 

 molecule of oxaloacetic acid is regenerated and can condense with a further 

 molecule of acetylcoenzyme A. 



The so-called malic enzyme which is closely related to malic dehydrogenase 

 can convert malic acid into pyruvic acid with DPN+ or TPN+. 



Mn ions 

 COOHCHo-CHOH COOH + TPN+ ^ — ^ 



CH;rCOGOOH + CO, + TPNH + H + 



Badin and Calvin (2) consider the reversal of this reaction, i.e., the reductive 

 carboxylation of pyruvic acid, may play a role in photosynthesis (see § 58). 

 The reaction discovered by Ochoa (20, 22, 23) is reversible; the presence o) 

 Mn ions is essential. When it proceeds from right to left and the malic acid 

 produced is converted by the malic dehydrogenase to oxaloacetic acid, the 

 over-all reaction is identical with the Wood-Werkman reaction discussed in 

 §45. 



pyruvic acid + CO, + TPNH + H+ ^ malic acid + TPN + 



malic acid + TPN+ -^ oxaloacetic acid + TPNH + H + 



pyruvic acid + CO 2 -^ oxaloacetic acid 



In various cells fixation of COo is a most important process.* It is mostly 

 due to reductive carboxylation, i.e., the reversal of oxidative decarboxylation 

 Other reactions of CO2 fixation are the Wood-Werkman reaction and the 

 carboxylation of phospho-^;?o/-pyruvic acid with inosine diphosphate (IDP). 



For literature on CO2 fixation, see Utter and Wood (32) and Vishniac et al, (33). 



