92 PHOTOSYNTHESIS 



It was discovered by Stephenson and Stickland and the 

 equiUbrium constant for the reaction with the bicarbonate 

 ion was determined by D. D. Woods. The system is freely 

 reversible with mixtures of CO2 and H2 and the determina- 

 tion of the equilibrium constant was an important direct 

 check upon the calculation of the free energy of formic acid 

 from thermochemical data. Its occurrence is limited to a 

 few groups of bacteria. 



Fortnic dehydrogenase. Two distinct enzymes have been 

 described, the one of bacterial origin requiring no coenzyme, 

 the other found in seeds of many plants requiring co- 

 enzyme I. The latter system will reduce the coenzyme prac- 

 tically to completion for the standard oxidation-reduction 

 potential of formate is about equal to that of the hydrogen 

 electrode. No hydrogen however is produced because the 

 enzvme is inert to this gas. In the case of the plant system 

 no evidence of reversibility could be obtained either by 

 Davison or by Vennesland (see Vennesland, 1952). Hence 

 this plant system does not react with CO2 to a measurable 

 extent. The formic dehydrogenase is thus quite distinct 

 from the formic hydrogenlyase. 



The malic enzyme and the isocitric system. These are con- 

 cerned in two of the best known reversible carboxylation 

 reactions and are found to be widely present in the leaves of 

 plants. They were originally obtained from animal tissues 

 by Ochoa; their presence in plants was established by the 

 work of Vennesland et al. and by Whatley (195 1). 



The malic enzyme catalyses the reaction 



COOH 



CH3 I 



I (Mn++) CH2 



H+ + CO2 + CO +TPNH ^ I +TPN+ (I) 

 1 CHOH 



COOH I 



COOH 



Pyruvic acid Malic acid 



The isocitric system catalyses the analogous reaction 



