94 PHOTOSYNTHESIS 



of reduced to oxidized coenzyme is maintained high and 

 this can be achieved in vitro by adding to the reaction 

 glucose-6-phosphate dehydrogenase in the presence of 

 glucose-6-phosphate. The reaction is then 



Glucose-6-phosphate+TPN+ -* 



6-phosphogluconic acid+TPNH+H + 



followed by I or II. Carboxylation will also be favoured by 

 removing one product of the reaction, as for example by 

 adding aconitase in II to remove isocitric acid as citric acid, 

 or in I adding fumarase to change malate to fumarate. 



Ochoa and colleagues investigated the effect of COg on 

 the rate of fixation in malate when the hexose monophos- 

 phate system was coupled with the malic enzyme. The con- 

 centration of COg in the gas phase for about half saturation 

 Avas found to be 17%. Although the activity of the system is 

 appreciable at very low CO2 pressures the saturation of the 

 enzyme requires a very high pressure of CO2, that is, com- 

 pared with the effect of COg on the rate of photosynthesis. 

 However, in the experiments of Elsden (loc. cit.) it was 

 found that in the formation of succinic acid by B. colt the 

 reaction appeared saturated at 5% COg. These facts are 

 given here to indicate a certain complexity in the determina- 

 tion of whether or not a given in vitro enzyme system can 

 be directly related to a physiological response. 



Oxalacetate decarboxylase. The original 'Wood-Werkman 

 reaction' (p. 89) represents a reversal of the decarboxyla- 

 tion of oxalacetate. In the decarboxylation reaction: 



HOOC— CH2— CO— COOH = CH3— CO— COOH + CO2 (6.4) 



the equilibrium is far over to the right. The reaction, which 

 can occur spontaneously in solution, is markedly accelerated 

 by the presence of a number of different metal ions; the 

 mechanism is connected with the formation of complex ions 

 with the metal and oxalacetate. In spite of this possibility of 

 non-enzymic reaction, specific decarboxylases for beta keto 

 acids are widely found in organisms. They are activated by 

 divalent metals, particularly by Mn++. Herbert (1951) ob- 



