REACTIONS OF CARBON DIOXIDE I03 



leaves of several plant species by Gibbs (1952) and by 

 Arnon (1952). With the plant preparations both DPN and 

 TPN are active. This system may provide one of the main 

 reduction steps in photosynthesis, where phosphoglyceric 

 acid is concerned. The simultaneous discovery by Needham 

 and Pillai and by Meyerhof and colleagues that phosphoryla- 

 tion of adenylic acid by inorganic phosphate was coupled to 

 the oxidation of triose phosphate formed the beginning of a 

 new epoch in our appreciation of the utilization of chemical 

 energy in cellular metabolism; further detailed knowledge of 

 the triose phosphate system is due to the work of Warburg 

 and Christian (1939). The general ideas of the relation be- 

 tween respiration and the utilization of the chemical energy 

 for synthetic processes we owe mainly to the work of Lip- 

 mann (1941). The process of phosphate group transfer has 

 as much relevance to cellular energetics as hydrogen trans- 

 fer. The two processes are intimately connected in aerobic 

 respiration and in glycolysis. 



The reduction of phosphoglyceric acid to triose phosphate 

 can occur with reduced DPN when there is a source of ATP 

 for the phosphate transferring step. Thus it can be seen how 

 both H transfer and group transfers involving phosphate are 

 important in mechanisms concerned with the reduction of 

 CO2 to the level of carbohydrate. Many experimental studies 

 have been directed towards finding out whether phosphoryla- 

 tion reactions are to be regarded as part of the photosynthetic 

 mechanism. The evidence in favour is strong; it is, however, 

 circumstantial because so far we have no knowledge of a 

 cell-free system involving phosphate which can be directly 

 related to the process of photosynthesis. 



Hexose and hexose phosphate dehydrogenases. The glucose 

 6-phosphate dehydrogenase has been shown to catalyse the 

 reaction: 



Glucose-6-phosphate + TPN + = 6-phosphogIuconate + TPNH + H + 



The enzyme is found to be present in widely different types 

 of organisms, including plants. The reaction would be the 

 first step in the oxidative breakdown of hexose by the trans- 

 aldolase transketolase cycle (p. 97). As written the reaction 



