CELLULAR METABOLISM 121 



Ochoa (68, 69) has thus shown a number of coupled reactions which 

 in in vitro experiments end in carboxylations : 



(i) a-ketoglutarate + CO2 + glucose 6-phosphate ^ i-isocitrate + 



phosphogluconate ; 

 (2) Malate + TPN ^ pyruvate + COo + TPNH, 



These reactions do not occur in the presence of pure isolated enzyme 

 systems. Carboxylation of a-ketoglutarate and pyruvate, difficult of at- 

 tainment because of the values of the equilibrium constants, was ren- 

 dered possible by coupling carboxylation with an oxidation-reduction 

 reaction. Whether such reactions do occur in living cells is problematic. 



Oxidations and Phosphorylations 



Whenever an important discovery is made, there spring forth applica- 

 tions, similarities, extensions. Years have to pass by before harvesting 

 from the melee the useful crop. The coupling between phosphorylation 

 and oxidation observed by Meyerhof (59) in his studies on the mecha- 

 nism of glycolysis was explained in 1939 by the striking demonstrations 

 in Warburg's laboratory (83) which culminated in 1942 in the isolation 

 and crystallization of the specific transphosphorylase (18). Warburg 

 and his coworkers showed that the aldehyde group in phosphoglyceralde- 

 hyde is not oxidized as aldehyde hydrate but as aldehyde phosphate. By 

 a non-enzymatic reaction glyceraldehyde 3-phosphate takes up another 

 phosphate group to give 1,3-diphosphoglyceraldehyde, which is oxidized 

 by DPN to 1,3-diphosphoglyceric acid: 



OH OH 



HX-0-P=0 H^C-O-P^rO 



I OH I \ 



HCOH HCOH OH 



I +DPN ^:±: | ^O -f DPN Hj 



HC-OH C^ 



'O-PO3H 



a-^-^2 



1 ^o« ^ 



0-P=:0 



The phosphate transfer enzyme, a Mg-protein, transfers the unstable 

 phosphate group into adenosinediphosphate to give ATP. This reaction 

 has been taken as a model of phosphate transfer from aceylphosphates : 



