no PROBLEMS OF PHOTOSYNTHESIS 



An equilibrium exists between the two triose phosphates 



Due to the triosephosphate isomerase, about 96% of phosphodihydroxy- 

 acetone is present at the equihbrium state so that the reaction practically pro- 

 ceeds from right to left. However, another enzyme, phosphoglyceraldehyde 

 dehydrogenase, drives the 3-phosphoglyceraldehyde so rapidly away that the 

 reaction proceeds from left to right. Warburg and Christian (36), who dis- 

 covered phosphoglyceraldehyde dehydrogenase, called this enzyme the "oxi- 

 dizing fermentation enzyme." With DPN+ and in the presence of inorganic 

 phosphate it catalyses the formation of 1,3-diphosphoglyceric acid, which is 

 afterwards dephosphorylated to produce 3-phosphoglyceric acid. 



HCO OCO-POsH2 



HCOH + H;iPO, + DPN+ ^ HCOH + DPNH + H + 



H2COPO3H2 H2COPO3H2 



3-phospho- 1 ,3-diphospho- 



glyceraldehyde glyceric acid 



OCO--PO-,H2 COOH 



HCOH + ADP ^ HCOH + ATP 



H2COPO..H2 H2COPO3H2 



1,3-diphospho- 3-phospho- 



glyceric acid glyceric acid 



In the first of these reactions an energy-rich phosphate bond is produced 

 which, according to the second reaction, is accumulated in ATP. The active 

 group of phosphoglyceraldehyde dehydrogenase has been considered by 

 Racker (24) to be the sulfhydryl group of its glutathion part. However, re- 

 cently, Warburg (38) confirmed the stoichiometric character of the aldehyde 

 oxidation in the presence of phosphate, which he and Christian (36) first 

 reported in 1939. 



The further oxidation of 3-phosphoglyceric acid is preceded by an intra- 

 molecular transfer of the phosphate group due to the action of phospho- 

 glyceromutase 



COOH COOH 



HCOH ^ HCOPO3H2 



H2COPO3H2 H2COH 



3-phospho- 2-phospho- 



glyceric acid glyceric acid 



