116 



PROBLEMS OF PHOTOSYNTHESIS 



The latter reaction discovered by Utter et al. (31) produces oxaloacetic acid 

 COOH COOH 



CO'-POsH., + GO2 + ID? ^ CO + IT? 



CHs 



CH2 



COOH 

 It is possible that ADP and not IDP may be the phosphate acceptor. 



During the breakdown of pyruvic acid 10 H atoms are transferred to the 

 respiratory chain. As the free energy decrease in oxidation of pyruvic acid 

 is about 275000 cal/mole, about 55000 cal/mole are liberated per 2 H atoms. 

 This amount of energy is accumulated in energy-rich phosphate bonds and it 

 is assumed that 3 molecules ATP are produced per 2 H atoms. This oxida- 

 tive phosphorylation is different to anaerobic phosphorylation in glycolysis, 

 the latter being substrate phosphoryladon, the chemistry of which has been 

 clearly defined (see § 44). In § 51 a third form of phosphorylation, due to 

 the action of light, is discussed. 



§ 47 The Pentose Phosphate Pathway 



In addition to glucose, various carbohydrates — tetroses, pentoses, hexoses, 

 heptoses — have been found to occur in living matter. These may also be 

 sources of energy or fulfil important biological functions ; thus, pentoses are 

 parts of nucleotides. The existence of ways of carbohydrate breakdown other 

 than glycolysis and the tricarboxylic acid cycle might therefore be suspected. 

 In fact, in the most varied types of cell it has been found that glucose-6- 

 phosphate can be oxidized in quite a different way. It had been noted that 

 glucose breakdown continues in various cell extracts even when potent in- 

 hibitors of glycolysis such as iodoacetate or NaF are added. Furthermore, 

 enzymes had been discovered which are TPN-specific whereas the enzymes 

 of glycolysis are mosdy DPN-specific. 



Warburg et al. {1)1) isolated in 1931 the enzyme glucose-6-phosphate de- 

 hydrogenase from yeast. This enzyme, which they called the "Zwischen- 

 ferment," catalyses, with TPN, the oxidation of glucose-6-phosphate to 6- 

 phosphogluconate. An intermediate product formed is the 5-lactone of 6- 

 phosphogluconic acid, the lactone ring of which is split by a lactonase (5). 



CHO OC— — I COOH 



HCOH 

 HOCH 

 HCOH 

 HCOH 



-2H 

 TPN+ 



HCOH 

 HOCH O 

 HCOH 

 HC 



+ H.0 



H2COPO3H2 



glucose-6- 

 phosphate 



H2COPO3H2 



8-lactone 



HCOH 



I 

 HOCH 



HCOH 



HCOH 



H2COPO3H2 



6-phospho- 

 gluconic acid 



