THE CHEMISTRY OF PHOTOSYNTHESIS 



117 



By means of 6-phosphogluconic acid dehydrogenase, TPN+ and TPP+, 

 6-phosphogluconic acid is converted by dehydrogenation and decarboxylation 

 to ribulose-5-phosphate or xylulose-5-phosphate. This is changed into ri- 

 bose-5-phosphate by phosphoriboisomerase. These reactions were also dis- 

 covered by Warburg and Christian (35). According to Horecker (11), the 

 reaction scheme can be written as follows (see also Dickens (6), Racker (25) 

 and Cohen (4)) : 



COOH 



I 

 HCOH 



I 

 HOCH 



HCOH 



HCOH 



I 

 H2COPO3H2 



6-phospho- 

 gluconic 

 acid 



COOH 

 HCOH 



-2H CO 

 TPN+ HCOH 

 HCOH 



H2COPO3H2 



3-keto-6- 



phosphoglu- 



conic acid 



CHoOH 



CO, CO 



TPP+ HCOH *- 



HCOH 



I 

 H2COPO3H2 



ribulose- 

 5-phosphate 



CHO 

 HCOH 

 HCOH 

 HCOH 



H2COPO3H2 



ribose- 

 5-phosphate 



From the pentose phosphate produced two C atoms are oxidized to COo, 

 leaving triose phosphate. As pentose phosphate is continuously regenerated, 

 there is a cyclic process using hexose phosphate and producing triose phos- 

 phate and CO2. These reactions do not involve the glycolytic pathway, so 

 that the pentose phosphate pathway is often denoted as the hexose phosphate 

 shunt. Two main reactions are responsible : transketolation and transaldola- 

 tion. 



The enzyme transketolase (TK) transfers the ketol group ( — CO CH2OH) 

 to aldehydes: 



CH2OH 



CH2OH 



CO 



I 

 HOCH 



I 

 HCOH 



I 

 H2COPO3H2 



xylulose- 

 5-phosphate 



CHO 



HCOH 



I 

 + HCOH 



I 

 HCOH 



I 

 H2COPO3H2 



ribose- 

 5-phosphate 



TK 



— > 



CO 



HOCH 



HCOH 



HCOH 



HCOH 



H2COPO3H2 



sedoheptulose- 

 7-phosphate 



CHO 

 + HCOH 



H2COPO3H2 



phosphoglycer- 

 aldehyde 



The enzyme thus splits xylulose-5-phosphate into a C3 fraction and a C2 frac- 

 tion. However, the C2 fraction is not produced as free glycolaldehyde but 

 reacts with ribose-5-phosphate to give the phosphate of a C7 sugar, sedoheptu- 

 lose-7-phosphate. Transketolase is also involved in the following conversion 



