PHOTOSYNTHESIS 



oxidation-reduction potential of the latter is now considered to 

 be —0.324 v. (15), there is a potential difference between cyto- 

 chrome and TPN+ of 0.65 v., or a negative free energy of 30.3 

 kcal. for the oxidation of one mole of TPNH by oxidized cyto- 

 chrome /. 



Very little is known about the mechanism of formation of 

 ATP during the oxidation of TPNH. There is enough energy 

 available in each of the steps suggested above to bring about 

 the formation of 2 moles of ATP with ease. We can suppose 

 that the general type of mechanism for the oxidative formation 

 of ATP might involve esterification of a hydroxyl group with 

 inorganic phosphate, dehydrogenation of an adjacent C — C or 

 C — N bond with a suitable oxidizing agent to form a "high- 

 energy phosphate group," a reaction with ADP to form ATP 

 and an unsaturated alcohol, and finally, reduction of the 

 unsaturated alcohol to the orginal substance. 



H 

 -C- 



H 



-C- 



H 



-f HOPO3H 



H 

 -C- 



OH 



H 

 -C- 

 I H 



OPO3H- 



+ H2O 



H 

 -G- 



H 

 -G 



— + R 



I H 

 OPO3H- 



■^ RH2 H C— C— 



I H 



OPO3H- 



-G^=G- 



I H 



OPO3H- 



+ ADP 



-C= 



OH 



=C h ATP 



H 



-G— C h R'H2 



I H 



OH 



H H 



-> — G G h R' 



I H 



OH 



-> RH2 + R' + ATP 



R + R 'H2 + ADP + HsPOr — 



The — CH2 group of the reacting alcohol can also be 

 — NH — . Considering the number of steps involved in the above 

 mechanism, we could expect that at least 5 kcal. negative free- 



59 



