226 REDUCTION OF CARBON DIOXIDE CHAP. 9 



volt; the succinate dehydrogenase transfers the hydrogen from succinate 

 to cytochrome c, whose potential is much higher — namely, + 0.27 volt. 

 The energy liberated in this transfer could well be used for the synthesis 

 of one molecule of a high-energy phosphate. 



In the respiration of dialyzed muscle extracts, five or six glucose 

 molecules were found to be phosphorylated to diphosphates simultane- 

 ously with the oxidation of one glucose molecule to carbon dioxide. 

 This indicates (Kalckar 1941) that ten, and perhaps all twelve hydrogen 

 transfers to dehydrogenases, which occur in the oxidation of one glucose 



molecule (CeHiaOe + 6 H2O + 12 Ed > 6 CO2 + 12 HaEc, where 



Ed = dehydrogenase), are associated with the production of one high- 

 energy phosphate ester (glucose serves, in these experiments, as the final 

 "phosphate acceptor," by taking phosphate over from the adenosine 

 triphosphate). 



In this way, as much as 20-25% of the combustion energy of glucose 

 could be converted into phosphate energy, to be utilized for muscular 

 work. The remaining 75-80% is liberated in the downward slide of the 

 hydrogen atoms from reduced dehydrogenases (whose potentials lie 

 between + 0.3 and volt) to oxygen (whose potential at pH 7 is — 0.81 

 volt). Some evidence speaks in favor of phosphorylations also being 

 associated with these stages of respiration (in which the largest part of 

 the combustion energy is liberated), but the nature and extent of these 

 phosphorylations is as yet unknown. 



5. Phosphorylation and Photosynthesis 



We found, in the preceding section, that phosphorylation permits 

 the oxidation of carbonyl groups to carboxyl groups without dissipation 

 of energy (and may have the same effect also on other steps in the 

 transfer of hydrogen from sugars to oxygen). Thus, phosphorylation 

 could help in bringing about the reversal of respiration in photosynthesis. 

 The possible role of phosphoric acid in photosynthesis (and chemo- 

 synthesis) was mentioned twice before: in chapter 8, in discussing the 

 mechanism of preliminary carbon dioxide fixation in photosynthesis; and 

 in chapter 5 (page 114), in discussing the mechanism of chemosynthesis 

 in Thiohacillus thiooxidans. We shall now see that phosphorylation 

 could also be used in the interpretation of the carbon dioxide reduction in 

 photosynthesis. This was first suggested by Ruben (1943), who thought 

 that the carboxyl group in the complex {CO2I may be phosphorylated 

 to facilitate its reduction. According to this hypothesis, the reductants, 

 {H} or HX, produced by the primary photochemical process (c/. Chapter 

 7) have reduction potentials of the order of those of the pyridinium 

 nucleotides {i. e., about + 0.3 volt), and are thus unable to reduce free 

 carboxyl groups, but may be able to reduce carboxyl phosphates. 



