PHOTOSYNTHESIS 



reducing agent (TPNH or DPNH) in plentiful supply, the j3-keto 

 acid might be reduced and then split by a rearrangement to one 

 molecule of PGA and one of phosphoglyceraldehyde. This 

 would provide a route for the formation of one molecule of 

 triose phosphate without the requirement of an ATP molecule. 

 This possibility should be kept in mind during the following 

 discussion of the energy requirements of the carbon reduction 

 cycle, which is based on the assumption that each molecule of 

 triose phosphate formed requires the supply of a molecule of 

 ATP. 



Estimation of the free energy change for the reaction of 

 RuDP with CO2 and water to produce two molecules of PGA 

 showed a net free-energy change at physiological conditions 

 which was either zero or negative (4), indicating that the 

 reaction would proceed under the influence of a suitable catalyst 

 without energetic coupling with some other reaction. It thus 

 appears that energy is supplied to the cycle in the form of TPNH 

 or ATP. Both are involved in the reduction of PGA to phos- 

 phoglyceraldehyde, whereas only ATP is needed for the phos- 

 phorylation of RuMP. For each carboxylation, using one 

 molecule of CO2 and producing two molecules of PGA, two 

 TPNH molecules and two ATP molecules would be converted 

 to TPN+ and ADP -f PO4 in the subsequent reduction of PGA 

 to phosphoglyceraldehyde. The enzymatic formation of RuDP 

 from ribulose monophosphate has been shown by Weissbach 

 et al. (63) to require a molecule of ATP. Since the enzymatic 

 conversion of triose phosphate to ribulose monophosphate proba- 

 bly does not require the expenditure of any ATP or other sub- 

 stances which supply energy through energetic coupling reactions 

 the net supply of energy to the carbon reduction cycle for each molecule of 

 CO2 reduced is that required for the formation of two molecules of 

 reduced TPN and three molecules of A TP as follows: 



2(TPN+ + H2O > TPNH + H+ + V2 O2) AF = +103 kcal. 



3 (ADP + PO; > ATP) AF = +32 kcal. 



which adds up to 135 kcal. (If one molecule of triose phosphate 



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