labile ^-keto acid. Evidence for the existence of this unstable 

 intermediate has been adduced from in vivo studies (5) . It 

 has not been isolated in the in vitro reaction with the enzyme 

 carboxydismutase. The product of the reaction in vitro is 2 

 molecules of 3-phosphoglyceric acid (PGA). The products 

 in intact photosynthesizing cells may be 2 molecules of PGA 

 or, as kinetic studies indicate (3), 1 molecule of PGA and 1 

 molecule of triose phosphate. 



Once formed, the PGA is transformed in two ways. 

 Some molecules are converted to products outside the cycle 

 while the remainder are reduced to 3-phosphoglyceraldehyde 

 via a reaction of the type shown in Eq. (4). The enzymes 

 responsiule for the two successive steps in the reduction are 

 probably similar to phosphoglycerylkinase (6) and triose 

 phosphate dehydrogenase (7-10). 



The next phase of the carbon reduction cycle is the 

 conversion of 5 molecules of triose phosphate to 3 molecules 

 of pentose phosphate by a series of reactions. These reactions 

 include condensations (aldolase), carbon-chain-length dismu- 

 tations (transketolase), removal of phosphate groups (phos- 

 phatase), and interconversions of different pentose phos- 

 phates (isomerase, epimerase). Enzyme systems that catalyze 

 reactions similar to these steps are listed in Table 2. The 

 sequence of steps may be seen in the cycle diagram (Figure 2). 



The various pentose phosphates are converted to ribu- 

 lose-5-phosphate. The final step is the formation of ribulose 

 diphosphate (RuDP) from ribulose-5-phosphate. This step 

 requires 1 molecule of ATP [Eq. (5)]. 



For every reaction in the cycle to occur at least once 

 (a complete turn of the cycle), the carboxylation reaction 

 must occur three times. The net result of each complete 

 turn of the cycle is the incorporation of 3 molecules of CO2 

 and the production of 1 three-carbon (or V2 six-carbon) or- 

 ganic molecule. Each complete turn of the cycle would re- 

 quire 6 molecules of TPNH or equivalent reducing cof actor 

 (2 per CO2) and 9 molecules of ATP, if each Ce carboxylation 

 product is split to 2 molecules of PGA and if all the PGA 



