EVOLUTION OF THE CO2 REDUCTION MECHANISM 



1697 



(pentose) 



(heptose) 



Scheme 36.IV. CO2 reduction mechanism with regeneration of C2 ac- 

 ceptor via Ct and C5 sugars (two carboxylations, Ci and C2, four reductions, 

 Ri, R2, R3, R4) (Calvin et al. 1951). 



reactions in scheme 36.V are disproportionations or condensations of sugars 

 (more precisely, sugar phosphates) without changes in the reduction level : 

 C3 + C4 -* C7; C3 + C7 -^ 2 Cs; Ce + C3 -^ C4 + C5. Malic acid must be 

 derived in scheme 36. V from an as yet unidentified C4 sugar (tetrose). 

 The C2-acceptor formation had been eliminated in passing from scheme 

 36.IV to scheme 36.V in favor of a direct carboxylation of pentose coupled 

 with splitting into two C3 fragments. (An unstable intermediate Ce com- 

 pound — on the reduction level of a uronic acid — could be postulated here.) 

 The simplification of the scheme to a single carboxylation and a single 

 hydrogenation makes it fundamentally similar to Gaffron, Fager and 

 Rosenberg's scheme 36.IIIB, with the important elaboration of the mecha- 

 nism by which % of the primarily formed triose is converted back into 

 phosphoglyceric acid. 



