EVOLUTION OF THE CO2 REDUCTION MECHANISM 1691 



Calvin and Benson assumed, when beginning their studies (1947), that 

 carbon dioxide and a hydrogen donor ("reducing power") must be drawn 

 into a cycle similar — but opposite in sense — to the known decarboxylation 

 and dehydrogenation cycles in respiration. At first, when they made no 

 distinction between "photosynthetic" and "respiratory" tracer uptake, 

 the occurrence of tagged succinic, malic, and fumaric acids, and the absence 

 of tagged C5 and Ce acids, led them to suggest that the carboxylation cycle 

 in photosynthesis is the reversal of the short C3-C4 decarboxylation cycle 

 shown in scheme 9. II (Vol. I, page 224) — rather than of the complete Krebs 

 cycle, illustrated by scheme 36.1 above. The early appearance of the 

 tracer in certain amino acids caused them to postulate side reactions of C3 

 and C4 acids in the cycle (such as pyruvic and oxalacetic acids, whose reduc- 

 tive amination leads to alanine and aspartic acid, respectively). This 

 scheme contained two primary carboxylation reactions — the Wood-Werk- 

 man reaction : 



(36.10) C*02 + CH3COCOOH , C*00HCH2C0C00H 



and the Lipmann reaction: 



(36.11) C*02 + CH3COOH + 2[H1 , CH3C0C*00H + H^O 



(cf. equations 36.1 and 36.2). 



Later (1948, 1949), when succinic acid faded out of the picture as a likely 

 intermediate of photosynthesis, and phosphogly eerie acid was found to be 

 the main primary carboxylation product, the specific cycle postulated in 

 1947 had to be altered. The initial C(14) fixation in carboxyl group of 

 phosphoglyceric acid, and its subsequent penetration into the two other 

 positions (Table 36. V) was taken as evidence that phosphoglyceric acid is 

 the pivot of the anabolic cycle — the role assumed in the catabolic cycle, 

 as represented in scheme 9. II, by pyruvic acid. (It will be noted that enol 

 pyruvic acid is a dehydration product of glyceric acid, and that the two 

 acids have the same reduction level.) In the catabolic cycle, 9. II, one 

 molecule of pyruvic acid accepts a second similar molecule and carries it 

 through a series of reactions, as a result of which it is completely torn apart 

 (all carbon being accepted by decarboxylases and later released as free 

 CO2 gas, and all hydrogen transferred to dehydrogenases and thence, 

 through a series of intermediate catalysts, to oxygen) . The other molecule 

 of pyruvic acid is regenerated at the end of the cycle, so that the same 

 series of reactions can be repeated. 



Calvin and co-workers thought (1948) that a similar cyclical mechanism, 

 running in the reverse sense, can start with one molecule of phosphoglyceric 

 acid and end with two such molecules, having assembled the second one 

 from carbon dioxide (via one or several carboxylases) and from hydrogen 

 atoms donated by water (via an unknown series of reactions, including at 



