1696 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



The relation of the Chicago model (36.13) to that preferred in Berkeley 

 in 1950 is best illustrated by schemes 36.IIIA and B. 



Ce < 2 C3 Ce < 3 Ce < 6 Cj 



(A) 



4C3 



(B) 



2C6 



+6C0j 



-^GC, 



4C2 <- 



^Q 



2C4 



Scheme 36. III. Transformation of the carbon chain in the carhoxylation cycles. 

 (A) Calvin, Bassham et al. (1950). (B) Fager, Rosenberg and Gaffron (1950). 



Calvin and co-workers sustained until 1953 the belief in the "second 

 carhoxylation" because of the kinetic evidence detailed in section 9. While 

 the question of the C3 acceptor (and, more generally, of the role of C4 com- 

 pounds in the reaction sequence of photosynthesis), thus remained in dis- 

 pute, a new important experimental finding was added by the Berkeley 

 group — the identification of sedoheptulose and ribulose phosphates as 

 early tagged products in photosynthesis (described in section 7 above). 

 These observations, and the indications that the C7 and C5 compounds may 

 serve as precursors of the C2 acceptor (rather than as intermediates in the 

 formation of the final products of photosynthesis), led Calvin and co- 

 workers to a new scheme for the transformation of the carbon chain in 

 photosynthesis. This mechanism is shown in scheme 36. IV. In this 

 scheme, the unknown acceptor C2 is regenerated in two successive reactions 

 — one half of it in the reductive splitting of a heptose, producing a pentose, 

 and the other half in a similar splitting of the pentose. The heptose is 

 supposed to be produced by reductive condensation of a triose and a C4 

 compound (oxalacetate?). The malate still appears as a by-product (and 

 a possible "bridge" to the respiratory system). 



The essential difference between scheme 36. IV and Calvin's earlier 

 schemes is the replacement of the direct split C4 ^- 2 C2 by a roundabout 

 mechanism C4 + C3 ^ C7 -^ C2 + C5 -* 2 C2 + C3, with a C3 (triose) 

 molecule acting as a catalyst and being regenerated at the end. 



When Bassham et al. (1954) finally decided — as mentioned in section 6 — 

 to follow Gaff ron et al. in postulating only one carhoxylation, scheme 36.IV 

 had to be changed, and evolved into scheme 36. V. Eliminating one car- 

 hoxylation permitted reduction of the number of hydrogenations from four 

 to one — the reduction of glyceric acid to glyceraldehyde. The rest of the 



