1692 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



least one photochemical step) . Of the two phosphoglyceric acid molecules 

 present at the end of the reaction cycle, one could be reduced to triose and 

 thence dimerized to hexose (reversal of glycolysis), while the other could 

 be returned into the cycle. 



The two reaction systems are presented in scheme 36.11, which omit 

 phosphorylations and leave open the specific nature of intermediates in the 

 two cycles. Calvin and co-workers did, however, speculate in some detail 

 on the nature of these intermediates. The first fact relevant to these specu- 

 lations was that after very brief photosynthesis in strong light, when 

 >95% of total tracer taken up was contained in phosphoglyceric acid, 

 practically all of it was located in the carboxyl group of this acid. This 

 made it likely that PGA is a direct product of carboxylation, and since it 

 is a C3 acid, leads to the assumption of a C2 carbon dioxide acceptor. 



Catabolic cycle 

 }/2 hexose 



Anabolic cycle 



-> triose 



^nexose <- 



triose 



-2[H] 



-HiO 



-> 2 pyruvate 



+3 H2O 



-3 CO2, -10[H] 



1 pyruvate 



+2[H] 

 2 glycerate 



-2 H2O 



+ 3 CO2, +10[H] 



-^ 1 glycerate 



Scheme 36.11. The catabolic cycle in respiration and the anabolic cycle in photosynthe- 

 sis, according to Calvin, Benson and co-workers (1948). 



Retaining, at first, the remainder of the originally postulated cycle, 

 Calvin and Benson (1949) suggested the interpolation between the last 

 C4 acid in this cycle (succinic acid) and pyruvic acid, of the following reac- 

 tion steps : 



OH 



CHsC/ 



acetyl phosphate 



+ 2[H1 



* CH3— C— H 



-H2O 



OPO3H2 

 phosphoacetaldehyde 



+ CO2 CH2— CH— COOH 



(36.12) CH2=CHOP03H2 > \ \ 



+ H2O OH OPO3H2 



2-phosphoglyceric 

 acid 



vinyl phosphate 

 (C2-acceptor) 



-H^O CH2=C— COOH 



> I 



OPO3H2 



2-pho-:pho-enol- 

 P3^ruvic acid 



This sequence was to replace a reversal of Lipmann's reaction (reduc- 

 tive carboxylation of acetate to pyruvate) in the original cycle; the other 



