Cs AND C7 SUGARS AS INTERMEDIATES 



1673 



It is easy to see that a pentose has the correct "reduction level" (L = 1.0, 

 c/. p. 109) to give glyceric acid (L = 0.833) after dilution with one molecule 

 CO2 {L = 0): 5/(5 + 1) = 0.833. The mechanism of reaction (36.7) 

 involves a not implausible internal dismutation-conversion of two neigh- 

 boring keto groups into a carboxyl and a hydroxyl group (c/. second arrow 

 in equation 36.8) : 



(36.8) 



CH2O© 



1 

 CHOH 



CHOH 



COOH 



:;ho© 



+CO1 I 

 > CH2OH 



I +HjO _^ 



C=0 CH(0H)2 



CH2O© 



C=0 

 CH2O© 



COOH 



I 

 -^2 CHOH 



CH2O© 



where © is a phosphate residue, H2PO3. 



Reaction (36.8) recently was observed in vitro by Weissbach, Smyrniotis 

 and Horecker (1954) with a leaf extract, and by Quayle, Fuller, Benson 

 and Calvin (1954) with a Chlorella extract as catalyst, making its inclusion 

 in the hypothetical reaction sequence of photosynthesis very plausible. 

 In spinach extracts, reaction (36.8) is stimulated by TPN and ATP, sug- 

 gesting that it may include a reversible oxidation-reduction stage. 



It will be noted that it is due to its occurrence as a diphosphate that 

 ribulose can be split into two molecules of PGA (in reaction 36.8 the prod- 

 ucts are one molecule of a-PGA and one molecule of /3-PGA) . 



As discussed in chapter 8 (section A4), carboxylation of RH to RCOOH 

 is, under standard conditions, a reaction with a total energy close to zero, 

 and a positive standard free energy of several kcal/mole (table 8. VIII) ; 

 consequently, the equilibrium usually lies on the side of decarboxylation. 

 The kinetics of CO2 fixation in photosynthesis, on the other hand, points to 

 CO2 fixation reaction with an equilibrium far on the side of synthesis, even 

 at 0.03% CO2 (c/. chapter 27). Even known reversible carboxylations, 

 such as that of pyruvic to oxalacetic acid, do not satisfy this requirement. 



Bassham et at. (1954) attempted to estimate the free energy of reaction 

 (36.8). Assuming pH 7, and concentrations of 5 X 10"* M for the pen- 

 tose, 10-=^ M for carbon dioxide, and 1.9 X 10"^ M for glycerate (for justi- 

 fication of these values, see section 9 below), they obtained AF = —7 

 kcal./mole, a value which would place the equilibrium far on the side of 

 carboxylation. 



Using standard bond energies (table 9. II) one notes that the dismuta- 

 tion reaction (second arrow in reaction 36.8) which is, in essence: 



