REACTIONS OF CARBON DIOXIDE 97 



carbohydrate. The cycle operating with three equivalents 

 of hexose would be summed as a reaction representing: the 

 formation of 3 molecules of CO2 and one molecule of triose 

 phosphate from one molecule of hexose phosphate. The 

 oxidation steps are the oxidation of glucose 6-phosphate 

 (described later) and the oxidative decarboxylation of 6- 

 phosphogluconate, TPN being involved in both steps. The 

 sedoheptulose 7-phosphate is formed in the 'transketolase' 

 reaction when ribulose 5 -phosphate is split to give triose 

 phosphate and a two-carbon fragment combined with the 

 enzyme. Transketolase has been obtained in crystalline form 

 from yeast and in purified form from spinach leaves. The 

 coenzyme, thiamine pyrophosphate (cocarboxylase), may be 

 separated reversibly. No reaction takes place unless an 

 aldose acceptor, such as ribose-5-phosphate, for the C2 

 fragment is present; the C7 compound is then formed. 

 Sedoheptulose 7-phosphate is broken down in an analogous 

 'transaldolase' reaction where a 4-carbon (erythrose) frag- 

 ment may be split off. Ribulose phosphate acting as acceptor 

 for the aldose fragment results in the formation of fructose 

 6-phosphate and triose phosphate. Starting with 3 equiva- 

 lents of phosphogluconic acid or hexose monophosphate, the 

 process may be represented as follows: 



3C6 = 3C5 + 3CO2 oxidation 

 C. + (C:) : ^cf ^ ""'} «--ketoIase 



2Cq = Cfi aldolase 



Ce = C3 +3CO2 



The (C2) and (C4) carbon residues are indicated only in the 

 form of their complex with an enzyme, the (C2) is most 

 probably attached via the thiamine pyrophosphate group. 

 This scheme, given here in a simplified form, shows the 

 possible conversions of the sugar phosphate esters and will 

 be referred to later (p. 109) in connexion with the inter- 

 pretation of results obtained on photosynthesis with tracer 

 carbon. There is no indication from these that a direct 



