1672 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



Table 36.VII 



C(14) Distribution in Products Formed by 5.4 Sec. Exposure of Scenedesmus to 

 C*02 DURING Steady-State Photosynthesis (after Bassham et al. 1954) 



Sedoheptulose and ribulose have similar configurations (at the C-5 and 

 C-6 atoms of the former and the C-3 and C-4 atoms of the latter). Cleavage 

 of an aldoheptose into a "biose" (glycolaldehyde) and a pentose appears 

 a likely reaction from recent chemical studies of this type of compound 

 by Horecker (cf. below) ; in the case of sedoheptulose the product of this 

 splitting will be ribose, which could isomerize to ribulose; a similar cleavage 

 of ribulose must lead to a "biose" and a triose. Benson et al. therefore 

 suggested that the function of sedoheptulose and ribulose in photosynthe- 

 sis is to serve as sources of the C2 body (whose carboxylation leads to PGA) . 



The location on the chromatogram suggests that ribulose is formed as a 

 diphosphate, and sedoheptulose as a monophosphate. This identification 

 of the esters was further supported (Benson, 1952) by a determination of 

 the ratio C(14):P(32) in the products of prolonged photosynthesis in 

 doubly tagged medium. This ratio was 0.9 in the fraction suspected to be 

 pentose diphosphate, as against 1.35 in the fraction containing PGA, 2.3 

 in that identified as glucose monophosphate, and 2.6 in that containing 

 fructose and sedoheptulose. The theoretical values for the first three 

 compounds (assuming complete isotopic equilibration with the medium) 

 were 2.5, 3.0 and 6.0, respectively. Since all values were smaller than ex- 

 pected, the saturation with C* must have been incomplete, but the order 

 of the ratios clearly points to the first-named fraction as the one containing 

 more phosphate residues than all the others. 



As mentioned above, when the C5 and C? intermediates were first found 

 it was suggested that they are the source of the then postulated "C2 ac- 

 ceptor" in photosynthesis. Subsequently, a simpler hypothesis was sug- 

 gested — that the C5 sugar phosphate itself serves as the carbon dioxide ac- 

 ceptor, giving rise to tw^o molecules of phosphoglycerate by hydrolytic 

 splitting : 



(36.7) 



+ HjO 



Cs + C, > 2 C3 



