CARBOXYLATION AND THE {CO2} COMPLEX 20] 



rate to become insensitive to the concentration of carbon dioxide long 

 before the complex, {CO2}, has been fully saturated; but this distortion 

 does not change the order of magnitude of the carbon dioxide concentra- 

 tion required for saturation. If the complex, {CO2}, is half-saturated at 

 carbon dioxide concentrations of the order of 10~^ mole per liter (0.03% 

 CO2 in the air), the free energy of its formation must be of the order of 

 — 6 kcal at room temperature (Ruben 1943, estimated ^F = — 2 kcal) — 

 a considerably more negative value than the free energies of carbamina- 

 tion and carboxylation quoted in the first part of this chapter; more 

 negative than even the free energy of association of carbon dioxide with 

 carbonic anhydrase (page 186). 



As an alternative to this "static" interpretation of the carbon dioxide 

 saturation of photosynthesis, we will consider in volume II, chapter 27, 

 the hypothesis of Franck, according to which this saturation is due 

 mainly to kinetic factors (slow rates of certain partial processes of photo- 

 synthesis which make the utilization of additional carbon dioxide im- 

 possible). The carboxylation equilibrium is assumed by Franck to lie 

 very far on the association side, even at the lowest carbon dioxide 

 pressures. This hypothesis implies that the free energy of carboxylation 

 is even more negative than the above calculated value of —6 kcal per 

 mole. 



Experiments with radioactive C*02, to be described below, make it 

 plausible that the complex ICO2) can be dissociated by evacuation; a 

 direct manometric determination of its CO2 tension could determine which 

 of the two alternative hypotheses is to be preferred. 



Ruben (1943) suggested that the strong affinity of the unknown 

 acceptor for carbon dioxide may be caused by a coupling between its 

 carboxylation and the hydrolysis of an "energy-rich" organic phosphate 

 which occurs with the liberation of 10 to 12 kcal per mole (c/. Chapter 9, 

 page 224). If the uptake of one molecule of carbon dioxide were coupled 

 with the formation of one molecule of inorganic phosphate from an 

 "energy-rich" phosphorylated molecule, the net change in free energy 

 could be of the magnitude required for the explanation of the early car- 

 bon dioxide saturation of photosynthesis. 



In chapter 5, the experiments of Vogler and Umbreit (1942) on 

 Thiohacillus thiooxidans have been described. After a period of sulfur 

 oxidation which is accompanied by a transfer of inorganic phosphate 

 from the medium into the cells, these bacteria prove to be capable of 

 absorbing a certain quantity of carbon dioxide in absence of sulfur and 

 oxygen. It was suggested on page 114 that this absorption probably is 

 a preliminary fixation (e. g., carboxylation) rather than a reduction of 

 carbon dioxide. It is accompanied by a release of inorganic phosphate 

 by the cell, and this can be considered as an argument in favor of Ruben's 



