SEVERO OCHOA 



Carbon Dioxide Fixation in Chemosynthesis 

 and Photosynthesis 



It is now known that photosynthesis can be divided into two 

 phases relatively independent of one another: (1) a so-called "dark" 

 reaction which occurs in the absence of light and consists of a reversible 

 fixation of carbon dioxide to form a carboxylic acid; and (2) the photo- 

 lytic fission of water, yielding hydrogen with liberation of oxygen. 

 Hydrogen produced by photolysis is used to reduce the products 

 formed by carboxylation. In chemosynthesis, hydrogen is obtained 

 by oxidation of inorganic compounds — a process that also supplies 

 energy (6,7,23-25). 



It would appear that essentially the same mechanisms that func- 

 tion in carbon dioxide fixation by heterotrophic organisms are operative 

 in both photosynthesis and chemosynthesis, with the difference that, 

 in the case of the heterotrophs, hydrogen and energy are derived by 

 oxidation of organic materials. The widespread occurrence in plants 

 of di- and tricarboxylic acids (malic, citric, isocitric) and of the enzymes 

 that participate in their metabolism (fumarase, malic dehydrogenase, 

 aconitase, isocitric dehydrogenase) lends support to such a view. Phos- 

 phorylation processes which, as we have seen, are essential for reductive 

 carboxylations, are connected with carbon dioxide fixation in the sulfur 

 oxidizing autotroph Thiobacillus thiooxidans, and photosynthetic organ- 

 isms may utilize radiant energy for the synthesis of energy-rich phos- 

 phate bonds (3,22,26). 



We cannot yet formulate in any detail the course of events in 

 photosynthesis and chemosynthesis, but, using the knowledge gained 

 by the study of the mechanisms of carbon dioxide fixation in hetero- 

 trophic organisms, we may attempt to draw a plausible picture of the 

 chemical events. Reversal of the oxidative degradation of foodstufTs, 

 i. e., of respiration, would now seem to be a definite possibility. 



We have seen that, on carboxylation and reduction, a-keto- 

 glutaric acid can be converted to citric acid, and have indicated that 

 the latter may be split to acetic and oxalacetic acids. Further, oxal- 

 acetic acid can be reduced to succinic acid by way of malic and fumaric 

 acids, and succinic acid could be converted to a-ketoglutaric acid by 

 reductive carboxylation, i. e., by reversal of reaction Illb. In this 

 way, a-ketoglutaric acid would be regenerated, while the acetic acid 



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