SEVERO OCHOA 



from a carboxylic acid, through the next higher a-keto acid, to an 

 a-hydroxy acid, which in turn would be carboxylated to the next higher 

 Q;-keto-/3-hydroxy acid, and so on. Such a sequence would be es- 

 sentially a reversal of a pathway of carbohydrate oxidation by way of 

 phosphohexonic acid, a-ketophosphohexonic acid, phosphopentonic 

 acid, etc., through alternating dehydrogenations and decarboxylations 

 (17). 



Obviously, the above schemes are only gross approximations. 

 The main point is that what we know about the mechanisms of carbon 

 dioxide assimilation by heterotrophic organisms strongly suggests that 

 all reactions involved in cellular respiration are essentially reversible. 

 Thus, the processes of both photosynthesis and chemosynthesis may rep- 

 resent reversals of the respiratory process not only from the standpoint 

 of energy but also from the standpoint of the enzymic mechanisms. 



References 



(1) Buchanan, J. M., Sakami, W., Gurin, S., and Wilson, D. W., J. Biol 

 Chem., 159,695 (1945). 



(2) Deffner, M., Ann., 536, 44 (1938). 



(3) Emerson, R. L., Stauffer, J. F., and Umbreit, W. W., Am. J. Botany, 

 31, 107 (1944). 



(4) Evans, E. A., Jr., and Slotin, L., J. Biol. Chem., 141, 439 (1941). 



(5) Evans, E. A., Jr., Vennesland, B., and Slotin, L., J. Biol. Chem., 147, 

 771 (1943). 



(6) Franck, J., and Gaffron, H., in Advances in Enzymology, Vol. I. Inter- 

 science, New York, 1941, p. 199. 



(7) Gaffron, H., Biol. Rev. Cambridge Phil. Soc, 19, 1 (1944). 



(8) Kalckar, H. M., Chem. Revs., 28, 71 (1941). 



(9) Kalnitsky, G., and Werkman, C. H., Arch. Biochem., 4, 25 (1944). 



(10) Kalnitsky, G., Wood, H. G., and Werkman, C. H., Arch. Biochem., 2, 

 269 (1943). 



(11) Krampitz, L. O., and Werkman, C. H., Biochem. J., 35, 595 (1945). 



(12) Krampitz, L. O., Wood, H. G., and Werkman, C. H., J. Biol. Chem., 

 147,243(1943). 



(13) Krebs, H. A., in Advances in Enzymology, Vol. III. Interscience, New 

 York, 1943, p. 191. 



(14) Lardy, H. A., and Elvehjem, C. A., Ann. Rev. Biochem., 14, 1 (1945). 



(15) Lewis, G. N., and Randall, M., Thermodynamics and the Free Energy of 

 Chemical Substances. McGraw-Hill, New York, 1923. 



184 



