[Reprinted from the Journal of the American Chemical Society. 76, 1760 {1954).) 

 CopyriKht lfl54 by the American Chemical Society and reprinted by permission of the copyright owner. 



[Contribution from Radiation Laboratory and Department of Chemistry, University of 



California, Berkeley) 



The Path of Carbon in Photosynthesis. XXI. The Cyclic Regeneration of Carbon 



Dioxide Acceptor^ 



By J. A. Bassham, A. A. Benson, Lorel D. Kay, Anne Z. Harris, A. T. Wilson and M. Calvin 



Received October 16, 1953 



Photosynthesizing plants have been exposed to C'Oj for short periods of time (0.4 to 15 sec.) and the products of carbon 

 dioxide reduction analyzed by paper chromatography and radioautography. Methods have been developed for the degra- 

 dation of ribulose and sedoheptulose. These sugars, obtained as their phosphate esters from the above C'»Oj exposures and 

 from other experiments, have been degraded and their distribution of radiocarbon determined. The distribution of radiocar- 

 bon in these sugars, and other data, indicate that sedoheptulose phosphate and ribulose diphosphates are formed during 

 photosynthesis from triose and hexose phosphates, the latter being synthesized, in turn, by the reduction of 3 phosphoglyceric 

 acid. Further evidence has been found for the previously proposed carboxylation of ribulose diphosphate to phosphoglyceric 

 acid. Free energy calculations indicate this step would proceed spontaneously if enzymatically catalyzed. The efficiency 

 of this cycle for reduction of CO2 to hexose would be 0.9 if the reduction of each molecule of PGA requires the concurrent 

 conversion of one molecule of ATP and one of DPN (red) to ADP, inorganic phosphate and DPN (ox.). 



Previously reported tracer studies of the path of action leading to phosphoglyceric acid (PGA)' 

 carbon in photosynthesis' led to the conclusion which is then reduced and condensed to fructose 

 that carbon is incorporated by a carboxylation re- (3, ^-he following abbreviations win be used throughout this paper: 



(1) The work described in this paper was sponsored by the U. S. PGA, phosphoglyceric acid; DHAP, dihydroxyacetone phosphate; 

 Atomic Energy Commission. This paper was presented before the FMP, fructose monophosphate; GMP, glucose monophosphate; 

 Division of Biological Chemistry. American Chemical Society, at the SMP, sedoheptulose monophosphate; RDP ribulose diphosphate; 

 124th National Meeting. Chicago. Illinois. September. 19.53. ADP, adenosine diphosphate. ATP adenosine triphosphate; DPN, 



(2) M Calvin. "The Harvey Lectures," Charles C Thomas Pub- diphosphopyridine nucleotide (Coenzyme I), oxidised form; DPNlHi], 

 lisbing Company. Spring&eld, 111., 1050-61, p. 218. diphosphopyridine nucleotide, reduced form. 



92 



