456 



M. Calvin and P. Massini: The Path of Carbon in Photosynthesis 



[ExpebientiaVol.VI 11/12) 



cofactor for the oxidative decarboxylation of pyruvic 

 acid to an active acetyl group' which is the one reaction 

 known to feed the Krebs cycle'. The mechanism of the 

 reaction may be written this way: 



CH, 



CH, HC - CH,-CH,-CH,-CO-Thiamin+ CH,-CO-COOH 



I I (Co-pyruvate oxidase) (Pyruvic acid) 



CH, 



/ \ 

 CH, CH- 



Cocnzymc A 



/ 



CH,-CO 



COOH 



CH, 



CH, CH-ff+ Acetyl CoA+ CO, 



Tlie reduced lipoic acid complex would then be reoxi- 

 dized to the disulfide form by a suitable oxidant (e.g. 

 pyridine or flavin nucleotides). In order that the oxi- 

 dation of pyruvic acid can proceed, the enzyme has to 

 be present in its oxidized form. If it is kept in its reduced 

 form under the influence of the light-produced reducing 

 f)ower, the reaction cannot proceed and the pyruvic 

 acid formed during photosynthesis will not find its way 

 into the respiratory cycle. The reaction is inhibited 

 because only a small amount of the enzyme catalyzing 

 it exists in the required form, most of it being kept in 

 the other form under the "pressure" of the reducing 

 power generated by the light energy. This recalls a 

 similar phenomenon which has been known for a long 

 time, i.e. the suppression of the fermentation of carbo- 

 hydrates in favor of their oxidation under aerobic con- 

 ditions (Pastruk effect). This effect has been explained 

 in a manner similar to the one used here to account for 

 the inhibition of the respiration of photosynthetic in- 

 termediates'. The reduction of acetaldehyde to alcohol 

 requires a dehydrogenase in its reduced form; under 

 aerobic conditions the dehj'drogenase exists primarily 

 in its oxidized form, and the acetaldehyde instead of 

 beijig reduced is oxidized to acetic acid. 



The sudden rise in phosphoglyceric acid and the 

 decrease in ribulose diphosjihate and sedoheptulose 



' L. J. Rfed, I. C. Cunsalus, et at.. J. Am. Chem. Soc. ?J, 5920 

 (1951). -E. L. Patterson, rf a/., J. Am. Chem. Soc. 7J, 5919 (1951|. 

 - I. C. GuNSALUS, I.. Struclia, and U. I. O Kane, ,1. Biol. Cheni. 

 J9<, 859 (1952).- L. J. Reed and B. G. DeBusk, J. Am. Chem. Soc. 

 r<, 3457 (1952). -M. W. Bullock, <( a/., J. Am. Chem. Soc. 7<, 3455 

 (1952). 



* S. OcpcoA, J. R. Stern, and M. C. Schm idfr. J. Biol. Chem. 

 /9J. 691 (1951). - S. KoRKEs, A.DelCamillo, I.C.Gvnsalus, and 

 S. OCHOA, J. Biol. Chem. /93, 721 (1951). 



• O. Meverhof, Amer. Scientist iO, 483 (1952). 



phosphate in the dark period, together with the obser- 

 vation that the dark rise in phosphoglyceric acid is 

 absent when the ribulose diphosphate concentration 

 was low during the light, confirms the earlier suggestion 

 that the phosphates of the C, and Cj sugars are pre- 

 cursors of the C, carbon dioxide acceptor*. ThLs, togeth- 

 er with evidence gathered in previous work* leads to 

 the following scheme for the photosynthetic cycle' 

 (Fig. 15). 



Upon this basis an attempt might be made to relate 

 the two effects as follows ; when the light is turned off, 

 the reduction reactions requiring light are stopped, 

 whereas cleavage and carboxylation reactions continue 

 until their substrates are exhausted. Presumably, this 

 would lead to a depletion of the Cj and C, sugars, the 

 synthesis of which requires reduction steps (particu- 

 larly the six-equivalents leading to the tetrose which 

 itself is a very small reservoir), and a rise of phospho- 

 glyceric acid, the further fate of which is also dej)endent 

 upon reduction. However, a number of arguments seem 

 to contradict this view : (I) The observation that plants 

 fix radiocarbon in the dark immediately following a light 

 period at low carbon dioxide concentration, to form 

 a similar pattern of compounds as the one found in 

 photosynthesis shows that the sequence following phos- 

 phoglyceric acid is not blocked at once upon cessation 

 of illumination, but that the cells contain sufficient 

 reducing power to transform some phosphoglyceric 

 acid intocarbohydrates , (2) the cleavageof the pentoses 

 and heptoses into the Cj carbon dioxide acceptor and 

 a triose and pentose respectively is dependent on a 

 reduction step as well. 



Fig. 15. 



We are thus led to the suggestion that the rise in 

 phosphoglyceric acid is not be explained by a mere 

 interruption of the sequence, but that the rate of pro- 

 duction of phosphoglyceric acid at some time in the 



' A. A. Benson, J. A. Bassham, M. Calvin, A. G. Hall, H. E. 

 HiRscH, S. Kawaguchi, V. H. Lvnch, and N. E. Tolbert, J. Biol. 

 Chem. ;9(i, 703 (1932). 



' S. Kawaguchi, A. A. Benson, M. Calvin, and P. M. Mayes, 

 J. Am. Chem. Soc. 7/, 4477 (1952). - M.Calvin, The Harvey 

 Lectures 46, 213-251, 1951, in press. 



' This scheme is intended to represent only changes in the carbon 

 skeletons. The reducing equivalents are indicated only to show redox 

 relationships between the known compounds. A number of the 

 isolated compounds are isoxiraers and have not been included. 



90 



