REACTIONS OF CARBON DIOXIDE 9I 



synthesis, the resuks obtained by Calvin et al. for photo- 

 synthesis in presence of tracer carbon have shown the rapid 

 appearance of labelled carbon in compounds previously 

 known to be concerned in respiration. The dark fixation of 

 CO2 was found by Calvin to be greatly increased in a green 

 plant immediately following illumination. As yet we do 

 not know whether there exists a single mechanism for the 

 fixation of carbon dioxide specifically associated with photo- 

 synthesis; all our present knowledge of the enzyme systems 

 concerned with the metabolism of carbon dioxide refers to 

 the dark processes. The study of these systems in the green 

 cell may form a significant contribution to the biochemistry 

 of photosynthesis. 



ENZYME SYSTEMS INVOLVING COg 



Carbonic anhydrase. This enzyme catalyses the reaction 

 between CO2 and water to give bicarbonate ions: 



CO2+H2O -^ H-+HCO3- 



It was discovered in mammalian blood by Meldrum and 

 Roughton and shown to be a zinc protein compound by 

 Keilin and Mann. Carbonic anhydrase activity has been 

 reported to be widely present in green plants but the 

 enzyme has not actually been isolated therefrom. It appears 

 unlikely to be a rate determining factor in photosynthesis. 

 The functional group of the enzyme seems to depend on 

 the zinc atom. The enzyme is the only zinc protein com- 

 pound of this type and the only metal enzyme known which 

 reacts directly with CO2. Since zinc is an essential element 

 for plant growth, we may speculate whether there are other 

 zinc protein compounds in the plant concerned with CO2 

 metabolism. Carbonic anhydrase is inhibited by cyanide, 

 and such a speculation has sometimes been invoked to 

 explain the eflfect of cyanide upon photosynthesis. 



Formic hydrogenlyase. This enzyme system catalyses the 

 reaction between COg and molecular hydrogen, or between 

 the bicarbonate ion and H2, to form formate: — 



CO2+H2 - H++HCO.,- 

 HCO3-+H2 -^ H.COO-+H2O 



