182 



FIXATION OF CARBON DIOXIDE 



CHAP. 8 



is established so rapidly that it can be studied, in aqueous solution, without interference 

 from the side of the more slowly estabhshed hydration equiUbrium. The constants 



(8.18) 



xV Carbarn. — 



NH 



coo- 



RCOO- 



[H+] 



[H2NRCOO-] [CO2] 



of alanine and glycine are, according to Stadie and O'Brien, of the order of 2.5 X IQ-^ 

 at 20° C. Consequently, half-saturation of these amino acids with carbon dioxide is 

 reached at [CO2] = 4 X 10"^ mole per liter {i. e., at a partial pressure of 90 mm.) 

 if pH = 8, and at a ten times smaller pressure if the pH is 9. 



Carbamination is of particular importance for the carbon dioxide transportation by 

 blood. The entire absorption of carbon dioxide by blood w^as attributed, until 1928, 

 to the carbonate-bicarbonate interconversion, although Bohr had postulated the 

 existence of a hemoglobin-carbon dioxide compound in 1905, and Siegfried had demon- 

 strated in the same year the formation of carbamates in blood serum. Kinetic studies 

 by Henriquez (1928, 1931), Margaria and Green (1933), and Meldrum and Roughton 

 (1933) have proved since that an important proportion of carbon dioxide is present in 

 the form of carbamate. The carbamination of blood was discussed quantitatively also 

 by Roughton (1935) and Stadie and O'Brien (1937). 



Despite the presence of carbonic anhydrase in blood, the carbamination equilibrium 

 can be studied, independently from the hydration equiUbrium, by poisoning this enzyme 

 with cyanide (0.05-0.1 mole/1.). At 0°, with poisoned enzyme, the half-saturation of 

 oxyhemoglobin is reached at about 10 mm. carbon dioxide in the air, and that of reduced 

 hemoglobin at about 30 mm. The heat of carbamination is considerable, about 17 kcal 

 per mole. Table 8.VII illustrates the role which carbamination plays in the carbon 

 dioxide balance of blood. 



Table 8.VII 



BiCAKBONATES AND CARBAMATES IN BlOOD 



Up to 20% of carbon dioxide in red blood cells is present as carbamate, and 45% 

 of the difference between the carbon dioxide content of these cells in venous and arterial 

 blood is caused by a shift of the carbamination equilibrium. 



The possible role of amino acids in the carbon dioxide absorption by plants will be 

 mentioned on pages 191 and 194. We must, at this point, cite a case of carbamination 

 which may conceivably be of importance for photosynthesis. For C — H bonds, the 

 substitution of a metal for hydrogen makes the affinity for carbon dioxide stronger, as 

 shown by the eager carboxylation of Grignard's reagents and other metalloorganic 

 compounds. Is it possible for nitrogen-metal bonds also to be more efficient carbon 



