328 VII. COMPARATIVE BIOCHEMISTRY OF HEMOGLOBINS 



At the loading tension pigments A and B are 95% saturated, but, 

 where B delivers 45% of its oxygen, A only delivers 12%. Pigments 

 B and C are both 50% saturated at the unloading tension, but on 

 oxygenation at the loading tension the saturation of C rises to only 

 80%, while B is 95% saturated. The sigmoid curve delivers 45% of 

 its oxygen between the loading and unloading tensions while the two 

 hyperbolic curves A and C deliver only 12% and 30%, respectively. 



In order to make up for the absence of the sigmoid dissociation 

 curve, an organism would have either to increase its respiratory 

 system relative to the rest of its body, or to increase the concentration 

 of carrier in the circulation. Both these adaptations may have dis- 

 advantages which are avoided by a carrier with a sigmoid dissociation 

 curve. 



Since the sigmoid dissociation curve is determined not only by the 

 structure of the protein, but also by the microenvironment, the vari- 

 ation in the position and shape of the curve which can be brought 

 about by the different synthetic capacity of different species is greater 

 than if variation were possible only in the structure of the protein 

 (c/. Section 9.3.). 



9.2. Interaction between Oxygen and Carbon 

 Dioxide Transport 



This problem has been thoroughly investigated by a number of 

 workers and the reader is referred elsewhere for detailed discussion 

 {1236, 21IfO, 2360). Hemoglobin not only assists in buffering the pH 

 changes brought about by the cyclic variation in the carbon dioxide 

 content of the blood, but in addition," as Henriques (12^1) and 

 Roughton (2360) have shown, forms carbamate compounds. The 

 effect that diminishing pH has on the affinity of hemoglobin for 

 oxygen has been discussed in Chapter VI. The slight diminution in 

 7?H facilitates the liberation of oxygen in the tissues and the reverse 

 shift in the lungs facilitates the oxygenation of hemoglobin. The 

 influence of shifts in pH of the cell, however, is probably of less 

 importance than the actual combination of carbon dioxide with the 

 hemoglobin molecule. 



The importance of certain firmly bound substances in the trans- 

 mission of heme-heme interactions has been discussed in Chapter 

 VI. Sidwell and co-workers (25^9) showed that, at constant pH, 

 increase in the bicarbonate concentration from to 0.02 moles per 



