Role of the α amino groups of the α and β chains of human hemoglobin in oxygen linked binding of carbon dioxide

Human hemoglobin was reacted with potassium cyanate and purified to yield three species, α₂(c)β₂(c), α₂(c)β₂, and α₂β₂(c), where (c) denotes specific reaction of cyanate with the α amino group of the particular chain. The effect of carbon dioxide on the oxygen affinity of these species in the presence and in the absence of 2,3 diphosphoglycerate was measured. Carbon dioxide has no effect on the oxygen affinity of α₂(c)β₂(c), confirming that the usual lowering of the oxygen affinity of carbon dioxide in normal hemoglobin is mediated by the α amino groups of the α and β chains. The lowering of the oxygen affinity of α₂β₂(c) by carbon dioxide was not affected by the presence or absence of 2,3 diphosphoglycerate, showing that 2,3 diphosphoglycerate does not interfere with the oxygen linked binding of carbon dioxide at the α chain α amino group. In α₂(c)β₂ there was a much larger effect of carbon dioxide on the oxygen affinity in the absence of 2,3 diphosphoglycerate than in α₂β₂(c); however, on addition of 2,3 diphosphoglycerate the effect of carbon dioxide on the oxygen affinity of α₂(c)β₂ was much smaller and similar to that occurring in α₂β₂(c). This shows that there is a large difference in the carbon dioxide binding constants of the β chain α amino group in the oxy and deoxy forms of human hemoglobin, and that 2,3 diphosphoglycerate suppresses this difference, probably by binding strongly to the β chain α amino group of deoxyhemoglobin and displacing any bound carbon dioxide.