The Carbon Dioxide Affinity of Various Human Hemoglobins * ( Received for publication

We have investigated the effect of molecular carbon dioxide, i.e., formation of carbamino compounds on (a) the partial pressure of oxygen at 50% saturation (&,,) of hemoglobin and (b) the Bohr effect when expressed as A log &o/ApH in the following human hemoglobins: Arr (a&), Frr (azrz), AI, (w&~‘~), and FIacetY1 (czz-r2acety1). Some experiments of this kind were also carried out with hemoglobin Hope ((Y&~~ G1y+Asp) and the mixed state hemoglobins (o~irrCN/3”)~ and (cr”P”‘CN)z. Hemoglobin AI, and FIacetY1 have blocked NHt-termini of p and y subunits, respectively, whereas in hemoglobin Hope a salt bridge exists between /3iz6 *W ‘O”and the NHz-terminal a-amino group of the fl subunits. Consequently, only the a-amino groups of the a! subunits are free to react with COz in these three hemoglobins. From the effect of COz on psO and Bohr effect we infer that, at physiological pH, 70 to 80% of oxylabile carbamate is formed at the /3 subunits of hemoglobin Aii. This conclusion is corroborated by the marked effect of 2,3-diphosphoglycerate on carbamate formation. This chemical appears to abolish carbamate formation at the p subunits of hemoglobin A II’ In the valency hybrid (a! “‘CNfl”)2, the effect of adding COz on psO was larger than in (c~“fl”‘CN)~. This indicates that changes of the tertiary structure of the /3 subunits, which occur on oxygen binding, have a more pronounced influence on carbamate formation than changes of the tertiary structure of the c~ subunits. Fetal hemoglobin FII has an intrinsically lower carbon dioxide affinity than hemoglobin Air. However, the reduction of this parameter by 2,3-diphosphoglycerate is much smaller than in hemoglobin AII. This explains the higher fraction of oxylabile carbamate found in fetal blood compared to adult blood under physiological conditions.