Comparative Study of Oxygen and Carbon Monoxide Binding

A spin label is attached covalently to a propionic acid group of the heme in either the a or B subunits of hemoglobin. Hemoglobins, so chemically modified, show functional properties similar to those of native hemoglobins. Since electron paramagnetic resonance is sensitive to the ligation state of hemoglobin, electron paramagnetic resonance changes have been used to “follow” the sequence of ligand binding to the a and f l subunits of hemoglobin. Studies on the binding of carbon monoxide with heme spin-labeled hybrid hemogiobins show that carbon monoxide binds preferably to the p chains of hemoglobin. A similar study is also carried out on the process of oxygen binding with heme spin-labeled hemoglobins. The results show that: 1) oxygen binding to hemoglobin is a random process in the absence of an allosteric effector; 2) 2,3-diphosphoglycerate causes preferential binding of oxygen to the a subunits of deoxyhemoglobin; and 3) a similar but more striking effect is observed in the presence of inositol hexaphosphate. These results, coupled with the oxygen equilibrium curves of these modified hemoglobins, are used to generate the parameters of the MonodWyman-Changeux model of oxygen binding of hemoglobin. It is concluded from the comparison of parameters that an allosteric effector stabilizes the T state of hemoglobin and decreases the oxygen affinity of the /3 subunits in the T state. The difference in the binding sequence of oxygen and carbon monoxide by hemoglobin suggests that the hemoglobin affinities for oxygen and carbon monoxide are controlled by different factors.