Examination of Haldane's first law for the partition of CO and O₂ to hemoglobin A₀

Abstract

SynopsisA study of the oxygen replacement reaction of carbon monoxide-saturated hemoglobin ( H b b ) was carried out using spectroscopic, calorimetric, and pH titration methods. Under fully saturated conditions the replacement reaction can be defined by a single partition constant over all ratios of bound oxygen to carbon monoxide. This indicates that under saturating conditions Haldane's fist law for the liiand binding of gas mixtures holds for any CO/Oz ratio.It further shows that there is no appreciable difference in relative CO-02 affinity between the a-and @-chains. The same partition coefficient was found to hold for different pH, buffer, and allosteric effector conditions. The lack of any pH dependence of the partition coefficient was confirmed by the absence of proton changes for the replacement reaction. The temperature dependence of the partition coefficient and calorimetric results yield a value for the enthalpy of the reaction of -3.65 f 0.29 kcal/mol/heme.

Carbon monoxide binding to human hemoglobin A0

“…Although the thermodynamic treatment of the CO-02 linkage to hemoglobin is straightforward, nevertheless only two edges of the thermodynamic triangle have been explored so far in a quantitative way, namely, the top edge, representing the 02 binding reaction (Imai, 1982;Chu et al, 1984;, and the diagonal edge, representing the competition reaction of CO and 02 in the fully saturated molecule (Wyman et al, 1982). Due to the very high affinity of CO for hemoglobin, no information has been available on the distribution of the remaining species of the thermodynamic triangle.3 02 and CO Binding Curves.…”

“…possible combinations of CO and 02. The equilibria between the species can be explored by saturating the molecule with one ligand and measuring its replacement through a series of increasing relative activities of the other ligand (Wyman et al, 1982). Figure 5a shows the optical density changes at the CO maximum absorbance (419 nm) as observed upon incremental dilution of the CO with 02.…”

“…However, a rigorous test of Haldane's laws is best obtained by examination of CO binding to the purified hemoglobin molecule. Preliminary work on the competition reaction between CO and 02 to human hemoglobin has demonstrated the validity of Haldane's prediction (Wyman et al, 1982) in the domain where the macromolecule is fully saturated (f = 4).…”

Carbon monoxide and oxygen binding to human hemoglobin F0

“…02-C0 Competition Experiment. The equilibria among the fully ligated species MX4, MX3Y, MX2Y2, MXY3, and MY" can be determined by saturating the molecule with one ligand and measuring its replacement through a series of increasing relative activities of the other ligand (Wyman et al, 1982;Di Cera et al, 1987a). In this experiment Hb was saturated with CO at 2.5% Pa (Pa = atmospheric pressure vapor pressure of H20), followed by dilution with 02 at pressure £a.…”

“…This implies that the 02-CO replacement reaction occurs at each site independently (Wyman et al, 1982;Di Cera et al, 1987a).…”