Binding of O2 and CO to hemes and hemoproteins

Abstract

Enthalpies and entropies have been determined for the reversible binding of O2 and CO to chelated protoheme, a compound having a covalently attached imidazole bound to the iron. The values, based upon 1 atm standard state, are delta HO2 = -14.0 kcal (1 kcal = 4.18 kJ)/mol, delta SO2 = -35 eu, delta HCO = -17.5 kcal/mol, delta SCO = -34 eu, delta H identical to O2 = 21 kcal/mol (dissociation), and delta H identical to CO = 25 kcal/mol (dissociation). The similarity of these values to those of high-affinity hemoproteins such as isolated hemoglobin chains or R-state hemoglobin (delta HO2 = -13.5, delta HCO = -17.5) show that this model compound accurately mimics the dynamic behavior of these hemoproteins, in contrast to the behavior of other, more elaborate, model compounds. The enthalpy of the replacement of O2 by CO, delta HM, is 3.5 kcal/mol, abut the same as that of R-state hemoglobin. This result obtained with the model compound which most resembles the hemoglobin active site, indicates that distal side steric effects in these hemoproteins neither decrease CO affinity nor differentiate between the binding of CO anmd O2. Consequences of these findings in the binding of O2 and CO to hemoproteins are discussed.