Binding of O2 and CO to hemes and hemoproteins.

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Abstract
Enthalpies and entropies were determined for the reversible binding of O2 and CO to chelated protoheme, a compound having a covalently attached imidazole bound to the Fe. The values, based upon I 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, .**GRAPHIC**. = 21 kcal/mol (dissociation) and .**GRAPHIC**. = 25 kcal/mol (dissociation). The similarity of these values to those of high-affinity hemoproteins such as isolated Hb chains or R-state Hb (.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, about the same as that of R-state Hb. This result, obtained with the model compound which most resembles the Hb active site, indicates that distal side steric effects in these hemoproteins neither decrease CO affinity nor differentiate between the binding of CO and O2. Consequences of these findings in the binding of O2 and CO to hemoproteins are discussed.