Subunit iron spin heterogeneity in human aquomethemoglobin A

On the basis of a reaction scheme in which the ligand binding steps are preceded by fast iron spin transitions (Okonjo, K.O. (1980) Eur. J. Biochem. 105, 329-334; Iwuoha, E.I. and Okonjo, K.O. (1985) Biochim. Biophys. Acta 829, 327-334), the spin equilibrium constants of methemoglobin subunits are calculated from kinetic and equilibrium binding parameters with azide ion as ligand. The results demonstrate the existence of thermodynamic spin heterogeneity within the tetramer.

Relaxation amplitude analysis of thiocyanate and formate binding to human aquomethemoglobin A

The kinetics of the reaction of thiocyanate and formate ions with aquomethemoglobin can be adequately accounted for by a scheme in which the ligand-binding step in both the alpha and beta subunits is preceded by a fast transition of the iron atom from high to low spin (Okonjo, K.O. (1980) Eur. J. Biochem. 105, 329-334). Amplitude expressions derived from this scheme are used to analyse the relaxation amplitude data for alpha and beta subunits within the methemoglobin tetramer. The mean of the reaction enthalpies for ligand binding by the subunits within the tetramer is in good agreement with the reaction enthalpy for ligand binding by the methemoglobin tetramer obtained from a Van't Hoff plot of equilibrium titration data.