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Role of the Relaxation of the Iron(ll1) Ion Spin States Equilibrium in the Kinetics of Ligand Binding to Methaemoglobin

Iwuoha, E. I. and Okonjo, K. O. (1993) Role of the Relaxation of the Iron(ll1) Ion Spin States Equilibrium in the Kinetics of Ligand Binding to Methaemoglobin. J. CHEM. SOC. FARADAY TRANS, 89 (21). pp. 3921-3924.

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Temperature-jump experiments of the reaction of the thiocyanate ion with human aquomethaemoglobin have been performed in the presence of a 10-fold excess of inositol hexakisphosphate (inositolP,). Two kinetic phases corresponding to the a and /3 subunits were observed. Kinetic parameters of the reaction were evaluated from the reciprocal relaxation times on the basis of a fast relaxation of the iron(iii) ion spin states equilibrium before binding of the ligand. The association, ki,, and dissociation, k-i,, rate constants determined were: k,, = 225 dm3 mol-' s-', k-aL = 1.52 s-', k,, = 2430 dm3 mol-' s-', k-pL = 6.51 s-' at 27"C, pH 6.44. There was good agreement between the equilibrium constant of the ligand binding step determined by static methods (Kequ = 204 & 11 dm3 mol-') and that evaluated from kinetic data [(KaLKpL)1'2 = 235 & 12 dm3 mol-'1. The value ksJkaL = 11 obtained ensured proper separation of the two kinetic phases. Analyses of the subunit relaxation amplitudes, a€,, , showed that inositolP, perturbed the absorption spectrum of the /3 subunits. This suggests that in the presence of the organic phosphate, methaemoglobin behaves as a protein with independent binding sites rather than as an allosteric molecule. The kinetic and relaxation amplitude spectral characteristics of the subunits, in the presence of inositolP, have demonstrated that the kinetic dynamics are effectively decoupled in a stable tetramer.

Item Type: Article
Subjects: Q Science > QD Chemistry
Divisions: Faculty of Engineering, Science and Mathematics > School of Chemistry
Depositing User: Mr Adewole Adewumi
Date Deposited: 27 Jun 2011 11:01
Last Modified: 13 Dec 2011 21:13

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