Okonjo, K. O. and Adediji, A. T. and Fodeke, A. A. and Adeboye, Omolara and Ezeh, C. V.
Transition of hemoglobin between two tertiary conformations: Determination of equilibrium and thermodynamic parameters from the reaction of 5,5′-dithiobis(2-nitrobenzoate) with the CysF9β sulfhydryl group.
The equilibrium constant of the reaction of 5,5′-dithiobis(2-nitrobenzoate) with the CysF9β sulfhydryl group of hemoglobin decreases by
2 to 3 orders of magnitude between pH 5.6 and 9. The reaction is coupled to the ionizations of two groups on the protein. At 25 °C one group has a pKa of 5.31±0.2 when hemoglobin is in its (tertiary) r conformation, typified by the thiolate anion form of CysF9β; this changes to 7.73±0.4 in the (tertiary) t conformation, typified by the mixed disulfide form of the sulfhydryl. The second group ionizes with a pKa of 7.11±0.4 in the r conformation; this changes to 8.38±0.2 in the t conformation. Krt, the equilibrium constant for the r←→t isomerization process, is 0.22±0.06.
The standard enthalpy and entropy changes for the isomerization are ΔHo rt=24.2 kJ mol−1 and ΔSo
rt=68.8 JK−1mol−1, respectively.
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