| Abstract
| - A novel model linking the thermodynamics and kinetics of hemoglobin's allosteric (R → T)and ligand binding reactions is applied to photolysis data for human HbCO. To describe hemoglobin'skinetics at the microscopic level of structural transitions and ligand-binding events for individual [ij]-ligation microstates (ijR → ijT, ijR + CO → (i+1)kR, and ijT + CO → (i+1)kT), the model calculates activationenergies, ijΔG‡, from previously measured cooperative free energies of the equilibrium microstates (Huang,Y., and Ackers, G. K. (1996) Biochemistry 35, 704−718) by using linear free energy relations (ijΔG‡ −01ΔG‡ = α[ijΔG − 01ΔG], where the parameter α, describing the variation of activation energy withreaction energy perturbation, can depend on the natures of both the reaction and the perturbation). The αvalue measured here for the allosteric dynamics, 0.21 ± 0.03, corresponds closely to values observedpreviously, strongly suggesting that the thermodynamic microstate energies directly underlie the allosterickinetics (as opposed to the α(ijΔGRT) serving merely as arbitrary fitting parameters). Besides systematizingthe study of hemoglobin kinetics, the utility of the microstate linear free energy model lies in the abilityto test microscopic aspects of allosteric dynamics such as the “symmetry rule” for quaternary changededuced previously from thermodynamic evidence (Ackers, G. K., et al. (1992) Science 255, 54−63).Reflecting a remarkably detailed correspondence between thermodynamics and kinetics, we find that akinetic model that includes the large free energy splitting between doubly ligated T microstates impliedby the symmetry rule fits the data significantly better than one that does not.
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