The kinetics of haemoglobin reactions

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Abstract
The data of Hartridge and Roughton (Proc. Royal Soc, 104: 395 (1923)) have been employed to calculate values of the energy and entropy of activation of the reactions of Hb with O2 and CO in aqueous soln. These values have been discussed with particular reference to the energies and entropies of the related equilibria and three conclusions have been reached. First, the reaction with CO appears to be relatively normal by the standards of chemical kinetics. Second, the reaction with O2 is distinctly abnormal, the difference between the activation energies between the forward and back reactions being widely different from the heat of reaction. This indicates that the activated complexes for the observed forward and back reactions are different, belonging to different steps in the chain of successive reactions postulated by Adair (Proc. Royal Soc. A, 109: 292 (1925)). Third, the association reactions are marked by their low value of the activation energy, but the dissociation reaction of HbO2 is marked by its high value of the entropy of activation. It is exceedingly unlikely that this is an unreal value, due to a chain reaction, nor does it appear that consideration of the rate of energy-distribution in the molecule is important. It is most probably due to a loosening of the internal structure of the protein on forming the activated complex.

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