Abstract
Abstract A model is presented which is able to describe satisfactorily the increase of the alkaline and acid Bohr effect due to the binding by human hemoglobin of 2,3-diphosphoglycerate. From an analysis of this extra Bohr effect it could be concluded that the histidines H21(143)β are very probably responsible for the increase in alkaline Bohr effect. The second ionization of the phosphate groups of 2,3-diphosphoglycerate is responsible for the increase in acid Bohr effect. For Kd the pH-independent part of the apparent association constant of the deoxyhemoglobin-diphosphoglycerate complex, we estimated a value of 8 x 105 m-1 at 25° and an ionic strength of 0.1. It appeared further that the binding of diphosphoglycerate by oxyhemoglobin is under these experimental conditions negligible as compared with the binding to deoxyhemoglobin. From binding experiments on deoxyhemoglobin with diphosphoglycerate at 4° and at an ionic strength of 0.1 a value for Kd of (6.5 ± 1.5) x 105 m-1 could be estimated. Measurements of the rate of reaction of 1-fluoro-2,4-dinitrobenzene with the α-NH2 groups of deoxyhemoglobin showed that in the presence of 2,3-diphosphoglycerate the reactivity of the α-NH2 group of the β chain was strongly depressed.
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