31P NMR study of the kinetics of binding of myo-inositol hexakisphosphate to human hemoglobin. Observation of fast-exchange kinetics in high-affinity systems

The association and dissociation kinetics of the complexes of myo-inositol hexakisphosphate (P6-inositol) with deoxyhemoglobin (Hb) and carboxyhemoglobin (HbCO) have been investigated by 31P NMR between pH 6.8 and pH 5.5. These complexes represent high-affinity systems with binding constants varying between 10(5) M-1 and 2 X 10(9) M-1. 31P NMR spectra of P6-inositol were recorded in the presence of hemoglobin as a function of the P6-inositol/hemoglobin molar ratio. It appeared that the exchange of the polyphosphate molecule between the solution and the central cavity binding site is fast on the NMR time scale. This observation cannot be reconciled with a single-step binding mechanism of P6-inositol to hemoglobin. Analysis of the spectra revealed the occurrence of additional binding of P6-inositol to both Hb and HbCO. This binding was also observed in pH-state experiments performed at low ionic strength. 31P NMR experiments carried out with hemoglobin of which the alpha-chain N termini were carbamylated, strongly suggest that these termini constitute the additional binding site for P6-inositol. A model is proposed which accounts for the enhancement of exchange kinetics in these high-affinity systems. In this model a rapid migration is assumed for P6-inositol between the central cavity binding site and an entry/leaving site on the hemoglobin molecule. Based on this model 31P NMR linewidths and chemical shift patterns for this three-site exchange problem were calculated.

The binding of chloride ions to ligated and unligated human hemoglobin and its influence on the Bohr effect

The contribution of the interaction of chloride ions with deoxy and oxyhemoglobin to the Bohr effect can be described by a simple binding model. Applying this model to experiment data reveals that at physiological pH and ionic strength about half of the release of Bohr protons is due to a difference in chloride ion binding to deoxy- and oxyhemoglobin. The chloride-independent part of the Bohr effect corresponds with the shift in pK which His-146 beta shows upon oxygenation. The proton absorptioon by hemoglobin observed upon oxygenation below pH 6 is apparently due to a chloride-ion-induced proton uptake, which is larger for oxyhemoglobin than for deoxyhemoglobin. The analysis of the experimental data indicates the existence of only two oxygen-linked chloride ion binding sites in both deoxy and oxyhemoglobin. In deoxyhemoglobin the binding sites most likely consist of Val-1 alpha of one chain and Arg-141 alpha of the partner chain. The sites in oxyhemoglobin consist of groups with a pK value in the neutral pH range; they do not contain lysyl or arginyl residues.