Interaction of Human Hemoglobin with Adenine Nucleotides

The interaction of deoxyhemoglobin with ATP, ADP, and AMP has been studied at pH 7.0, 25°, Γ/2 = 0.12 m . Under these conditions, ATP and ADP each form a 1:1 complex with the deoxyhemoglobin tetramer and they compete with 2,3-diphosphoglyceric acid, presumably for the same site on the protein. The order of binding affinities is 2,3-diphosphoglyceric acid > ATP > ADP > AMP. The free energy of binding is approximately proportional to the net charge on the phosphate ligand, although other factors are also operative. Addition of ATP or ADP or AMP to oxyhemoglobin reversibly converts the protein to the deoxy form, thereby showing that these nucleotides bind to deoxyhemoglobin with an overwhelming preference. A discussion of these results is given.