The Reaction of n-Butyl Isocyanide with Human Hemoglobin

The α and β chains within the human hemoglobin tetramer possess different reactivities toward n-butyl isocyanide. Identification of the kinetic components involved in the binding of the fourth ligand molecule is made possible by intrinsic spectral differences between the α and β chains. In the last step in ligand binding, the association rate constants for the α and β chains were determined to be 35 x 103 m-1 sec-1 and 240 x 103 m-1 sec-1, respectively; the dissociation constants were 0.32 sec-1 and 4.00 sec-1, respectively. These kinetic parameters were little affected by heme concentration and pH. Similarly, inorganic phosphate and 2,3-diphosphoglycerate had little effect. However, inositol hexaphosphate did influence the parameters through its ability to bind to the liganded form of hemoglobin. Under all the experimental conditions investigated the β chain association and dissociation rate constants were both significantly greater than the corresponding α chain constants. The equilibrium constants for both chains were roughly equivalent.