The uptake of protons by heme-linked ionizable groups on azide binding to methemoglobin
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When azide ion reacts with methemoglobin in unbuffered solution the pH of the solution increases. This phenomenon is associated with increases in the pK values of heme-linked ionizable groups on the protein which
give rise to an uptake of protons from solution.
We have determined as a function of pH the proton uptake, Ah', on azide binding to methemoglobin at 20°C.
Data for methemoglobins A (human), guinea pig and pigeon are fitted to a theoretical expression based on the
electrostatic effect of three sets of heme-linked ionizable groups on the binding of the ligand. From these fits the
pK values of heme-linked ionizable groups are obtained for liganded and unliganded methemoglobins. In unliganded methemoglobin pK1, which is associated with carboxylic acid groups, ranges between 4.0 and 5.5 for the three methemoglobins; pK,, which is associated with histidines and terminal amino groups, ranges from 6.2 to 6.7. In liganded methemoglobin pK1 lies between 5.8 and 6.3 and pK, varies from 8.1 to 8.5. The pH dependences of the apparent equilibrium constants for azide binding to the three methemoglobins at 20°C are well accounted for with the pK values calculated from the variation of Ah' with pH.
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QD Chemistry