The effect of heme-linked ionizable groups on cyanide binding to methemoglobin

The pH dependence of the kinetics of the binding of cyanide ion to methemoglobins A and S and to guinea pig and pigeon methemoglobins appears to be not directly correlated with the net charges on the proteins. The kinetics can, however, be adequately explained in terms of three sets of heme-linked i...

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Bibliographic Details
Published in:The Journal of biological chemistry 1986-08, Vol.261 (23), p.10576-10581
Main Authors: Vega-Catalan, F J, Odeyemi, O J, Okonjo, K O
Format: Article
Language:English
Subjects:
Animals
Biological and medical sciences
Columbidae
Cyanides - metabolism
Fundamental and applied biological sciences. Psychology
Guinea Pigs
Heme - metabolism
Hydrogen-Ion Concentration
Interactions. Associations
Intermolecular phenomena
Kinetics
Methemoglobin - metabolism
Molecular biophysics
Protein Binding
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Summary:The pH dependence of the kinetics of the binding of cyanide ion to methemoglobins A and S and to guinea pig and pigeon methemoglobins appears to be not directly correlated with the net charges on the proteins. The kinetics can, however, be adequately explained in terms of three sets of heme-linked ionizable groups with pK1 ranging between 4.9 and 5.3, pK2 between 6.2 and 7.9, and pK3 between 8.0 and 8.5 at 20 degrees C. pK1 is assigned to carboxylic acid groups, pK2 to histidines and terminal amino groups, and pK3 to the acid-alkaline methemoglobin transition. Kinetic second order rate constants have also been determined for the binding of cyanide ion by the four sets of methemoglobin species present in solution. The pKi values and the rate constants of methemoglobin S are strikingly different from those of methemoglobin A. This result is explained in terms of different electrostatic contributions to the free energy of heme linkage arising from differences in the environments of ionizable groups at the surfaces of the two molecules.
ISSN:0021-9258
1083-351X
DOI:10.1016/S0021-9258(18)67423-X