J. Biol. Chem., Vol. 261, Issue 23, 10576-10581, 08, 1986
The effect of heme-linked ionizable groups on cyanide binding to methemoglobin
FJ Vega-Catalan, OJ Odeyemi and KO Okonjo
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.
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