Vega-Catalan, F. J. and Odeyemi, O. J. and Okonjo, K. O.
The Effect of Heme-linked Ionizable Groups on Cyanide Binding
The Journal of Biological Chemistry, 261 (23).
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, howeverb, e adequately explained in
terms of three sets of heme-linked ionizable groups
with pK, ranging between 4.9 and 5.3, pK, between
6.2 and 7.9, and pK3 between 8.0 and 8.5 at 20 “C.
pK1 is assigned to carboxylic acid groups, pKZ to histidines
and terminal amino groups, and pK3 to the acidalkaline
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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