J. Biol. Chem., Vol. 256, Issue 20, 10267-10271, 10, 1981
Studies on cobalt myoglobins and hemoglobins. The interaction of molecular oxygen with leghemoglobin
M Ikeda-Saito, H Hori, T Inubushi and T Yonetani
Cobalt-substituted leghemoglobin (CoLb) was prepared from cobaltous
protoporphyrin IX and apo-leghemoglobin a, and its functional and
spectroscopic properties were examined. The oxygen affinity of CoLb was
pH-dependent. The partial oxygen pressure at half-saturation was decreased
from 14.3 torr at pH 8 to 3.5 Torr at pH 3.5 with a pK value of 5.8 at 15
degrees C. The oxygen association and dissociation rates were determined by
the temperature jump-relaxation technique. The association rate was
essentially pH-independent, and the relatively high oxygen affinity of CoLb
was found to be due to the fast oxygen association rate in comparison with
cobalt myoglobin. The oxygen dissociation rate was pH-dependent with a pK
value of 5.7 at 15 degrees C, suggesting that the pH dependence of the
oxygen dissociation rate is responsible for the pH-dependent change in the
oxygen affinity of CoLb. The EPR spectrum of oxyCoLb exhibited a
pH-dependent change with a pK value of 5.7 at 15 degrees C. The EPR
spectrum at 77 K of oxyCoLb at neutral pH exhibited a small effect upon the
replacement of H2O with D2O, but it was drastically altered in the acidic
pH region. The deoxy EPR spectrum was pH-independent between pH 4 and 9.
These observations indicate that in oxyCoLb, the bound oxygen is
interacting with the distal amino acid group with a pK value of 5.7 and
that this interaction is responsible for the increased oxygen affinity in
the acidic pH region.
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