JBC, Vol. 250, Issue 18, 7093-7098, Sep, 1975
Thermodynamical studies of oxygen equilibrium of hemoglobin. Nonuniform heats and entropy changes for the individual oxygenation steps and enthalpy-entropy compensation
K. Imai and T. Yonetani
Precise oxygen equilibrium curves of human adult hemoglobin were determined
by the automatic recording method at several temperatures in the presence
and absence of 2,3-diphosphoglycerate (DPG) or inositol hexaphosphate (IHP)
with 0.05 M 2,2-bis(hydroxymethyl)-2,2',2''-nitrolotriethanol (bis-tris)
buffers (pH 7.4) containing 0.1 M Cl-. The equilibrium data were analyzed
according to the Adair scheme, and the heats, deltaHi (i = 1,2,3,4) and the
entropy changes, deltaSi (i = 1,2,3,4), for the individual oxygenation
steps were obtained. The shape of the equilibrium curve varies on
temperature changes whether DPG or IHP is present or absent. In
consequence, the deltaHi value depends on i and on the presence of DPG and
IHP. Behavior of deltaSi is similar to that of deltaHi. The similar
behavior of deltaHi and deltaSi resulted in a compensation phenomenon. The
contribution of T cdeltaSi to the free energy change is compensated by the
contribution of deltaHi at the first three oxygenation steps but not at the
fourth step, and for i = 1,2, and 3 changes of T cdeltaSi value upon the
addition of DPG and IHP are compensated by accompanied changes of deltaHi
value, where T c (= 260 K) is the compensation temperature. A major part of
both the enthalpy-entropy compensation and nonuniformity of deltaHi and
deltaSi appears to be attributable to contributions of the oxygen-linked
binding of Cl-, DPG and IHP, by hemoglobin. The present results do not
necessarily support the earlier idea of Wyman that the cooperative
oxygenbinding is essentially an entropy effect.
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