section 28.2
Functional Aspects of Hemoglobin
P143 His
Schematic representation of the side chains of the /3 subunits of human hemoglobin that participate in binding to
2,3-DPG. The binding cavity is lined with eight positive charges (four from each
subunit) that react with five negative
charges on 2,3-DPG. Fetal hemoglobin binds 2,3-DPG much less tightly than does maternal hemoglobin because its
chains (the counterpart of /3-chains) contain Ser at yl43 in place of His at /3143.
an intermediate in glycolysis (Chapter 13). The rearrange-
ment, catalyzed by bisphosphoglycerate mutase, requires
3-phosphoglycerate as cofactor and is allosterically stim-
ulated by 2-phosphoglycerate (Figure 28-9). Inorganic
phosphate appears to be a negative allosteric modifier.
2,3-DPG is also a cofactor for the phosphoglycerate
mutase of glycolysis. Bisphosphoglycerate phosphatase
converts 2,3-DPG to 3-phosphoglycerate. Identical elec-
trophoretic and chromatographic patterns and copurifica-
tion of the two activities suggest that the catalytic sites for
bisphosphoglycerate mutase and phosphatase may reside
in the same protein. This hypothesis is supported by the
report of an individual with an extremely low intraery-
throcytic concentration of 2,3-DPG whose red blood cells
lacked both bisphosphoglycerate mutase and phosphatase
activities. Trace amounts of 2,3-DPG were present to act
as cofactor for phosphoglycerate mutase and permit gly-
colysis to proceed. The 2,3-DPG deficiency diminished
oxygen delivery to the tissues and produced a mild ery-
throcytosis. There was no hemolysis, and the disorder was
clinically silent. Patients who have pyruvate kinase de-
ficiency have above-normal levels of 2,3-DPG, whereas
those who have hexokinase deficiency have below-normal
levels. Appropriate erythropoietic responses are seen in
both types (see below). The concentration of 2,3-DPG in
the red cell can be altered by 15-25% in less than 12 hours.
The most probable mechanisms involved are summarized
1. The binding of 2,3-DPG to deoxyhemoglobin
decreases the amount of free 2,3-DPG available for
participation in other reactions and causes increased
2,3-DPG synthesis at the expense of 1,3-bisphospho-
glycerate. A decrease in oxygen saturation of
hemoglobin may act in the same way.
2. The intraerythrocytic pH affects 2,3-DPG
concentration. A decrease in pH increases the amount
of bound 2,3-DPG by increasing the concentration of
deoxyhemoglobin, which acts as described in the
preceding paragraph. An increase in pH stimulates
glycolysis, which tends to increase the concentration
of all glycolytic intermediates, including 2,3-DPG. A
decrease in pH within the physiological range also
decreases the activity of bisphosphoglycerate mutase
and increases the activity of bisphosphoglycerate
3. As erythrocytes age
in vivo,
their oxygen affinity
increases. The concentration of 2,3-DPG in young red
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