section 28.2

Functional Aspects of Hemoglobin

653

P143 His

FIGURE 28-8

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

/8

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

y

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

below.

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

phosphatase.

3. As erythrocytes age

in vivo,

their oxygen affinity

increases. The concentration of 2,3-DPG in young red