The Enzymes of 2,3-Diphosphoglycerate Metabolism in the Human Red Cell

Summary

2,3-Diphosphoglycerate is synthesized in red cells by a specific mutase and hydrolyzed by a specific phosphatase. Diphosphoglycerate mutase catalyzes the synthesis of 2,3-diphosphoglycerate from 1,3-diphosphoglycerate and 3-phosphoglycerate. The maximum activity of the enzyme in the mature human erythrocyte is 4 μmoles per minute per ml of cells at 37°. Studies with the purified enzyme showed activation by 2-phosphoglycerate and glycolate 2-P. 2,3-Diphosphoglycerate is a competitive inhibitor of 1,3-diphosphoglycerate (K_m 0.5 μM) with a K₁. of 0.8 μM. Due to the high level of 2,3-diphosphoglycerate in the mature red cell (4–6 mM), the rate of the diphosphomutase will be described by the equation for competitive inhibition:

$$ \frac{v}{V} = 1/1 + \frac{{0.5\mu {\text{M}}}}{{\left[ {1,3 - {\text{diPGA}}} \right]}}\left( {1 + \frac{{\left[ {2,3 - {\text{diPGA}}} \right]}}{{0.8\mu {\text{M}}}}} \right) $$

The activity of 2,3-diphosphoglycerate phosphatase in human red cells is 1 m¼mole per minute per ml of red cells at 37°. The rate is increased in the presence of certain activators. The system activated by phosphate and chloride (0.1 M) has a maximal velocity 14 times higher. Glycolate 2-P activates 1350-fold and does not require chloride. Since the K_a for phosphate, 0.7 mM, is higher than the normal intracellular level of 0.4 mM, changes in inorganic phosphate in the cell will change the rate. The data are compatible with the following velocity equation:

$$ \frac{{\text{v}}}{{\text{V}}}{\text{ = 0}}{\text{.07 + }}\left( {\frac{{{\text{0}}{\text{.93}}}}{{{\text{1 + }}\frac{{{\text{0}}{\text{.7mM}}}}{{\left[ {{\text{Pi}}} \right]}}}}} \right) $$