| Abstract
| - Hexokinase I governs the rate-limiting step of glycolysis in brain tissue, being inhibited by itsproduct, glucose 6-phosphate, and allosterically relieved of product inhibition by phosphate. On the basisof small-angle X-ray scattering, the wild-type enzyme is a monomer in the presence of glucose andphosphate at protein concentrations up to 10 mg/mL, but in the presence of glucose 6-phosphate, is adimer down to protein concentrations as low as 1 mg/mL. A mutant form of hexokinase I, specificallyengineered by directed mutation to block dimerization, remains monomeric at high protein concentrationunder all conditions of ligation. This nondimerizing mutant exhibits wild-type activity, potent inhibitionby glucose 6-phosphate, and phosphate reversal of product inhibition. Small-angle X-ray scattering datafrom the mutant hexokinase I in the presence of glucose/phosphate, glucose/glucose 6-phosphate, andglucose/ADP/Mg2+/AlF3 are consistent with a rodlike conformation for the monomer similar to thatobserved in crystal structures of the hexokinase I dimer. Hence, any mechanism for allosteric regulationof hexokinase I should maintain a global conformation of the polypeptide similar to that observed incrystallographic structures.
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