Regulation of human erythrocyte hexokinase. The influence of glycolytic intermediates and inorganic phosphate.
Human erythrocyte hexokinase (ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1) was inhibited competitively with respect to MgATP2- by glucose-6-P (Ki - 10.8 muM) and fructose-6-P (Ki = 160 muM). Low concentrations of inorganic phosphate were competitive with respect to glucose-6-P and fructose-6-P, although higher concentrations of Pi were not able to overcome completely the inhibition by the hexose phosphates. The results are consistent with a model in which hexokinase exists in equilibrium either as free or phosphate-associated enzyme, the latter having a reduced but still substantial affinity for hexose phosphate. An alternative explanation could be found in the presence of two different enzymes, one with a high affinity for glucose-6-P being sensitive to regulation by Pi, one with a lower affinity for glucose-6-P being insensitive to Pi. A similar but less pronounced effect of Pi, was found on the inhibition by 2,3-diphosphoglycerate (Ki = 4.0 mM). Pi in the absence of inhibitor was also a competitive inhibitor with respect to MgATP2- (Ki = 20 mM). Furthermore a competitive inhibition with respect to MgATP2- was found by fructose 1,6-diphosphate (Ki = 4.3 mM), glycerate-3-P (Ki = 3.8 mM), glycerate-2-P (Ki = 12.5 mM), MgADP- (Ki = 1.0 mM) and MgAMP (Ki = 1.7 mM).
D000255:Adenosine Triphosphate / Q000378:metabolism; D004163:Diphosphoglyceric Acids / Q000494:pharmacology; D004912:Erythrocytes / Q000201:enzymology*; D005636:Fructosephosphates / Q000494:pharmacology; D005958:Glucosephosphates / Q000494:pharmacology; D006019:Glycolysis; D006593:Hexokinase / Q000037:antagonists & inhibitors / Q000097:blood*; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D010710:Phosphates / Q000494:pharmacology*
D004163:Diphosphoglyceric Acids; D005636:Fructosephosphates; D005958:Glucosephosphates; D010710:Phosphates; D000255:Adenosine Triphosphate; D006593:Hexokinase; D008274:Magnesium