Allosteric inhibition of brain hexokinase by glucose 6-phosphate in the reverse reaction.
A study of the reverse reaction of rat brain hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) has been performed using a photometric method based on a mutarotase-glucose oxidase-peroxidase-chromogen system to trap and visualize glucose, plus a glycerol kinase-glycerol system to trap ATP. Glucose 6-phosphate or 2-deoxyglucose 6-phosphate were used as phosphoryl donors at different concentrations of ADP. Variation of glucose 6-phosphate concentrations resulted in a biphasic curve from which apparent Km and Ki values of ca. 0.2 mM were calculated. In contrast, variation of 2-deoxyglucose 6-phosphate concentrations resulted in Michaelian kinetics with an apparent Km of 2 mM. The Km value for MgADP was 16 mM irrespective of the nature and concentration of the hexose 6-phosphate substrate. These results are fully consistent with an allosteric site for glucose 6-phosphate as an explanation for the inhibition of animal hexokinases by glucose 6-P and further indicate that the maximal rate is the parameter affected. From these observations and previous knowledge, the possible occurrence in animal hexokinases of a regulatory site for ATP to account for the competition between glucose 6-phosphate and ATP in the forward reaction is postulated.
D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001921:Brain / Q000201:enzymology*; D019298:Glucose-6-Phosphate; D005958:Glucosephosphates / Q000494:pharmacology*; D006593:Hexokinase / Q000037:antagonists & inhibitors*; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D051381:Rats; D013237:Stereoisomerism
D005958:Glucosephosphates; D019298:Glucose-6-Phosphate; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D006593:Hexokinase; D008274:Magnesium