Glucose-6-phosphate Glc

Brain hexokinase is inhibited by its product glucose-6-phosphate and to a lesser extent by adenosine diphosphate. The isoenzyme of hexokinase found in brain may be soluble in the cytosol or be attached firmly to mitochondria [2 and references therein]. An equilibrium exists between the soluble and the bound enzyme. The binding changes the kinetic properties of hexokinase and its inhibition by Glc-6-P resulting in a more active enzyme. The extent of binding is inversely related to the ATP ADP ratio, i.e. conditions in which energy utilization... 

Frc-6-P = fructose-6-phosphate Glc-1-P = glucose-1-phosphate PPP = pentose phosphate pathway UTP = uridyl triphosphate... 

Glucose (Glc) is taken up and phosphorylated into glucose-6-phosphate (Glc6P), with consumption of ATP. Isomerization and phosphorylation afford fructose-l,6-bisphosphate (Frul,6P2), which is cleaved into two triose molecules D-glyceraldehyde-3-phosphate (GA3P) and dihydroxyacetone monophosphate (DHAP). These are equilibrated by triose phosphate isomerase as only GA3P is metabolized further, except approximately 5 mol% of DHAP that leaks out of the pathway via reduction to glycerol, which is excreted as a side-product. 

D-Glucose - - 2 ATP D-Glucose 6-phosphate UTP - - D-glucosyl phosphate UDP-Glc - - D-glucose 6-phosphate ... 

Fig. 6.1 Reaction scheme of the system under study. Glc, glucose HK, hexokinase PGI, phos-phoglucose isomerase PFK, phosphofructokinase ALD, aldolase TIM, triose phosphate isomerase G3PDH, glycerol 3-phosphate dehydrogenase 1,3BPG 1,3-bisphosphoglycerate PGK, phospho-glycerate kinase G6P, glucose 6-phosphate F6P, fructose 6-phosphate ATP, adenosine triphosphate ADP, adenosine diphosphate PEP, phospho-enolpyruvate F1,6BP, fructose 1,6-bisphosphate DHAP, dihydroxyacetone phosphate G3P, glycerol 3-phosphate GAP, glyceraldehyde...

Figure 5. Enzymatic reactions relating to FDP production. Glc = glucose G6P = glucose 6-phosphate Fru = fructose F6P = fructose 6-phosphate FDP = fructose 1,6-diphosphate AcOP = acetyl phosphate AK = acetate kinase GlcK = glucokinase HK = hexokinase PFK = phosphofructokinase.

Fig. 15.2. Pathways of fructose metabolism. 15.4, Fructose-1-phosphate aldolase 15.5, fructose-1,6-diphosphatase 15.7, D-glycerate kinase. F-l-P, Fructose-1-phosphate F-6-P, fructose-6-phosphate F-l,6-DiP, fructose-1,6-diphosphate Glc-6-F, glucose-6-phosphate ATP, adenosine triphosphate ADP, adenosine diphosphate...

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