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| Names | |
|---|---|
| IUPAC names D-Glucopyranose 6-phosphate 6-O-Phosphono-D-glucopyranose | |
| Identifiers | |
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3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
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| KEGG | |
| MeSH | Glucose-6-phosphate |
| UNII | |
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| Properties | |
| C6H13O9P | |
| Molar mass | 260.136 |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Glucose 6-phosphate (G6P, sometimes called theRobison ester) is aglucose sugarphosphorylated at the hydroxy group on carbon 6. This dianion is very common incells as the majority of glucose entering a cell will become phosphorylated in this way.
Because of its prominent position in cellularchemistry, glucose 6-phosphate has many possible fates within the cell. It lies at the start of two majormetabolic pathways:glycolysis and thepentose phosphate pathway.
In addition to these two metabolic pathways, glucose 6-phosphate may also be converted toglycogen orstarch for storage. This storage is in theliver andmuscles in the form of glycogen for most multicellularanimals, and inintracellular starch or glycogen granules for most other organisms.
Within a cell, glucose 6-phosphate is produced by phosphorylation ofglucose on the sixth carbon. This is catalyzed by theenzymehexokinase in most cells, and, in higher animals,glucokinase in certain cells, most notably liver cells. One equivalent ofATP is consumed in this reaction.
| D-Glucose | Hexokinase | α-D-Glucose 6-phosphate | |
 |  | ||
| ATP | ADP | ||
 | |||
| Glucose 6-phosphatase | |||
CompoundC00031 atKEGG Pathway Database.Enzyme2.7.1.1 atKEGG Pathway Database.CompoundC00668 atKEGG Pathway Database.ReactionR01786 atKEGG Pathway Database.
The major reason for the immediate phosphorylation of glucose is to prevent diffusion out of the cell. The phosphorylation adds a chargedphosphate group so the glucose 6-phosphate cannot easily cross thecell membrane.
Glucose 6-phosphate is also produced duringglycogenolysis fromglucose 1-phosphate, the first product of the breakdown ofglycogen polymers.
When the ratio ofNADP+ toNADPH increases, the body needs to produce more NADPH (a reducing agent for several reactions like fatty acid synthesis and glutathione reduction inerythrocytes).[1] This will cause the G6P to be dehydrogenated to6-phosphogluconate byglucose 6-phosphate dehydrogenase.[1] This irreversible reaction is the initial step of the pentose phosphate pathway, which generates the useful cofactor NADPH as well asribulose-5-phosphate, a carbon source for the synthesis of other molecules.[1] Also, if the body needsnucleotide precursors of DNA for growth and synthesis,G6P will also be dehydrogenated and enter the pentose phosphate pathway.[1]
If the cell needs energy or carbon skeletons for synthesis, then glucose 6-phosphate is targeted forglycolysis.[2] Glucose 6-phosphate is first isomerized tofructose 6-phosphate byphosphoglucose isomerase, which usesmagnesium as acofactor.[2]
| α-D-Glucose 6-phosphate | Phosphoglucose isomerase | β-D-Fructose 6-phosphate | |
|  | ||
 | |||
| Phosphoglucose isomerase | |||
CompoundC00668 atKEGG Pathway Database.Enzyme5.3.1.9 atKEGG Pathway Database.CompoundC05345 atKEGG Pathway Database.ReactionR00771 atKEGG Pathway Database.
This reaction converts glucose 6-phosphate tofructose 6-phosphate in preparation for phosphorylation tofructose 1,6-bisphosphate.[2] The addition of the second phosphoryl group to produce fructose 1,6-bisphosphate is an irreversible step, and so is used to irreversibly target the glucose 6-phosphate breakdown to provide energy for ATP production viaglycolysis.
Click on genes, proteins and metabolites below to link to respective articles.[§ 1]
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If blood glucose levels are high, the body needs a way to store the excess glucose. After being converted to G6P, the molecule can be turned intoglucose 1-phosphate byphosphoglucomutase. Glucose 1-phosphate can then be combined withuridine triphosphate (UTP) to formUDP-glucose, driven by the hydrolysis of UTP, releasing phosphate. Now, the activated UDP-glucose can add to a growing glycogen molecule with the help ofglycogen synthase. This is a very efficient storage mechanism for glucose since it costs the body only one ATP to store the one glucose molecule and virtually no energy to remove it from storage. Glucose 6-phosphate is anallosteric activator of glycogen synthase, which makes sense because when the level of glucose is high the body should store the excess glucose as glycogen. On the other hand, glycogen synthase is inhibited when it is phosphorylated by protein kinase during times of high stress or low levels of blood glucose, viahormone induction byglucagon oradrenaline.
When the body needs glucose for energy,glycogen phosphorylase, with the help of anorthophosphate, can cleave away a molecule from the glycogen chain. The cleaved molecule is in the form of glucose 1-phosphate, which can be converted into G6P by phosphoglucomutase. Next, the phosphoryl group on G6P can be cleaved by glucose 6-phosphatase so that a free glucose can be formed. This free glucose can pass through membranes and can enter the bloodstream to travel to other places in the body.
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Liver cells express the transmembrane enzymeglucose 6-phosphatase in the endoplasmic reticulum. The catalytic site is found on the lumenal face of the membrane, and removes the phosphate group from glucose 6-phosphate produced duringglycogenolysis orgluconeogenesis. Free glucose is transported out of the endoplasmic reticulum viaGLUT7 and released into the bloodstream viaGLUT2 for uptake by other cells. Muscle cells lack this enzyme, so myofibers use glucose 6-phosphate in their own metabolic pathways such as glycolysis. Importantly, this prevents myocytes from releasing glycogen stores they have obtained into the blood.
