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Glucose-6-phosphate



Glucose-6-phosphate
IUPAC name (3,4,5,6- tetrahydroxytetrahydropyran- 2-yl) methoxyphosphonic acid
Identifiers
CAS number 56-73-5
PubChem 208
MeSH Glucose-6-phosphate
SMILES C(C1C(C(C(C(O1)O)O)O)O)OP(=O)(O)O
Properties
Molecular formula C6H13O9P
Molar mass 260.136
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Glucose 6-phosphate (also known as Robison ester) is glucose sugar phosphorylated on carbon 6. This compound is very common in cells as the vast majority of glucose entering a cell will become phosphorylated in this way.

Because of its prominent position in cellular chemistry, glucose 6-phosphate has many possible fates within the cell. It lies at the start of two major metabolic pathways:

In addition to these metabolic pathways, glucose 6-phosphate may also be converted to glycogen or starch for storage. This storage is in the liver and muscles in the form of glycogen for most multicellular animals, and in intracellular starch or glycogen granules for most other organisms.

Additional recommended knowledge

Contents

Production of glucose 6-phosphate

From glucose

Within a cell, glucose 6-phosphate is produced by phosphorylation of glucose on the sixth carbon. This is catalyzed by the enzyme hexokinase in most cells, and, in higher animals, glucokinase in certain cells, most notably liver cells. One molecule of ATP is consumed in this reaction.

D-Glucose Hexokinase α-D-Glucose 6-phosphate
 
ATP ADP
 
 

Compound C00031 at KEGG Pathway Database. Enzyme 2.7.1.1 at KEGG Pathway Database. Compound C00668 at KEGG Pathway Database. Reaction R01786 at KEGG Pathway Database.

The major reason for the immediate phosphorylation of glucose is to prevent diffusion out of the cell. The phosphorylation adds a charged phosphate group so the glucose 6-phosphate cannot easily cross the cell membrane.

From glycogen

Glucose-6-phosphate is also produced during glycogenolysis from glucose-1-phosphate, the first product of the breakdown of glycogen polymers.

Fate 1: Pentose Phosphate Pathway

When the ratio of NADP+ : NADPH increases, the body realizes it needs to produce more NADPH (a reducing agent for several reactions like fatty acid synthesis and glutathione reduction in erythrocytes). This will cause the G6P to be dehydrogenated by glucose 6-phosphate dehydrogenase. This reversible reaction is the initial step of the pentose phosphate pathway, which generates the useful cofactor NADPH as well as ribulose 5-phosphate, a carbon source for the synthesis of other molecules. Also, if the body needs nucleotide precursors of DNA for growth and synthesis, G6P will also be dehydrogenated and enter the pentose phosphate pathway.

Fate 2: Glycolysis

If the cell needs energy or carbon skeletons for synthesis then glucose 6-phosphate is targeted for glycolysis. Glucose 6-phosphate is first isomerized to fructose-6-phosphate by phosphoglucose isomerase.

α-D-Glucose 6-phosphate Phosphoglucose isomerase β-D-Fructose 6-phosphate
 
 
  Phosphoglucose isomerase

Compound C00668 at KEGG Pathway Database. Enzyme 5.3.1.9 at KEGG Pathway Database. Compound C05345 at KEGG Pathway Database. Reaction R00771 at KEGG Pathway Database.

This reaction converts glucose 6-phosphate to fructose 6-phosphate in preparation for phosphorylation to Fructose-1,6-bisphosphate. The addition of the 2nd 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 via glycolysis.

Fate 3: Storage as Glycogen

If blood glucose levels are high, the body needs a way to store the excess glucose. After being converted to G6P, phosphoglucose mutase (isomerase) can turn the molecule into glucose-1-phosphate. Glucose-1-phosphate can then be combined with uridine triphosphate (UTP) to form UDP-glucose. This reaction is driven by the hydrolysis of pyrophosphate that is released in the reaction. Now, the activated UDP-glucose can add to a growing glycogen molecule with the help of glycogen synthase. This is a very efficient storage mechanism for glucose since it costs the body only 1 ATP to store the 1 glucose molecule and virtually no energy to remove it from storage. It is important to note that glucose-6-phosphate is an allosteric 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 blood glucose levels (via hormone induction by glucagon or adrenaline).

When the body needs glucose for energy, glycogen phosphorylase, with the help of an orthophosphate, 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.

Fate 4: Dephosphorylation and Release into Bloodstream

Liver cells possess glucose-6-phosphatase, which removes the phosphate group from glucose-6-phosphate produced during glycogenolysis or gluconeogenesis. The free glucose is sent into the bloodstream for uptake by other cells.

References

  • Berg, Jeremy M.; Tymoczko, Stryer (2002). Biochemistry, 5th, New York: W.H. Freeman and Company. ISBN 0-7167-3051-0. 

See also

 v  d  e 
Glycolysis Metabolic Pathway
Glucose Hexokinase Glucose-6-phosphate Phosphoglucoisomerase Fructose 6-phosphate Phosphofructokinase Fructose 1,6-bisphosphate Fructose bisphosphate aldolase Dihydroxyacetone phosphate Glyceraldehyde 3-phosphate Triosephosphate isomerase Glyceraldehyde 3-phosphate Glyceraldehyde phosphate dehydrogenase
ATP ADP ATP ADP NAD+ + Pi NADH + H+
+ 2
NAD+ + Pi NADH + H+
1,3-Bisphosphoglycerate Phosphoglycerate kinase 3-Phosphoglycerate Phosphoglycerate mutase 2-Phosphoglycerate Enolase Phosphoenolpyruvate Pyruvate kinase Pyruvate Pyruvate dehydrogenase Acetyl-CoA
ADP ATP H2O ADP ATP CoA + NAD+ NADH + H+ + CO2
2 2 2 2 2 2
ADP ATP H2O
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Glucose-6-phosphate". A list of authors is available in Wikipedia.
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