Glucose 6-phosphate
  Glucose-6-phosphate | |
| Chemical name D-glucose-6-phosphate | |
| General | |
|---|---|
| Chemical formula | C6H13O9P |
| Molecular weight | 260.14 g/mol |
| Appearance | ? |
| CAS number | 56-73-5 |
| MSDS | Glucose-6-phosphate MSDS |
| Other names | |
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| Physical properties | |
| Density and phase at STP | ? |
| Solubility | ? |
| Specific gravity | ? |
| Crystal structure | ? |
| pH (10% solution with water) (pKa) |
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| Acidity constant (pKa) |
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| Thermal decomposition | ? K (? °C) |
| Phase behavior | |
| Melting point | ? |
| Boiling point | ? |
| Triple point | ? |
| Critical point | ? |
| Heat of fusion (ΔfusH) |
? |
| Entropy of fusion (ΔfusS) |
? |
| Heat of vaporization (ΔvapH) |
? |
| Safety | |
| Ingestion | ? |
| Inhalation | ? |
| Skin | ? |
| Eyes | ? |
| Flash point | ? |
| Autoignition temperature | ? |
| Explosive limits | ? |
| OSHA Permissible Exposure Limit (PEL) |
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| NIOSH Immediate Danger to Life and Health (IDLH) |
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| Precautions | |
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| Solid properties | |
| Standard enthalpy change of formation (ΔfH0solid) |
? |
| Standard molar entropy (S0solid) |
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| Heat capacity (Cp) |
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| Liquid properties | |
| ΔfH0liquid | ? |
| S0liquid | ? |
| Cp | ? |
| Density | ? |
| Gas properties | |
| ΔfH0gas | ? |
| S0gas | ? |
| Cp | ? |
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Except where noted, all data was produced under conditions of standard temperature and pressure. | |
Glucose-6-phosphate is a phosphorylated glucose molecule on carbon 6. When glucose enters a cell, it is immediately phosphorylated to G6P. This is catalyzed with hexokinase enzymes, thus consuming one ATP. A major reason for immediate phosphorylation of the glucose is so that it cannot diffuse out of the cell. The phosphorylation adds a charged group so the G6P cannot easily cross cell membranes. G6P can travel down two metabolic pathways, glycolysis and the pentose phosphate pathway. In addition to the metabolic pathways, G6P can also be stored as glycogen in the liver if blood glucose levels are high.
Its empirical formula is C6H13O9P.
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). 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.
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. 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 exces 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 a during times of high stress or low blood glucose levels.
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 blood stream to travel to other places in the body.
References
1. Berg, Tymoczko, Stryer, "Biochemistry", 5th edition, 2002, W.H. Freeman and Company, New York. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=stryer.TOC&depth=10