IUPACName=(3,4,5,6- tetrahydroxytetrahydropyran- 2-yl) methoxyphosphonic acid
Section1= Chembox Identifiers
Section2= Chembox Properties
Section3= Chembox Hazards
Glucose 6-phosphate (also known as Robison ester) is
glucosesugar phosphorylatedon 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
Pentose phosphate pathwayIn addition to these metabolic pathways, glucose 6-phosphate may also be converted to glycogenor starchfor storage. This storage is in the liverand muscles in the form of glycogen for most multicellular animals, and in intracellularstarch or glycogen granules for most other organisms.
Production of glucose 6-phosphate
Within a cell, glucose 6-phosphate is produced by phosphorylation of
glucoseon the sixth carbon. This is catalyzed by the enzyme hexokinasein most cells, and, in higher animals, glucokinasein certain cells, most notably liver cells. One molecule of ATP is consumed in this reaction.
The major reason for the immediate phosphorylation of glucose is to prevent diffusion out of the cell. The phosphorylation adds a charged
phosphategroup so the glucose 6-phosphate cannot easily cross the cell membrane.
Glucose-6-phosphate is also produced during
glycogenolysisfrom glucose-1-phosphate, the first product of the breakdown of glycogenpolymers.
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-phosphateby phosphoglucose isomerase.
This reaction converts glucose 6-phosphate to
fructose 6-phosphatein 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 glucagonor 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 glycogenolysisor gluconeogenesis. The free glucose is sent into the bloodstream for uptake by other cells.
Pentose phosphate pathway
Glucose-6-phosphate dehydrogenase deficiency
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