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Concept
Glucose 6-phosphate
Summary
Glucose 6-phosphate (G6P, sometimes called the Robison ester) is a glucose sugar phosphorylated at the hydroxy group on carbon 6. This dianion is very common in cells as the 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: glycolysis and the pentose phosphate pathway.
In addition to these two 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.
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 equivalent 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 phosphate group so the glucose 6-phosphate cannot easily cross the cell membrane.
Glucose 6-phosphate is also produced during glycogenolysis from glucose 1-phosphate, the first product of the breakdown of glycogen polymers.
Pentose phosphate pathway
When the ratio of NADP+ to NADPH increases, the body 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 to 6-phosphogluconate by glucose 6-phosphate dehydrogenase. This irreversible 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.
Official source
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