Glucose cycle

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Glucose cycle

The glucose cycle (also known as the hepatic futile cycle) occurs primarily in the liver

Liver

The liver is a vital organ present in vertebrates and some other animals; it has a wide range of functions, a few of which are detoxification, protein synthesis, and production of biochemicals necessary for digestion....
and is the dynamic equilibrium between glucose

Glucose

Glucose , a monosaccharide also known as grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology....
and glucose 6-phosphate. This is important for maintaining a constant concentration of glucose in the blood stream.

glucose cycle is required for one of the liver functions; the homeostasis

Homeostasis

Homeostasis is the property of a system, either open system or closed system, that regulates its internal environment and tends to maintain a stable, constant condition....
of glucose in the blood stream. When blood glucose levels are in excess, glucose can be stored in the liver as glycogen

Glycogen

Glycogen is a polysaccharide of glucose which functions as the secondary short term energy storage in animal cells. It is made primarily by the liver and the muscles, but can also be made by the brain and stomach....
.

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Encyclopedia

The glucose cycle (also known as the hepatic futile cycle) occurs primarily in the liver

Liver

Glucose

Function

The glucose cycle is required for one of the liver functions; the homeostasis

Homeostasis

Glycogen

Catabolism

Catabolism is the set of metabolic pathways which break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides and amino acids, respectively....
and glucose may re-enter the blood stream.

The catabolic process occurs at the nonreducing end of glycogen. A phosphate group breaks the bond between C 1 of a glucose ring and the O that connects it to the next(phosphorolysis). One glucose unit is thus splitted off. Glycogen (with n glucose units) is converted into G-1-P(a PO4 group now attaches to C1 where O used to ) and glycogen (with n-1 glucose units) by enzyme glycogen phosphorylase. G-1-P is then converted into G-6-P by enzyme phosphoglucomutase. A water molecule hydrolyses G-6-P to glucose, the enzyme is glucose-6-phosphatase.

Cell specificity

When glucose enters a cell it is rapidly converted to glucose 6-phosphate, by hexokinase

Hexokinase

A hexokinase is an enzyme that phosphorylation a six-carbon sugar, a hexose, to a hexose phosphate. In most tissues and organisms, glucose is the most important substrate of hexokinases, and glucose-6-phosphate the most important product....
. The glucose cycle can occur in liver cells due to a liver specific enzyme glucose-6-phosphatase, which catalyse the dephosphorylation of glucose 6-phosphate back to glucose.

Glucose-6-phosphate is the product of glycogenolysis

Glycogenolysis

Glycogenolysis is the catabolism of glycogen by removal of a glucose monomer through cleavage with inorganic phosphate to produce glucose-1-phosphate....
or gluconeogenesis

Gluconeogenesis

Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactic acid, glycerol, and glucogenic amino acids....
, where the goal is to increase free glucose in the blood due body being in catabolic state. Other cells such as muscle and brain cells do not contain glucose 6-phosphatase. As a result, any glucose 6-phosphate produced in those cells is committed to cellular metabolic pathways, primarily pentose phosphate pathway or glycolysis.

Regulation of glucose cycle

Flux through the glucose cycle is regulated by several hormones including insulin

Insulin

Insulin is a hormone with extensive effects on both metabolism and several other body systems . Insulin causes most of the body's cells to take up glucose from the blood , storing it as glycogen in the liver and muscle, and stops use of fat as an energy source....
and glucagon

Glucagon

Glucagon is an important hormone involved in carbohydrate metabolism. Produced by the pancreas, it is released when the glucose level in the blood is low , causing the liver to convert stored glycogen into glucose and release it into the bloodstream....
as well as allosteric regulation of both hexokinase and glucose 6-phosphatase.

Diseases associated with glucose cycle

A deficiency in glucose 6-phosphatase that disrupts the liver glucose cycle, can lead to von Gierke's disease

Glycogen storage disease type I

Glycogen storage disease type I or von Gierke's disease, is the most common of the glycogen storage diseases. This genetic disease results from deficiency of the enzyme glucose-6-phosphatase....