Glucose cycle
Encyclopedia
The glucose cycle occurs primarily in the liver
and is the dynamic equilibrium between glucose
and glucose 6-phosphate. This is important for maintaining a constant concentration of glucose in the blood stream.
of glucose in the blood stream. When blood glucose levels are in excess, glucose can be stored in the liver as glycogen
. When glucose levels in the blood stream are low the glycogen can be catabolised
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 split 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.
. 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
or gluconeogenesis
, 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.
and glucagon
as well as allosteric regulation of both hexokinase and glucose 6-phosphatase.
.
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and is the dynamic equilibrium between glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and glucose 6-phosphate. This is important for maintaining a constant concentration of glucose in the blood stream.
Function
The glucose cycle is required for one of the liver functions; the homeostasisHomeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
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 molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
. When glucose levels in the blood stream are low the glycogen can be catabolised
Catabolism
Catabolism is the set of metabolic pathways that 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...
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 split 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 changed to glucose 6-phosphate, by hexokinaseHexokinase
A hexokinase is an enzyme that phosphorylates 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 conversion of glycogen polymers to glucose monomers. Glycogen is catabolized 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 lactate, 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 insulinInsulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
and glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
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 diseaseGlycogen 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. This deficiency impairs the ability of the liver to produce free glucose from glycogen and from...
.