Glucose cycle

The glucose cycle (also known as the hepatic futile 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.

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

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

Regulation of glucose cycle
Flux through the glucose cycle is regulated by several hormones including insulin and glucagon 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