Glycogenesis

Glycogenesis is the process of glycogen synthesis, in which glucose molecules are added to chains of glycogen for storage. This process is activated during rest periods following the Cori cycle, in the liver, and also activated by insulin in response to high glucose levels, for example after a carbohydrate containing meal.

Steps

Glucose is converted into glucose-6-phosphate by the action of glucokinase or hexokinase.
Glucose-6-phosphate is converted into glucose-1-phosphate by the action of Phosphoglucomutase, passing through an obligatory intermediate step of glucose-1,6-bisphosphate.
Glucose-1-phosphate is converted into UDP-glucose by the action of Uridyl Transferase (also called UDP-glucose pyrophosphorylase) and pyrophosphate is formed, which is hydrolyzed by pyrophosphatase into 2 molecules of Pi.
Glucose molecules are assembled in a chain by glycogen synthase, which must act on a pre-existing glycogen primer or glycogenin (small protein that forms the primer). The mechanism for joining glucose units is that glycogen synthase binds to UDPG, causing it to break down into an oxonium ion, also formed in glycogenolysis. This oxonium ion can readily add to the 4-hydroxyl group of a glucosyl residue on the 4 end of the glycogen chain.
Branches are made by branching enzyme (also known as amylo-α(1:4)->α(1:6)transglycosylase), which transfers the end of the chain onto an earlier part via α-1:6 glucosidic bond, forming branches, which further grow by addition of more α-1:4 glucosidic units.

Control and regulation

Glycogenesis responds to hormonal control.

One of the main forms of control is the varied phosphorylation of glycogen synthase and glycogen phosphorylase. This is regulated by enzymes under the control of hormonal activity, which is in turn regulated by many factors. As such, there are many different possible effectors when compared to allosteric systems of regulation.

Epinephrine (Adrenaline)

Glycogen phosphorylase is activated by phosphorylation, whereas glycogen synthase is inhibited.

Glycogen phosphorylase is converted from its less active b form to an active a form by the enzyme phosphorylase kinase. This latter enzyme is itself activated by protein kinase A and deactivated by phosphoprotein phosphatase-1.

Protein kinase A itself is activated by the hormone adrenaline. Epinephrine binds to a receptor protein that activates adenylate cyclase. The latter enzyme causes the formation of cyclic AMP from ATP; two molecules of cyclic AMP bind to the regulatory subunit of protein kinase A, which activates it allowing the catalytic subunit of protein kinase A to dissociate from the assembly and to phosphorylate other proteins.

Returning to glycogen phosphorylase, the less active form (b) can itself be activated without the conformational change. 5'AMP acts as an allosteric activator, whereas ATP is an inhibitor, as already seen with phosphofructokinase control, helping to change the rate of flux in response to energy demand.

Epinephrine not only activates glycogen phosphorylase but also inhibits glycogen synthase. This amplifies the effect of activating glycogen phosphorylase. This inhibition is achieved by a similar mechanism, as protein kinase A acts to phosphorylate the enzyme, which lowers activity. This is known as co-ordinate reciprocal control. Refer to glycolysis for further information of the regulation of glycogenesis.

Insulin

Insulin has an antagonistic effect to adrenaline. When insulin binds on the G protein-coupled receptor, the alpha subunit of GDP in the G protein changes to GTP and dissociates from the inhibitory beta and gamma subunits. The alpha subunit binds on adenylyl cyclase to inhibit its activity. Thus less cAMP then less protein kinase A will be produced. Thus glycogen synthase, one of the targets of protein kinase A, will be in non-phosphorylated form, which is the active form of glycogen synthase. Active glycogen synthase can decrease the blood glucose level after a full meal

Calcium ions

Calcium ions or cyclic AMP (cAMP) act as secondary messengers. This is an example of negative control. The calcium ions activate phosphorylase kinase. This activates glycogen phosphorylase and inhibits glycogen synthase.

External links

The chemical logic of glycogen synthesis

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