Glycogenesis
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Glycogenesis is the process of glycogen synthesis, in which glucose molecules are added to chains of glycogen. This process is activated by insulin in response to high glucose levels.
The first step involves the synthesis of UDP-glucose from glucose-1-phosphate and UTP:
- Glucose 1-phosphate + UTP + H2O → UDP-glucose + 2 Pi
This reaction is catalyzed by UDP-glucose pyrophosphorylase. This reaction would be reversible if it were not for the rapid exergonic hydrolysis (hence the need for water) of pyrophosphate to orthophosphate (catalyzed by pyrophosphatase).
In the second reaction, UDP-glucose is transferred to the hydroxyl group of the existing chain, forming an α-1,4-glycosidic link. This reaction is catalyzed by glycogen synthase. Glycogen synthase can only add to a chain that contains at least four units. Therefore, the protein glycogenin is used as a "primer-molecule." α-1,6 Links are created by glycogen branching enzyme.
Glycogen synthase is activated by phosphoprotein phosphatase. This enzyme is activated by insulin.
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[edit] Control and regulation
Glycogenesis responds to both hormonal and electrical 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 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.
[edit] 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. Adrenaline binds to a receptor protein which activates adenylate cyclase. This in turn activates the secondary messenger system, by causing the formation of cyclic AMP, which acts allosterically to activate protein kinase A
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 this helps to change the rate of flux in response to energy demand.
Adrenaline not only activates glycogen phosphorylase, but also inhibits glycogen synthase. This amplifies the effect of activating glycogen phosphorylase. This inhibition is achievied by a similar mechanism, as protein kinase A acts to phosphorylate the enzyme and this lowers activity. This is known as co-ordinate reciprocal control.
[edit] Insulin
Insulin has an antagonistic effect compared to adrenaline. The glycogen synthase enzyme can be kept in a low activation form by insulin, which switches off one of its kinase enzymes – glycogen synthase kinase 3.
[edit] Calcium ion
Calcium ion, like cyclic AMP (cAMP), acts as a secondary messenger. This is an example of negative control. The calcium ions activates phosphorylase kinase. This activates glycogen phosphorylase and inhibits glycogen synthase.
[edit] External links
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