Glycogen Metabolism I





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Uploaded on Oct 20, 2010

This course is part of a series taught by Kevin Ahern at Oregon State University on General Biochemistry. For more information about online courses go to http://ecampus.oregonstate.edu/ http://www.youtube.com/playlist?list=...

1. The structure of glycogen consists of units of glucose linked in the alpha 1-4 configuration with branches linked in the 1-6 configuration.

2. Glycogen differs from starch in the amount of branching (much more).

3. Glycogen is a storage form of energy that can yield ATP very quickly, because glucose-1-phosphate can be released very quickly.

4. You should know the function/activites of the enzymes in glycogen breakdown - glycogen phosphorylase, phosphoglucomutase, and debranching enzyme.

5. Glycogen phosphorylase action on glycogen yields glucose-1-phosphate. Glycogen phosphorylase exists in two forms - phosphorylase a and phosphorylase b. Phosphorylase a differs from phosphorylase b only in that phosphorylase a contains two phosphates and phosphorylase b contains none. Phosphate is added to glycogen phosphorylase by the enzyme phosphorylase kinase.

6. Glucose-6-phosphate (G6P) has many different fates and sources. First, breakdown of glycogen produces G1P, which is readily converted to G6P. G6P can then go three different directions. In muscle and brain (and most other tissues), G6P enters glycolysis. In liver only, G6P enters gluconeogenesis and is converted to glucose for export to the bloodstream. In other tissues, G6P enters the pentose phosphate pathway and is oxidized to produce NADPH.

7. Breakdown of glycogen by glycogen phosphorylase involves phosphorolysis (use of a phosphate to cleave molecules) instead of hydrolysis. The advantage of this is that the energy of the alpha1-4 bond is used to add phosphate to glucose (forming G1P) instead of using a triphosphate to do so. This saves energy for cells.

8. Glycogen phosphorylase catalyses phosphorolysis of glycogen to within 4 residues of a branch point and then stops. Further metabolism of glycogen requires action of Debranching Enzyme. Debranching enzyme removes three of the remaining four glucoses at a branch point and transfers them to another chain in a 1-4 configuration. The remaining glucose in the 1-6 configuration at the branch point is cleaved in a hydrolysis reaction to yield free glucose. It is the only free glucose released in glycogen metabolism.

9. Phosphoglucomutase interconverts G1P and G6P via a G1,6BP intermediate. The reaction is readily reversible (Delta G zero prime near zero) and the direction of the reaction depends on the concentration of substrates.

10. Glycogen phosphorylase is present in two forms, GPa (glycogen phosphorylase a) and GPb (glycogen phosphorylase b). They differ in phosphorylation. GPa is phosphorylated and GPb is not.1. GPb is converted into GPa by phosphorylation at two sites. Covalent modifications are DIFFERENT from allosteric controls, which interconvert the R and T states of BOTH GPa and GPb.

11. GPb is inhibited by ATP and G6P (converts R state to T state). When the body is at rest, ATP and G6P levels are high enough to turn GPb off. GPb is ONLY converted to the R state (activated) when AMP is present in sufficiently high concentrations.

12. Glucose converts GPa from the R to the T state (inhibition). Nothing is required to convert GPa from the T to the R state, so if glucose is absent (which it usually is), then GPa will be in the R state.

13. Consequently, in normally resting cells, GPa is usually in the R state (active) and GPb is usually in the T state (inactive). Thus, processes like phosphorylation/dephosphorylation which interconvert GPa and GPb have major effects on whether or not glycogen breakdown is occuring.

14. The enzyme responsible for phosphorylating glycogen phosphorylase (thus converting GPb to GPa) is known as (glycogen) phosphorylase kinase. This interesting enzyme is activated by two different mechanisms - phosphorylation and/or calcium ions.

15. Calcium ions bind to phosphorylase kinase by virtue of the fact that the protein calmodulin (which binds calcium ions) is a subunit of phosphorylase kinase.

16. Note that calcium ions are a factor in muscular contraction. Release of calcium occurs with muscular contraction. Thus, when ATP is needed, phosphorylase kinase is activated by calcium. Activation of phophorylase kinase causes phosphorylation of GPb, forming GPa, favoring glycogen breakdown and, ultimately, ATP production.

17. Calcium alone only partly activates phosphorylase kinase. Full activation of phosphorylase kinase requires that the protein be phosphorylated as well. Phosphorylation of phosphorylase kinase is catalyzed by protein kinase A. Phosphorylation of phosphorylase kinase alone can also partly activate phosphorylase kinase, just as was the case with calcium. Either binding of calcium ion or phosphorylation can happen first. There is no required order to the binding.

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