Glycogen phosphorylase

Glycogen phosphorylase is one of the phosphorylase enzymes (EC 2.4.1.1). It breaks up glycogen into glucose subunits. Glycogen is left with one less glucose molecule, and the free glucose molecule is in the form of glucose-1-phosphate. In order to be used for metabolism, it must be converted to glucose-6-phosphate by the enzyme phosphoglucomutase.

Glycogen phosphorylase can only act on linear chains of glycogen (a 1-4 glycosidic linkage). Its work will immediately come to a halt four residues away from a 1-6 branch (which are exceedingly common in glycogen). In these situations, a debranching enzyme is necessary, which will straighten out the chain in that area. Additionally, an alpha 1-6 glucosidase enzyme is required to break the remaining 1-6 residue that remains in the new linear chain. After all this is done, glycogen phosphorylase can continue.

An insulin stimulated enzyme known as phosphoprotein phosphatase (PP-1) inactivates glycogen phosphorylase to prevent glycogen break up.

Glycogen phosphorylase has a pyridoxal phosphate (PLP) at each catalytic site. Pyridoxal phosphate links with basic residues and covalently forms a Schiff Base which helps attack the substrate. PLP is covalently linked to the protein via the epsilon-amino group of a lysyl residue. PLP therefore stabilizes the attacking phosphate group.

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Regulation

The enzyme is regulated by both allosteric control and reversible phosphorylation, which is a kind of covalent regulation.

Hormones such as adrenaline and glucagon regulate glycogen phosphorylase, using a secondary messenger amplification system. Adrenaline activates adenylate cyclase, which increases levels of cAMP. cAMP in turn activates protein kinase A. This phosphorylates phosphorylase kinase, which is required for the activation of the glycogen phosphorylase. Phosphorylase kinase does this by converting glycogen phosphorylase from the inactive form of phosphorylase b to the active form of glycogen phosphorylase a. Glycogen phosphorylase is phosphorylated on its seryl 14 side chain.

Calcium ions also indirectly activate glycogen phosphorylase, as they activate the phosphorylase kinase enzyme.

The inactive glycogen phosphorylase is not completely inactive, and can be activated by 5'AMP, which reflect energy demand. ATP opposes this activation, reflecting sufficient energy. This prevents the release of G1P when it is not needed to enter glycolysis.

Liver

The hepatic glycogen phosphorylase a (the active form of the enzyme by phosphorylation) a is converted from relax form to the tense form to relaxed form if the allosteric inhibitor is present. The significance of the regulation is to salvage the glycogen when the glucose is in high level.

Muscle

The glycogen phosphorylase b (the inactive form of the enzyme due to dephosphorylation) is converted from relax form to tense form if the allosteric inhibitor, ATP and glucose-6-phosphate is present. The significance of the regulation is to salvage the glycogen when the energy status is high and the glucose supply to the muscle cells is high.

When AMP is concentrated, the glycogen phosphorylase b in tense conformation is converted to the relaxed form. It is to increase the glucose-6-phosphate supply for increasing the rate of glycolysis and thus the rate of ATP formation.

See also

• McArdle's Disease