7 Glycogen Metabolism: Key Enzymes, Pathways, and Diseases

Glycogen

The storage form of glucose in the body.

Liver and Muscle

The primary storage sites for glycogen.

Glycogenesis

The process of glycogen synthesis.

Glycogenolysis

The process of glycogen breakdown.

Hexokinase or Glucokinase

The enzyme that phosphorylates glucose to glucose-6-phosphate.

Phosphoglucomutase

The enzyme that converts glucose-6-phosphate to glucose-1-phosphate.

UDP-glucose phosphorylase

The enzyme that synthesizes UDP-glucose.

Glycogen synthase

The enzyme that adds glucose units to glycogen.

α-1,4 glycosidic bonds

The type of glycosidic bonds created by glycogen synthase.

Branching enzyme

The enzyme that creates glycogen branches.

Amylo-α(1→4)→α(1→6)-transglycosylase

The systematic name of the branching enzyme.

Tyrosine-194

The amino acid to which the first glucose residue of glycogen is attached in glycogenin.

Primer

The role of glycogenin in glycogen synthesis.

Cytosol

The location where glycogen synthesis occurs.

Activates

The effect of insulin on glycogen synthase.

Inhibit

The effect of glucagon and epinephrine on glycogen synthase.

Glycogen phosphorylase

The enzyme that catalyzes the first step of glycogenolysis.

Pyridoxal phosphate (PLP)

The coenzyme for glycogen phosphorylase.

Debranching enzyme

The enzyme that removes glycogen branches.

Transferase and Amylo-α(1→6)-glucosidase

The two activities of the debranching enzyme.

Oligo-α(1→4)→α(1→4)-glucantransferase

The alternative name for the transferase activity of the debranching enzyme.

Free glucose

The product released by amylo-α(1→6)-glucosidase.

Glucose-6-phosphatase

The enzyme that converts glucose-6-phosphate to glucose in the liver.

Endoplasmic reticulum

The location of glucose-6-phosphatase.

Glucagon

The hormone that activates glycogenolysis in the liver.

Epinephrine

The hormone that activates glycogenolysis in muscle.

cAMP

The second messenger that activates PKA in glycogenolysis.

Active

The state of glycogen phosphorylase when phosphorylated.

Inactive

The state of glycogen synthase when phosphorylated.

Dephosphorylation

The process through which insulin activates glycogen synthase.

Phosphorylation

The process through which epinephrine activates glycogen phosphorylase.

Enzymes

What the storage granules of glycogen also contain for its metabolism.

Glycogen granules

The location where glycogen is stored.

Cytoplasm

The part of a cell where glycogen granules are located.

Branched carbohydrate polymer

A type of carbohydrate structure characterized by multiple branching points.

UDP-glucose

The donor of glucose in glycogen synthesis.

Solubility

The property of glycogen that increases due to branching.

8-12 glucose residues

The number of glucose residues at which glycogen branches occur.

3 glucose residues

The number of glucose residues transferred by the debranching enzyme's transferase activity.

Amylo-α(1→6)-glucosidase

The enzyme that cleaves the α-1,6 bond at a glycogen branch point.

Von Gierke's disease

A glycogen storage disease caused by glucose-6-phosphatase deficiency.

Cori disease

A glycogen storage disease caused by debranching enzyme deficiency.

Andersen disease

A glycogen storage disease caused by branching enzyme deficiency.

Muscle glycogen phosphorylase

The enzyme whose deficiency causes McArdle disease.

Lysosomal α-1,4-glucosidase

The enzyme whose defect causes Pompe disease.

Glucose-1-phosphate

The form in which glycogen phosphorylase releases glucose.

Muscle

The only tissue that uses glucose released from glycogen breakdown for its own use.

Blood

The glucose levels maintained by glycogen breakdown in the liver.

Phosphorylase b

The form of glycogen phosphorylase in muscle activated by AMP.

Liver

The primary organ that glucagon acts on.

O-glycosidic bond

The type of bond that attaches the first glucose to glycogenin.

Tyrosine

The residue to which glucose is attached by glycogenin.

Homodimeric protein

The classification of glycogenin based on its structure.

4:6 transferase activity

The activity that catalyzes the creation of a glycogen branch.

Limit dextrin

The product of glycogen breakdown that stops four residues away from a branch.

Fed state

The physiological state in which glycogen synthesis is favored.

Fasting state

The physiological state in which glycogenolysis is favored.

PKA

The protein kinase activated by glucagon to phosphorylate and activate glycogen phosphorylase.

Kinase

An enzyme that transfers phosphate groups from high-energy donor molecules, like ATP, to specific substrates.

Phosphorylase

PKA phosphorylates and activates this kinase.

Protein phosphatase

Insulin activates glycogen synthase through this enzyme.

β-adrenergic

Epinephrine acts on the muscle through these receptors.

Catabolic

Glycogen degradation is an example of this pathway.

Anabolic

Glycogen synthesis is an example of this pathway.

Glycogen phosphorylase kinase

Protein kinase A phosphorylates and activates this enzyme.

AMP

The allosteric activator of glycogen phosphorylase in muscle.

ATP

High levels of this inhibit glycogen phosphorylase.

Glucose

High levels of this inhibit glycogen breakdown in the liver.

11

The branching enzyme requires a chain of at least this many glucose residues.

4

Glycogen phosphorylase stops cleaving when it is this many residues from a branch point.

Glucokinase

Glycogenolysis provides glucose quickly because it bypasses this enzyme.

Blood glucose

Glycogen serves as a buffer for this.

Blood glucose maintenance

The glucose equivalent produced from glycogenolysis in the liver is primarily used for this.

PKA; Ca2+

Phosphorylase kinase is activated by these.

α-1,4

Glycogen synthase catalyzes the formation of these linkages.

Phosphate

Glycogen phosphorylase catalyzes the addition of this group to glycogen.

6-8

The branching enzyme transfers a block of this many glucose units.

Spherical particles

Glycogen granules appear as these under a microscope.

Glycogenin

Glycogen synthesis begins with the self-glucosylation of this.

UTP

The energy for glycogen synthesis comes from this.

Dephosphorylated

Glycogen synthase is active in this form.

Neural

The muscle form of glycogen phosphorylase is regulated by these signals.

Storage; Mobilization

Glycogen metabolism is tightly regulated to balance these.