Glycogen

What is glycogen?

A branched polymer of glucose used for energy storage.

What is the main function of glycogen?

Store glucose for later use.

Where is glycogen stored in cells?

Cytosol as glycogen granules.

Which tissues commonly store glycogen?

Liver and skeletal muscle.

What is glycogenolysis?

Breakdown of glycogen.

What is glycogenesis?

Synthesis of glycogen.

Are glycogen synthesis and breakdown the same pathway reversed?

No; they use separate enzymes and reactions.

Which metabolite connects glycogen to other pathways?

Glucose-6-phosphate.

What three pathways connect to glucose-6-phosphate?

Glycolysis, glycogen metabolism, and pentose phosphate pathway.

What type of molecule makes up glycogen?

Glucose.

What bonds form the straight chains of glycogen?

Alpha-1,4 glycosidic bonds.

What bonds form glycogen branch points?

Alpha-1,6 glycosidic bonds.

What is the reducing end of glycogen associated with?

Glycogenin.

What are nonreducing ends?

Ends where glucose units are added or removed.

Why is glycogen highly branched?

Branches allow rapid glucose addition and removal.

How many glucose units are associated with glycogen branch points?

Approximately 10%.

What is the main enzyme for glycogen breakdown?

Glycogen phosphorylase.

Where does glycogen phosphorylase remove glucose?

Nonreducing ends.

What bonds does glycogen phosphorylase cleave?

Alpha-1,4 glycosidic bonds.

What does glycogen phosphorylase use to break bonds?

Inorganic phosphate (Pi).

What is the main product of glycogen phosphorylase?

Glucose-1-phosphate.

Why does glycogen phosphorylase use Pi instead of water?

It conserves energy and avoids wasting ATP.

Can glycogen phosphorylase remove branch points?

No; debranching enzymes are required.

What is the product of alpha-1,6 branch hydrolysis?

Free glucose.

What is the difference between straight-chain and branch removal?

Straight chains yield glucose-1-P; branch hydrolysis yields glucose.

Which enzyme converts glucose-1-P to glucose-6-P?

Phosphoglucomutase.

Why is glucose-6-P important after glycogen breakdown?

It can enter glycolysis or other pathways.

What happens to glycogen-derived G6P in muscle?

It enters glycolysis to produce ATP.

What happens to glycogen-derived G6P in liver?

It can become free glucose for release into blood.

What other pathway can G6P enter?

Pentose phosphate pathway.

How many subunits does glycogen phosphorylase have?

Two identical subunits.

How many amino acids are in each glycogen phosphorylase subunit?

840.

What two types of regulatory sites does glycogen phosphorylase have?

Catalytic and allosteric sites.

What are the two main conformations of glycogen phosphorylase?

T (tense) and R (relaxed).

Which phosphorylase conformation is less active?

T state.

Which phosphorylase conformation is more active?

R state.

What are the two ways glycogen phosphorylase is regulated?

Covalent phosphorylation and allosteric regulation.

Which amino acid is phosphorylated in glycogen phosphorylase?

Serine 14 (Ser14).

What does Ser14 phosphorylation do?

Stabilizes the active R state.

What is glycogen phosphorylase a?

The phosphorylated form, generally more active.

What is glycogen phosphorylase b?

The dephosphorylated form, generally less active.

Which phosphorylase form favors the R state?

Phosphorylase a.

Which phosphorylase form favors the T state?

Phosphorylase b.

What molecule allosterically activates muscle phosphorylase?

AMP.

How does AMP affect phosphorylase?

Stabilizes the active R state.

Why does AMP activate glycogen breakdown?

It signals low cellular energy.

How does high glucose affect liver phosphorylase a?

Inhibits it by favoring the T state.

How does glucose-6-P affect phosphorylase?

Favors the inactive T state.

How does low ATP affect muscle phosphorylase b?

Favors activation through AMP signaling.

What enzyme phosphorylates glycogen phosphorylase?

Phosphorylase kinase.

What does phosphorylase kinase convert?

Phosphorylase b into phosphorylase a.

What is the result of phosphorylase kinase activation?

Increased glycogen breakdown.

How is phosphorylase kinase activated hormonally?

Through a cAMP and protein kinase A signaling cascade.

What type of receptor regulates hormonal glycogen breakdown?

G-protein-coupled receptor (GPCR).

What hormones stimulate glycogen breakdown?

Glucagon and epinephrine.

What enzyme produces cAMP from ATP?

Adenylyl cyclase.

What does cAMP activate?

Protein kinase A (PKA).

What does PKA activate during glycogen breakdown?

Phosphorylase kinase.

What is the hormone signaling sequence for glycogen breakdown?

Hormone → GPCR → cAMP → PKA → phosphorylase kinase → phosphorylase.

Why is hormonal signaling called a cascade?

Each step amplifies the signal.

What is the activated glucose donor for glycogen synthesis?

UDP-glucose.

What does UDP-glucose contain?

Glucose attached to uridine diphosphate.

What is the function of UDP-glucose?

Donates glucose to a growing glycogen chain.

Which enzyme synthesizes UDP-glucose?

UDP-glucose pyrophosphorylase.

What molecules form UDP-glucose?

Glucose-1-P + UTP.

What is released during UDP-glucose formation?

Pyrophosphate (PPi).

What happens to PPi after UDP-glucose formation?

It is hydrolyzed into 2 Pi.

Why is PPi hydrolysis important?

It helps drive UDP-glucose formation forward.

Which enzyme forms alpha-1,4 bonds during glycogen synthesis?

Glycogen synthase.

Where does glycogen synthase add glucose?

Nonreducing ends.

What does glycogen synthase release after glucose transfer?

UDP.

Which enzyme forms alpha-1,6 bonds during glycogen synthesis?

Branching enzyme.

What does the branching enzyme do?

Transfers a chain segment to form a new branch.

About how many glucose units does branching enzyme transfer?

Approximately 7 glucose units.

Why are new glycogen branches useful?

They create more nonreducing ends.

How much energy is needed to store one G6P-derived glucose in glycogen?

1 ATP equivalent.

Why is ATP needed for glycogen storage?

To regenerate UTP from UDP.

What reaction regenerates UTP?

UDP + ATP → UTP + ADP.

What enzyme regulates glycogen synthase through phosphorylation?

Protein kinase A (PKA).

What happens when glycogen synthase is phosphorylated?

Its activity decreases.

What happens when glycogen synthase is dephosphorylated?

Its activity increases.

What enzyme activates glycogen synthase by dephosphorylation?

Protein phosphatase.

How does phosphorylation affect glycogen phosphorylase?

Generally activates it.

How does phosphorylation affect glycogen synthase?

Generally inhibits it.

How does dephosphorylation affect glycogen metabolism?

Favors synthesis over breakdown.

Why are glycogen synthase and phosphorylase oppositely regulated?

To avoid simultaneous glycogen synthesis and breakdown.

Which enzyme builds glycogen?

Glycogen synthase.

Which enzyme breaks down glycogen?

Glycogen phosphorylase.

What is the key difference between glycogen synthesis and breakdown?

Synthesis uses UDP-glucose; breakdown uses Pi.

How is galactose converted into glucose-derived intermediates?

Through UDP-sugar intermediates.

Is glucose-1-P directly produced from galactose-1-P?

No; UDP-sugar conversions are required.