Glycogen Synthesis

Explain the biological purpose of glycogen synthesis

Stores excess glucose as a rapidly mobilizable polymer

Explain why glycogen is an effective storage form of glucose

Highly branched → many non-reducing ends → rapid synthesis/breakdown

Explain why glycogen synthesis occurs in liver and muscle

Liver maintains blood glucose; muscle stores glucose for local ATP use

Explain why glucose must be activated before glycogen synthesis

Direct glucose addition is energetically unfavorable

Explain the reaction catalyzed by UDP-glucose pyrophosphorylase

Glucose-1-P + UTP → UDP-glucose + PPi

Explain how PPi hydrolysis drives UDP-glucose formation

PPi → 2 Pi makes reaction irreversible

Explain the role of glycogenin

Autoglycosylates Tyr residue to form primer

Explain why glycogen synthase cannot initiate glycogen synthesis

Requires pre-existing glucose chain

Explain the reaction catalyzed by glycogen synthase

Adds glucose from UDP-glucose to non-reducing end via α-1,4 linkage

Explain the catalytic strategy of glycogen synthase

SN2-like glycosyl transfer; UDP is leaving group

Explain why glycogen synthase forms only α-1,4 linkages

Active site geometry restricts bond type

Explain the reaction catalyzed by the branching enzyme

Transfers 6–7 glucose units to form α-1,6 branch

Explain why branching increases glycogen solubility

Reduces crystallization; increases surface area

Explain how branching enhances glycogen metabolism

More ends → faster synthesis and breakdown

Explain how glycogen synthase is regulated by phosphorylation

Active when dephosphorylated

Explain how insulin promotes glycogen synthesis

Activates PP1 → dephosphorylates glycogen synthase

Explain how glucose-6-phosphate regulates glycogen synthase

Allosteric activator

Explain why glycogen synthesis is inhibited during fasting

Low insulin; high glucagon/epinephrine

Explain how glycogen synthesis is coordinated with glycolysis

Both activated by insulin in fed state

Explain why glycogen synthesis does not occur simultaneously with glycogen breakdown

Reciprocal regulation prevents futile cycling

Summarize glycogen synthesis in one sentence

Insulin-stimulated pathway that converts excess glucose into a branched polymer for storag