Glycogen Metabolism Knowt Deck

Structure of glycogen

Glycogen is a branched glucose polysaccharide stored mainly in liver and muscle. It contains α(1→4) glycosidic bonds forming linear chains and α(1→6) glycosidic bonds forming branch points.

Role of branching in glycogen

Branching increases solubility and provides more non-reducing ends so glycogen can be rapidly synthesized and degraded.

Key enzymes of glycogen breakdown

Glycogen phosphorylase removes glucose as G1P from α(1→4) bonds; Debranching enzyme transfers 3 residues then hydrolyzes α(1→6) bonds; Phosphoglucomutase converts G1P ⇌ G6P.

Function of glycogen phosphorylase

Cleaves α(1→4) linkages using Pi to release G1P until reaching 3–4 residues from a branch point.

Function of debranching enzyme

Has two activities: transferase moves 3 residues to another chain; glucosidase hydrolyzes the α(1→6) bond releasing free glucose.

Role of phosphoglucomutase

Converts G1P to G6P (or reverse) through reversible isomerization.

Fate of G6P in muscle

Enters glycolysis to produce ATP; starting from G6P gives 3 net ATP instead of 2.

Fate of G6P in liver

Is dephosphorylated to free glucose so it can leave the liver and regulate blood sugar.

Regulation of glycogen phosphorylase

Phosphorylated form = active (R state); dephosphorylated = inactive (T state); activated by glucagon in liver and epinephrine in liver + muscle; allosteric regulators include G6P.

Activation by glucagon

Glucagon (low blood sugar) activates glycogen phosphorylase in the liver via phosphorylation.

Activation by epinephrine

Epinephrine activates glycogen phosphorylase in both liver and muscle during fight-or-flight.

Key enzymes of glycogen synthesis

Phosphoglucomutase (G6P⇌G1P)

Role of UDP-glucose in glycogen synthesis

Activated glucose donor used by glycogen synthase; formed from G1P + UTP → UDP-glucose + 2Pi.

Function of glycogen synthase

Regulated enzyme that adds glucose from UDP-glucose to non-reducing ends

Function of branching enzyme

Transfers 6–7 residues to form new α(1→6) branch points.

Regulation of glycogen synthase

Dephosphorylated = ON; phosphorylated = OFF; insulin activates it during high blood sugar.

Role of insulin in glycogen metabolism

Insulin (high blood sugar) activates glycogen synthase to store glucose as glycogen.

Role of glucagon in glycogen metabolism

Glucagon (low blood sugar) activates glycogen phosphorylase to release glucose from glycogen.

Role of epinephrine in glycogen metabolism

Epinephrine promotes rapid glycogen breakdown in muscle and liver for immediate energy.

Glycogenin priming

Glycogenin initiates glycogen synthesis by attaching the first glucose to its Tyr194 residue and adding 6–7 more to form a primer chain for glycogen synthase.

Function of glycogenin

Acts as both an enzyme and a scaffold; creates the initial short glucose chain required before glycogen synthase can act.

Why glycogen needs glycogenin

Glycogen synthase cannot start a chain from scratch; it needs a pre-existing oligosaccharide

Reciprocal regulation of glycogen metabolism

When glycogen synthesis is ON (insulin)