Glycogen Metabolism Lecture

Practice flashcards covering the structural characteristics, enzymatic pathways, and regulatory mechanisms of glycogen synthesis and degradation.

Glycogen

A branched polymer of glucose involving $\alpha$-(1-4) linkages with branch points occurring every 8 to 12 residues via $\alpha$-(1-6) linkages.

Glycogenolysis

The biochemical pathway responsible for the breakdown of glycogen into glucose units, primarily occurring in the liver and skeletal muscle.

Glycogen Phosphorylase

A homodimer enzyme containing a PLP prosthetic group that catalyzes the phosphorolysis of glycogen to produce $\alpha$-D-Glucose-1-phosphate.

Phosphorolysis

The cleavage of a chemical bond coupled to a group transfer to an oxygen atom of a phosphate group ($P_i$), which allows glucose release without the consumption of ATP.

Phosphoglucomutase

A reversible isomerase that moves a phosphate group between the C1 and C6 positions ($\text{Glucose-1-phosphate} \rightleftharpoons \text{Glucose-6-phosphate}$).

Transferase (Debranching)

One of the two activities of the debranching enzyme that moves three of the last four glucose residues from a branch point to the end of a glycogen strand.

$\alpha$-1-6 glucosidase

An enzyme activity that cleaves the final glucose residue at an $\alpha$-1-6 branch point, releasing a free (unphosphorylated) glucose molecule.

Glucose-6-Phosphatase

A liver-specific enzyme located in the endoplasmic reticulum lumen that hydrolyzes glucose-6-phosphate to produce free glucose for release into the bloodstream.

Glycogenesis

The process of glycogen synthesis from glucose-6-phosphate, which requires several ATP equivalents for activation.

Glycogenin

A protein primer that autoglycosylates to form a core of at least 4 glucose residues, serving as the starting point for glycogen synthase.

UDP-glucose pyrophosphorylase

An enzyme that catalyzes the formation of UDP-glucose from UTP and glucose-1-phosphate, driven by the rapid hydrolysis of the resulting pyrophosphate ($PP_i$).

Glycogen synthase

The regulatory transferase that adds glucose from UDP-glucose to the C4 terminal hydroxyl group of a growing glycogen chain to form an $\alpha$-1,4 glycosidic linkage.

Branching enzyme

An enzyme that creates $\alpha$-1,6 branches by moving a block of approximately 7 glucose residues, increasing the number of non-reducing ends for faster synthesis and degradation.

Phosphorylase $a$

The active, phosphorylated form of glycogen phosphorylase, which is stimulated by hormones like epinephrine and glucagon.

Synthase $a$

The active, unphosphorylated form of glycogen synthase, which is stimulated by the hormone insulin.

Reciprocal Regulation

A metabolic control strategy where the same signal (e.g., phosphorylation) concurrently activates one pathway (degradation) while inactivating the opposing pathway (synthesis).

PLP (Pyridoxal phosphate)

The prosthetic group required by glycogen phosphorylase for its catalytic function.

Glucagon

A hormone released by the pancreas during fasting that triggers a cAMP-mediated cascade in the liver to increase glycogen degradation and blood glucose levels.

Epinephrine

A hormone released by the adrenal medulla during stress or exercise that stimulates glycogen breakdown in both liver and skeletal muscle.