Lecture 29 - Glycogen Breakdown and Synthesis

Vocabulary flashcards covering the structural features, enzymatic machinery, and reciprocal hormonal and allosteric regulation of glycogen breakdown and synthesis.

Glycogen

A highly branched homopolymer of glucose found in all tissues, primarily stored in the liver and skeletal muscle.

Glycogen Storage Particle

A cellular storage complex containing a central glycogenin protein surrounded by $\alpha\text{-}1,4$-linked glucose chains, with $\alpha\text{-}1,6$ branches occurring every $10$ residues and nonreducing ends exposed on the surface.

$\alpha\text{-}1,4$-Glycosidic Bond

The primary covalent linkage connecting linear glucose residues in glycogen, cleaved by glycogen phosphorylase during breakdown and formed by glycogen synthase during synthesis.

$\alpha\text{-}1,6$-Glycosidic Bond

The covalent linkage responsible for forming branch points in glycogen, occurring approximately every $10$ residues.

Liver Glycogen

A glycogen reserve broken down to release free glucose into the bloodstream, maintaining blood glucose levels to nourish tissues such as the brain and red blood cells.

Muscle Glycogen

A glycogen store mobilized internally to generate glucose 6-phosphate for glycolysis, supplying ATP directly for muscle contraction.

Glycogen Phosphorylase

The key regulatory enzyme of glycogenolysis that catalyzes phosphorolysis of $\alpha\text{-}1,4$-glycosidic bonds from the nonreducing ends to produce glucose 1-phosphate.

Phosphorolysis

The cleavage of a covalent bond by the addition of inorganic orthophosphate ($\text{HPO}_4^{2-}$), yielding a phosphorylated product (glucose 1-phosphate) while conserving energy without expending ATP.

Glucosyltransferase

A remodeling enzyme that shifts a small oligosaccharide near a branch point to a nearby chain, exposing the remaining $\alpha\text{-}1,6$-linked residue to debranching enzyme.

$\alpha\text{-}1,6$-Glucosidase

A debranching enzyme that hydrolytically cleaves the $\alpha\text{-}1,6$ bond at a branch point, releasing one molecule of free glucose.

Glucose 1,6-Bisphosphate

The intermediate formed during the phosphoglucomutase reaction when a catalytic phosphoserine transfers its phosphoryl group to glucose 1-phosphate.

Phosphoglucomutase

A phosphoserine-dependent mutase that interconverts glucose 1-phosphate and glucose 6-phosphate via a glucose 1,6-bisphosphate intermediate.

Glucose 6-Phosphatase

A liver endoplasmic reticulum enzyme that hydrolyzes glucose 6-phosphate into free glucose and inorganic phosphate for release into the bloodstream.

Phosphorylase $a$

The phosphorylated, catalytically active form of glycogen phosphorylase.

Phosphorylase $b$

The unphosphorylated, generally less active form of glycogen phosphorylase.

Liver Glycogen Phosphorylase Isoform

An isozyme whose default state is phosphorylase $a$ in the active $R$ state (usually ON), which is allosterically inhibited by glucose shifting it to the $T$ state.

Muscle Glycogen Phosphorylase Isoform

An isozyme whose default state is phosphorylase $b$ in the inactive $T$ state (usually OFF), which is allosterically activated by high concentrations of AMP and inhibited by ATP and glucose 6-phosphate.

Phosphorylase Kinase

An enzyme activated by Protein Kinase A that phosphorylates phosphorylase $b$, converting it into the more active phosphorylase $a$.

Protein Phosphatase 1 (PP1)

An insulin-stimulated phosphatase that removes phosphoryl groups from phosphorylase kinase and glycogen phosphorylase to inactivate breakdown, while activating glycogen synthase.

UDP-Glucose (Uridine Diphosphate Glucose)

The activated glucosyl donor substrate required by glycogen synthase for glycogen polymer elongation.

UDP-Glucose Pyrophosphorylase

The enzyme that synthesizes UDP-glucose and pyrophosphate ($\text{PP}_i$) from glucose 1-phosphate and UTP.

Pyrophosphate Hydrolysis

The exergonic breakdown of pyrophosphate ($\text{PP}_i + H_2O \rightarrow 2\,P_i$) that drives the net synthesis of UDP-glucose toward completion.

Glycogen Synthase

The key regulatory enzyme of glycogen synthesis that transfers a glucosyl unit from UDP-glucose to the C-4 terminal hydroxyl group of an existing glycogen chain, creating an $\alpha\text{-}1,4$-glycosidic bond.

Glycogenin

A dimeric protein that initiates glycogen synthesis by autocatalytically synthesizing an oligosaccharide primer of $10\text{--}20$ glucose units per subunit.

Branching Enzyme

An enzyme that forms $\alpha\text{-}1,6$ branches by cleaving an $\alpha\text{-}1,4$ bond, transferring an oligosaccharide block of approximately $7$ glucose residues, and attaching it via an $\alpha\text{-}1,6$ linkage.

Glycogen Synthase $a$

The unphosphorylated, catalytically active state of glycogen synthase.

Glycogen Synthase $b$

The phosphorylated, inactive or less active state of glycogen synthase, which can be allosterically activated by glucose 6-phosphate.

Reciprocal Regulation

A regulatory mechanism where signaling events (such as phosphorylation triggered by glucagon or epinephrine) activate glycogen breakdown while simultaneously inactivating glycogen synthesis, preventing futile cycles.