So i have biochemistry viva tomorrow and the topic is gylcogenolysis give best explanation and all notes with all question related to it

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

Loading Image

I now have everything I need to write a comprehensive viva preparation guide. Let me compile the full notes.

🧬 Glycogenolysis - Complete Viva Notes

What is Glycogenolysis?

Glycogenolysis is the breakdown (degradation) of stored glycogen to release glucose. It is NOT a reversal of glycogenesis - it uses a completely separate set of cytosolic enzymes. The primary product from α(1→4) bonds is glucose 1-phosphate, and free glucose is released from each α(1→6) branch point.

Occurs mainly in liver (to maintain blood glucose) and skeletal muscle (for energy during exercise)
Liver glycogen: responds to fasting/feeding
Muscle glycogen: responds to exercise/contraction

Step-by-Step Enzymatic Reactions

Step 1 - Glycogen Phosphorylase (Chain Shortening)

Cleaves α(1→4) glycosidic bonds from the nonreducing ends by phosphorolysis (using inorganic phosphate, Pi)
Product: Glucose 1-phosphate (not free glucose - no ATP wasted)
Stops when 4 glucosyl units remain at each branch point → structure called limit dextrin
Phosphorylase cannot act on branch points - it stops 4 residues away
Requires pyridoxal phosphate (PLP, vitamin B6) as a coenzyme

Reaction: Glycogen (n residues) + Pi → Glycogen (n-1 residues) + Glucose 1-phosphate

Step 2 - Debranching Enzyme (Branch Removal)

The debranching enzyme is a bifunctional single protein with two distinct enzymatic activities:

Activity 1 - 4:4 Transferase (oligo-α(1→4)→α(1→4)-glucantransferase):

Transfers the outer 3 of 4 glucosyl residues from the branch to a nonreducing end of another chain
Breaks an α(1→4) bond and makes a new α(1→4) bond (does NOT yield free glucose or glucose 1-P)

Activity 2 - α(1→6)-Glucosidase (amylo-α(1→6)-glucosidase):

Hydrolyzes the remaining 1 glucosyl residue attached at the α(1→6) branch point
Releases free glucose (not glucose 1-phosphate!)
This is the ONLY step that produces free glucose in glycogenolysis

Remember: ~8:1 ratio of glucose 1-P to free glucose (since branches occur every ~8-12 residues)

Step 3 - Phosphoglucomutase

Converts Glucose 1-phosphate → Glucose 6-phosphate
Requires glucose 1,6-bisphosphate as intermediate cofactor

Step 4 - Fate of Glucose 6-Phosphate

Tissue	What happens	Why
Liver	Glucose 6-phosphatase removes Pi → free glucose → enters blood	Liver has glucose 6-phosphatase
Muscle	Enters glycolysis directly as glucose 6-P	Muscle lacks glucose 6-phosphatase

Key exam point: Muscle glycogenolysis cannot raise blood glucose because muscle lacks glucose 6-phosphatase.

Regulation of Glycogenolysis

Regulation occurs at two levels: (1) covalent (hormonal) and (2) allosteric.

Diagram: Hormonal (Covalent) Regulation Cascade

Covalent regulation of glycogenolysis - Lippincott

A. Covalent (Hormonal) Regulation - The Cascade

Hormonal triggers: Glucagon (liver), Epinephrine (liver + muscle)

Cascade (memorize this sequence):

Glucagon / Epinephrine
    ↓  (binds GPCR)
Adenylyl cyclase activated → ATP → cAMP ↑
    ↓
PKA (Protein Kinase A) activated [inactive R₂C₂ → active 2C after cAMP binds R subunits]
    ↓  (phosphorylates)
Phosphorylase kinase b (inactive) → Phosphorylase kinase a (active)
    ↓  (phosphorylates)
Glycogen phosphorylase b (inactive) → Glycogen phosphorylase a (active)
    ↓
GLYCOGENOLYSIS ↑

Amplification cascade: A few hormone molecules → millions of glucose 1-P molecules. Each step amplifies the signal enormously.

PKA also simultaneously:

Phosphorylates glycogen synthase → inactivates it (stops glycogenesis)
Phosphorylates inhibitor-1 → inhibits protein phosphatase-1 (prevents inactivation of phosphorylase)

Insulin counteracts glycogenolysis:

Activates phosphodiesterase → breaks down cAMP
Activates protein phosphatase-1 → dephosphorylates and inactivates phosphorylase a → phosphorylase b (inactive)

B. Allosteric Regulation

Allosteric effector	Effect on phosphorylase	Tissue
AMP	Activates phosphorylase b (without phosphorylation)	Muscle only
ATP, Glucose 6-P, Glucose	Inhibits phosphorylase a	Muscle (ATP, G6P); Liver (glucose too)
Ca²⁺ (via calmodulin)	Activates phosphorylase kinase b (without phosphorylation)	Muscle + Liver

Calcium's Role in Muscle:

Nerve stimulation → Ca²⁺ released from sarcoplasmic reticulum
Ca²⁺ binds to calmodulin (δ subunit) of phosphorylase kinase
Activates phosphorylase kinase b without phosphorylation
This links muscle contraction directly to glycogenolysis

AMP's Role in Muscle:

During extreme anoxia/ATP depletion: AMP ↑
AMP directly activates glycogen phosphorylase b (bypasses the whole cascade)

C. Glycogenolysis in Liver Can Be cAMP-Independent

α₁-adrenergic receptor stimulation by norepinephrine (or epinephrine) → IP₃ pathway → Ca²⁺ mobilization from ER → Ca²⁺/calmodulin activates phosphorylase kinase
Also activated by vasopressin, oxytocin, and angiotensin II (via Ca²⁺ or PIP₂ pathway)
(Harper's Illustrated Biochemistry, 32nd Ed)

Liver vs. Muscle Glycogenolysis - Key Differences

Feature	Liver	Muscle
Purpose	Maintain blood glucose	Fuel for contraction
Hormone trigger	Glucagon (primary), epinephrine	Epinephrine, neural Ca²⁺
Glucose 6-phosphatase	Present → free glucose released	Absent → G6P stays in cell
Raises blood glucose?	YES	NO
AMP activates phosphorylase b?	No	Yes (myophosphorylase)
cAMP-independent pathway?	Yes (α₁-adrenergic/Ca²⁺)	No

Glycogen Storage Diseases (GSDs) - Viva Favorites!

The diagram below shows the key GSD types and where they block glycogen degradation:

Glycogen degradation and GSD diagram - Lippincott

Type	Name	Enzyme Deficient	Tissue	Key Features
I (Ia)	Von Gierke disease	Glucose 6-phosphatase	Liver + Kidney	Severe fasting hypoglycemia, hepatomegaly + renomegaly, lactic acidemia, hyperlipidemia, hyperuricemia, fatty liver
I (Ib)	Von Gierke Ib	G6P translocase	Liver + Kidney	Same as Ia + neutropenia + recurrent infections
II	Pompe disease	Lysosomal acid α-glucosidase (acid maltase)	ALL organs (heart, muscle, liver)	ONLY lysosomal GSD; massive cardiomegaly; hypotonia; infantile form often fatal by 2 years; enzyme replacement therapy (ERT) available
III	Cori disease (Forbes)	Debranching enzyme (4:4 transferase + α-1,6-glucosidase)	Liver + Muscle	Fasting hypoglycemia; abnormal glycogen (short outer chains, many branches)
IV	Andersen disease	Branching enzyme (enzyme of SYNTHESIS)	Liver	Liver cirrhosis - fatal in early childhood; glycogen has long unbranched chains
V	McArdle disease	Muscle phosphorylase (myophosphorylase)	Muscle	Muscle cramps + weakness on exercise; myoglobinuria; NO rise in blood lactate on exercise; liver enzyme normal; relatively benign
VI	Hers disease	Liver phosphorylase	Liver	Mild fasting hypoglycemia; hepatomegaly; benign
IX	Phosphorylase kinase deficiency	Phosphorylase kinase	Liver (X-linked) or Muscle	Similar to Hers disease

Most GSDs are autosomal recessive. Exceptions: Type IX (phosphorylase kinase - X-linked), Danon disease (X-linked).

Likely Viva Questions & Model Answers

Basic Mechanism

Q: What is the first enzyme of glycogenolysis? A: Glycogen phosphorylase. It cleaves α(1→4) glycosidic bonds by phosphorolysis, producing glucose 1-phosphate.

Q: Why is the product glucose 1-phosphate and not free glucose? A: Phosphorolysis uses inorganic phosphate (Pi) instead of water. This traps glucose in the cell as glucose 1-phosphate (charged, cannot cross membranes) and conserves the energy of the glycosidic bond. No ATP is consumed.

Q: What is a limit dextrin? A: The residual glycogen structure after phosphorylase has cleaved all available α(1→4) bonds, leaving 4 glucosyl residues on each chain before a branch point. Phosphorylase cannot degrade it further.

Q: What are the two activities of the debranching enzyme? A: (1) 4:4 transferase - transfers 3 of 4 residues from a branch to another chain; (2) amylo-α(1→6)-glucosidase - hydrolyzes the remaining α(1→6)-linked residue to yield free glucose.

Q: What is the ratio of glucose 1-phosphate to free glucose in glycogenolysis? A: Approximately 8:1 (or 9:1). The ratio reflects the average chain length between branch points.

Q: What coenzyme does glycogen phosphorylase require? A: Pyridoxal phosphate (PLP), a derivative of vitamin B6. It acts as an acid-base catalyst.

Regulation

Q: How does glucagon stimulate glycogenolysis? A: Glucagon binds GPCR → Gs protein activates adenylyl cyclase → ATP → cAMP → PKA activated → phosphorylates phosphorylase kinase b → active phosphorylase kinase a → phosphorylates glycogen phosphorylase b → active phosphorylase a → glycogenolysis.

Q: What is the difference between phosphorylase a and phosphorylase b? A: Phosphorylase a is the phosphorylated, active form. Phosphorylase b is the dephosphorylated, inactive form (though it can be activated allosterically by AMP in muscle).

Q: How does calcium activate glycogenolysis in muscle? A: During neural stimulation, Ca²⁺ is released from the sarcoplasmic reticulum. Ca²⁺ binds to calmodulin (the δ-subunit of phosphorylase kinase b), activating it without phosphorylation. Activated phosphorylase kinase then phosphorylates glycogen phosphorylase b → active a form.

Q: How does AMP activate glycogenolysis in muscle? A: Under extreme anoxia/ATP depletion, AMP accumulates and directly activates glycogen phosphorylase b allosterically without requiring phosphorylation, bypassing the entire hormonal cascade.

Q: How does insulin inhibit glycogenolysis? A: Insulin activates phosphodiesterase (degrades cAMP), activates protein phosphatase-1 (dephosphorylates and inactivates phosphorylase a), and increases glucose uptake → raises glucose 6-phosphate (allosteric inhibitor of phosphorylase a).

Q: Can glycogenolysis in liver be activated without cAMP? A: Yes. Epinephrine/norepinephrine acting via α₁-adrenergic receptors activates the IP₃/Ca²⁺ pathway → Ca²⁺/calmodulin activates phosphorylase kinase independent of cAMP. Also activated by vasopressin, oxytocin, and angiotensin II.

Q: Why does epinephrine cause glycogenolysis in both liver and muscle but glucagon only in liver? A: Because glucagon receptors are present only on hepatocytes, not on muscle cells. Epinephrine binds β-adrenergic receptors on both liver and muscle (via cAMP pathway).

Clinical / Applied

Q: Why can't muscle glycogenolysis raise blood glucose? A: Muscle lacks glucose 6-phosphatase. Therefore, glucose 6-phosphate cannot be converted to free glucose and must be used within the muscle via glycolysis.

Q: What is McArdle disease? What is its key biochemical feature? A: Type V GSD - deficiency of muscle phosphorylase (myophosphorylase). Key feature: failure of blood lactate to rise during exercise (because muscle cannot break down glycogen → no glucose → no glycolysis → no lactate).

Q: What is Von Gierke disease? A: Type I GSD - deficiency of glucose 6-phosphatase (Type Ia) or its translocase (Ib). Both glycogenolysis and gluconeogenesis are impaired (both produce glucose 6-P which cannot be released as free glucose). Features: severe fasting hypoglycemia, massive hepatomegaly, lactic acidemia, hyperlipidemia, hyperuricemia.

Q: Which is the only lysosomal GSD? A: Pompe disease (Type II GSD) - deficiency of lysosomal acid α-glucosidase (acid maltase). It is the only GSD involving lysosomes.

Q: What disease results from branching enzyme deficiency? A: Andersen disease (Type IV GSD) - a disease of glycogen SYNTHESIS (not breakdown). The abnormal unbranched glycogen deposited triggers liver cirrhosis and is fatal in early childhood.

Q: What is the signal amplification cascade and why is it important? A: The multi-step cascade (hormone → cAMP → PKA → phosphorylase kinase → phosphorylase) amplifies the signal at each step. A few hormone molecules binding their receptors ultimately activate many phosphorylase molecules, allowing a rapid, large release of glucose.

Quick Memorization Mnemonics

GSD types for liver GSDs (hypoglycemia): Type I (G6Pase), Type III (debrancher), Type VI (liver phosphorylase), Type IX (phosphorylase kinase), Type 0 (glycogen synthase)

Phosphorylase "b" = inactive, "a" = active (b is "bad"/inactive, a is "active") Glycogen synthase is opposite: "a" form = active (dephosphorylated), "b" form = inactive (phosphorylated)

Debranching enzyme activities:

4:4 transferase = moves 3 residues
α-1,6-glucosidase = releases free glucose

Blood lactate test distinguishes:

McArdle (no rise) vs. normal (lactate rises on exercise)

Summary Flowchart

GLYCOGEN
    |
    | ① Glycogen phosphorylase (cleaves α-1,4 bonds)
    | Coenzyme: PLP (Vit B6)
    ↓
LIMIT DEXTRIN (4 residues before branch)
    |
    | ② Debranching Enzyme: 4:4 transferase activity
    |    (transfers 3 residues to adjacent chain)
    |
    | ③ Debranching Enzyme: α-1,6-glucosidase activity
    |    (releases 1 free glucose from branch point)
    ↓
GLUCOSE 1-PHOSPHATE (major product, ~8/9 of total)
+ FREE GLUCOSE (~1/9 of total, from branch points)
    |
    | Phosphoglucomutase
    ↓
GLUCOSE 6-PHOSPHATE
    |              |
  LIVER         MUSCLE
    |              |
G6Pase         Glycolysis
    ↓
FREE GLUCOSE → Blood

Sources: Lippincott Illustrated Reviews: Biochemistry, 8th Ed; Harper's Illustrated Biochemistry, 32nd Ed; Basic Medical Biochemistry: A Clinical Approach, 6th Ed; Robbins & Kumar Basic Pathology

Good luck on your viva tomorrow! Know the enzyme sequence cold, be ready to explain regulation at both covalent and allosteric levels, and be prepared to link any GSD to its deficient enzyme, product accumulation, and clinical features.

This is a shared conversation. Sign in to Orris to start your own chat.