What is glycogenesis?
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Glycogenesis is the intracellular synthesis of glycogen from glucose.
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What is glycogenesis?
Glycogenesis is the intracellular synthesis of glycogen from glucose.
Where does glycogenesis occur?
In the cytosol of the liver and muscles.
What percentage of liver weight is glycogen approximately?
About 8–10% of liver weight is glycogen.
What percentage of muscle weight is glycogen approximately?
About 1–2% of muscle weight is glycogen.
How is UDP‑glucose formed?
UDP‑glucose is formed from glucose‑1‑phosphate + UTP catalyzed by UDP‑glucose pyrophosphorylase (releasing PPi).
What is the role of liver glycogen?
Liver glycogen serves as a glucose reserve to maintain blood glucose between meals.
When is liver glycogen typically depleted?
Liver glycogen is depleted after prolonged fasting (~18 hours).
What is the role of muscle glycogen?
Muscle glycogen is a fuel reserve used for ATP generation in muscle during contraction (for the muscle’s own use).
When is muscle glycogen typically depleted?
Muscle glycogen is depleted after prolonged exercise.
What is the key enzyme of glycogen synthesis and its function?
Glycogen synthase is the key enzyme; it transfers glucose units from UDP‑glucose to a glycogen primer, forming α‑1,4 glycosidic bonds.
What bond is formed when glycogen synthase adds glucose?
A C1 (from UDP‑glucose) → C4 linkage forming an α‑1,4‑glucosidic bond.
What is 'immature glycogen' as mentioned in the notes?
After elongation by glycogen synthase up to a minimum of ~11 glucose units, the product is termed immature glycogen.
What does the branching enzyme do?
The branching enzyme (Amylo‑1,4 → 1,6 transglucosidase) transfers a segment (minimum 6 glucose residues) of an elongated chain to create an α‑1,6 linkage, forming a new branch point.
Which bond does the branching enzyme create?
The branching enzyme creates α‑1,6‑glucosidic bonds at new branch points.
How is glycogen synthase regulated?
Glycogen synthase is regulated by covalent modification (hormones: insulin, glucagon) and allosteric regulation (notably G‑6‑P).
What is glycogenolysis?
Glycogenolysis is the intracellular breakdown of glycogen.
Where does glycogenolysis occur?
In the cytosol of the liver and muscles.
What is the rate‑limiting enzyme of glycogenolysis and its action?
Glycogen phosphorylase is the rate‑limiting enzyme; it cleaves α‑1,4 bonds in the presence of Pi to release glucose‑1‑phosphate.
How close does glycogen phosphorylase get to a branch point before stopping?
It stops 4 glucose residues away from an α‑1,6 branch point.
What is the role of α‑1,4 ↔ α‑1,4 glucan transferase in glycogenolysis?
The transferase moves a trisaccharide from one branch to another, exposing the α‑1,6 branching point for debranching.
What does the debranching enzyme (amylo‑1,6‑glucosidase) do?
The debranching enzyme hydrolyzes the α‑1,6 bond, releasing the last glucose at the branch as free glucose.
How does the liver produce free glucose from glycogen?
In the liver, G‑6‑phosphatase converts G‑6‑P to free glucose, which can then be released into the blood.
Why can't muscle release glucose into the blood from glycogen?
Muscle lacks G‑6‑phosphatase, so glycogen-derived G‑6‑P is retained and used for muscle ATP production rather than released as free glucose.
What is an advantage of phosphorolytic cleavage (by phosphorylase) over hydrolytic cleavage?
Phosphorolytic cleavage yields G‑6‑P which can enter glycolysis in muscle without needing hexokinase phosphorylation, thereby saving 1 ATP.
Can G‑6‑P diffuse out of muscle cells?
No — G‑6‑P is membrane‑impermeable, so it cannot diffuse out of muscle; free glucose can diffuse but muscle does not produce it due to lack of G‑6‑phosphatase.
Which hormones activate glycogen phosphorylase?
Norepinephrine, epinephrine, and glucagon activate glycogen phosphorylase (note: glucagon acts on the liver, not muscle).
How does insulin affect glycogen phosphorylase?
Insulin promotes dephosphorylation and inactivation of glycogen phosphorylase.
Name key allosteric regulators of glycogenolysis.
ATP and G‑6‑P inhibit glycogenolysis; AMP and Ca2+ (in muscle) activate it.
Do muscles have glucagon receptors?
No — muscles have no receptor for glucagon; glucagon acts primarily on the liver.
What enzyme converts fructose to fructose‑1‑phosphate?
Fructokinase converts fructose to fructose‑1‑phosphate.
Which enzyme cleaves fructose‑1‑phosphate and what are the products?
Aldolase B cleaves fructose‑1‑phosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde, which enter glycolysis.
How is glyceraldehyde processed to enter glycolysis?
Glyceraldehyde is phosphorylated to glyceraldehyde‑3‑phosphate, which enters glycolysis.
What is the polyol pathway for fructose synthesis?
The polyol pathway converts glucose → sorbitol (via aldose reductase, using NADPH) and sorbitol → fructose (via sorbitol dehydrogenase, using NAD+). This occurs in seminal vesicles for sperm nutrition.
Which cofactors are used by aldose reductase and sorbitol dehydrogenase?
Aldose reductase uses NADPH (→ NADP+); sorbitol dehydrogenase uses NAD+ (→ NADH).
What causes Von Gierke disease?
Von Gierke disease is caused by a deficiency of glucose‑6‑phosphatase in the liver.
Key clinical features of Von Gierke disease?
Features include severe fasting hypoglycemia, hyperlipidemia, and ketosis.
What is McArdle syndrome caused by?
McArdle syndrome is due to deficiency of muscle glycogen phosphorylase.
What is the main clinical consequence of McArdle syndrome?
Reduced glycogen breakdown in muscle leads to decreased ATP during exercise, causing exercise intolerance and muscle cramps.
What causes essential fructosuria?
Essential fructosuria is caused by a hereditary deficiency of fructokinase and is usually benign/asymptomatic.
What causes hereditary fructose intolerance (HFI)?
HFI is caused by deficiency of aldolase B in the liver. ,
What are the biochemical consequences of hereditary fructose intolerance?
In HFI, fructose‑1‑phosphate accumulates, leading to depletion of inorganic phosphate (Pi), inhibition of glycogen phosphorylase, and episodes of hypoglycemia that can progress to coma or death.
Why does accumulation of fructose‑1‑phosphate cause hypoglycemia?
Accumulated fructose‑1‑phosphate sequesters Pi, impairing glycogen phosphorylase and ATP production, which reduces glucose release and leads to hypoglycemia.
What is the immediate product released by glycogen phosphorylase during glycogen breakdown?
The immediate product is glucose‑1‑phosphate (G‑1‑P).
Which enzyme converts G‑1‑P to G‑6‑P?
Phosphoglucomutase converts G‑1‑P to G‑6‑P during glycogen metabolism.
Where in the body is the polyol pathway physiologically important for fructose production?
It is physiologically important in the seminal vesicles, producing fructose for sperm nutrition.
Which enzyme in glycolysis is effectively bypassed when muscle uses glycogen‑derived G‑6‑P?
Hexokinase is bypassed because glycogenolysis produces G‑6‑P directly, which enters glycolysis without phosphorylation.
Name two allosteric inhibitors of glycogen phosphorylase.
ATP and glucose‑6‑phosphate (G‑6‑P) are allosteric inhibitors.
Name two allosteric activators of glycogen phosphorylase in muscle.
AMP and calcium (Ca2+) activate glycogen phosphorylase in muscle.
State one reason why liver and muscle glycogen serve different physiological roles.
Liver glycogen maintains blood glucose for the whole body, whereas muscle glycogen fuels muscle ATP production locally because muscle lacks G‑6‑phosphatase. [pages 1 and 2]
Which enzyme is responsible for converting G‑6‑P to free glucose in the liver?
Glucose‑6‑phosphatase converts G‑6‑P to free glucose in the liver.
