Lecture 25

Question 1

what is glycogen

Answer 1

polymer of glucose

“Starch for Animals & Fungi”

Question 2

Advantage of a polymer?

Answer 2

0.01 µM glycogen(insoluble) = 400 mM glucose(soluble)

→ Can store lots of glucose with little effect on osmotic pressure

Question 3

Glycogen granules

Answer 3

Mini-compartments containing 20-40 glycogen molecules, and enzymes for glycogen synthesis & degradation.

Question 4

Why glycogen but not fat?

Answer 4

Fat
• Synthesis & breakdown is slow
• Contributes to long-term energy homeostasis during starvation

Glycogen
• Synthesis & breakdown is rapid
• Helps maintain constant blood glucose level when fasting

Question 5

Glycogen – Where is it?

Answer 5

Glycogen is stored in liver and skeletal muscle; has multiple functions.

Question 6

glycogen to blood glucose

Answer 6

Glycogen –> Glucose-1-P –> Glucose-6-P –>Glucose –>Blood Glucose

Question 7

glycogen in liver

Answer 7

• glycogen = up to10% of wet weight
• Exportable glucose reservoir
• Exhausted in 12-24 h

Question 8

glycogen to energy

Answer 8

Glycogen
Glucose-1-P
Glucose-6-P
Energy

Question 9

glycogen in skeletal muscle

Answer 9

• glycogen - 1-2% of wet weight
• usable energy reservoir
• exhausted in ~1h

Question 10

Glycogen Structure

Answer 10

Glycogen uses two kinds of glycosidic bonds.

Question 11

no reducing ends

Answer 11

• Glycogen degradation and synthesis occurs

Question 12

branching occurs where

Answer 12

Branching occurs every 8-14 units at α-1,6 linkages.
plants: starch —> α-amylose (not branched)
amylopectin (branched every 24-30 units)

Question 13

Why is glycogen so highly branched?

Answer 13

~2,000 non-reducing ends per glycogen molecule available for degradation
= rapid release of glucose !

Question 14

Glycogen Breakdown aka glycogenolysis

Answer 14

look at power point

Question 15

Glycogen Breakdown Overview of 3 Key Enzymes

1. Glycogen phosphorylase

Answer 15

Cleaves (α-1,4) linkages from non-reducing ends until it
reaches four units from a branch point

Cleaves (α-1,4) linkages:
glycogenn + Pi —> glucose-1-P + glycogenn-1

Glycogen phosphorylase does NOT use H2O to cleave (hydrolyze) the glycosidic bond. It uses phosphate to generate a phosphorylated product, AKA: phosphorolysis

Why use phosphorolysis?
→ Pi is abundant & no ATP required
→ glucose-1 phosphate can’t exit cell

Question 16

Glycogen Breakdown Overview of 3 Key Enzymes

2. Debranching enzyme

Answer 16

• Transfers a block of three units to the non-reducing end of the chain
• Cleaves the last remaining (a-1,6)–linked glucose

3 units of one branch are transferred onto another

α-1,6 linkage hydrolyzed to yield unphosphorylated glucose

open for phosphorolysis

Question 17

Glycogen Breakdown Overview of 3 Key Enzymes

3. Phosphoglucomutase

Answer 17

Converts glucose-1-P into glucose-6-P

A phosphorylated serine on the enzyme participates in phosphate exchange.

Question 18

cofactor for glycogen phosphorylase

Answer 18

Pyridoxal-5- phosphate (PLP) is an essential
cofactor for glycogen phosphorylase.

PLP, a derivative of vitamin B6

PLP serves as a prosthetic group. The phosphate of PLP is involved in acid/base catalysis by glycogen phosphorylase.

PLP is covalently bound to glycogen phosphorylase via a Schiff base at Lys 680.

Question 19

glycogen phosphorylase inhibitor

Answer 19

1,5-Gluconolactone

Mimics structure of the intermediate

Question 20

[G6P] determines reaction direction

Answer 20

(fasted state)

Question 21

Glycogen Breakdown in summary

Answer 21

3 enzymes
2 products

Glucose units obtained from glycogen:
~ 92% glucose-1-phosphate
~ 8% glucose (Must be phosphorylated before use!)

Question 22

Glycogen Synthesis (AKA glycogenesis)

Answer 22

3 steps, 3 enzymes - Not just the opposite of breakdown!

Question 23

synthesis overview

Answer 23

1. Activation of glucose
   Glucose-1-P + UTP —> UDP-Glucose + PPi
   UDP-glucose-pyrophosphorylase
2. Formation of an α-1,4 bond
   UDP-Glucose + glycogenn —> glycogenn+1 + UDP
   glycogen synthase
3. Formation of an α-1,6 bond
   branching enzyme

Question 24

1. Activation of Glucose with UDP

Answer 24

Glucose-1-P + UTP —> UDP-Glucose + PPi
UDP-glucose-pyrophosphorylase
∆G ~ 0

PPi –(H2O)–> 2Pi
pyrophosphatase
ΔG = -19 kJ/mol

Question 25

Where does glucose-1-P come from?

Answer 25

→ phosphoglucomutase

Question 26

What other modifications are used to “activate” metabolites?

Answer 26

→ acetyl CoA ~ acetate | → ATP ~ Pi

Question 27

Glycogen Synthase

Answer 27

Forms α-1,4 bonds at non-reducing ends Result: Linear enlargement of existing glycogen molecule ...

Question 28

Glycogenin:

Answer 28

a primer for glycogen synthase auto-catalytic tyrosine glycosyl-transferase – The enzyme IS the substrate! After glycogenin “seeds” itself with glucose chains, glycogen synthase can bind & polymerize non-reducing ends.

Question 29

Branching Enyzme

Answer 29

α-1,4 linkage is cleaved to yield a 7-unit chain α-1,6 bond forms between 7-unit chain & another glycosyl unit ∆G of hydrolysis (kJ/mol) α-1,4 = -15.5 α-1,6 = - 7.1

Question 30

Glycogen Synthesis summary

Answer 30

2 branches per chain, ~13 units in a branch * Enough non-reducing ends to provide lots of glucose rapidly * Long chains provide enough glucose to sustain escape response…

Question 31

von gierke's disease

Answer 31

breakdown enzyme deficiency: glucose-6-phosphatase tissue: liver

Question 32

andersen's disease

Answer 32

synthesis enzyme deficiency: amylo-(1,4-1,6)-transg.. (branching enzyme) tissue: liver, probably all organs

Question 33

McArdle's disease

Answer 33

breakdown enzyme deficiency: glycogen phosphorylase tissue: muscle

Question 34

hers' disease

Answer 34

breakdown enzyme deficiency: glycogen phosphorylase tissue: liver

Question 35

Type I: Von Gierke’s disease

Answer 35

* Glucose-6-phosphatase deficiency * Most common GSD Clinical Signs • Glycogen accumulation in the liver and kidneys • Liver enlargement • Hypoglycemia Treatment & Prognosis • Feed cornstarch to maintain blood glucose • Avoid sugars that → glucose-6-P • Prognosis good if diagnosed before damage occurs RECALL: muscle lacks glucose 6- phosphatase ``` Glycogen Glucose-1-P Glucose-6-P (problem in this step going to next) Glucose blood glucose ```

Question 36

Type IV: Andersen’s disease - Branching enzyme deficiency

Answer 36

Very long unbranched chains have impaired solubility Symptoms & Prognosis • Long unbranched glycogen precipitates in liver and heart • Liver cirrhosis, muscle weakness • Most succumb in early childhood; one cause of miscarriage

Question 37

Type V: McArdle’s disease

Answer 37

MUSCLE glycogen phosphorylase deficiency ``` Symptoms & Prognosis • Glycogen accumulates in muscle • Poor acute exercise tolerance, cramping • Prognosis OK with exercise training & sucrose supplementation ``` Glycogen (problem in this step going to next) Glucose-1-P Glucose-6-P

Question 38

Type VI: Hers’ disease – | LIVER glycogen phosphorylase deficiency

Answer 38

Symptoms & Prognosis • Can be asymptomatic • Mild liver enlargement, hypoglycemia, low muscle tone, elevated ketone bodies (less fat more glucose) • Prognosis good; symptoms usually stabilize by adulthood ``` Glycogen (problem in this step going to next) Glucose-1-P Glucose-6-P Glucose blood glucose ```

Question 39

Two phosphorylase genes, two phosphorylase deficiencies:

Answer 39

Type V: McCardle’s Disease • Muscle glycogen phosphorylase deficiency • Exercise intolerance, muscle pain, fatigue, cramps • Associated with nearly 100 mutations in the muscle phosphorylase gene Type VI: Hers’ Disease • Liver glycogen phosphorylase deficiency • Enlarged liver, hypoglycemia, elevated ketone bodies • Associated with >17 mutations in the liver phosphorylase gene

Question 40

Key Points Glycogen Overview

Answer 40

– A polymer provides glucose and energy (G-1-P) – α-1,4 and α-1,6 linkages; non-reducing end – Different roles in liver and skeletal muscle

Question 41

Key Points Glycogen breakdown (Glycogenolysis)

Answer 41

– Phosphorylase (Uses phosphate for phosphorolysis; cleave α-1,4 ) – Debranching enzyme (Transferase and hydrolyze α-1,6) – Phosphoglucomutase (The fate of G-6-P)

Question 42

Key Points (Glycogenesis)

Answer 42

– Activating glucose (UDP-G pyrophosphorylase; requires UTP) – Glycogen Synthase (Extension of existing chain; – Branching enzyme (Cleave α-1,4 to form α-1,6) – 1 chain, two branches, ~13 units per branch (advantage = ?)

Question 43

Key Points Glycogen Storage diseases

Answer 43

– Deficiencies in glucose-6-P phosphatase, branching enzyme, | glycogen phosphorylase in liver and muscle