Flashcards in Lecture 21: Glycogen Coordination & Fuel Metabolism Deck (63)
Can the brain use fats for fuel? Why/Why not?
Only very few FAs can cross the BB (essential ones like omega-3 and 6s), but even then they are too slow to be a steady supply for the brain
Are ketone bodies hydrophilic or hydrophobic?
Hydrophilic (very negative!)
Which are more oxidized: fats or proteins?
How much more energy does fat provide compared to glycogen?
6x the metabolic energy
Why do the high levels of blood glucose in diabetes cause damage to vascular and neurological systems?
Because glucose can be deprotonated and act as a Nu attacker on various macromolecules like collagen
What is the maintained physiological concentration of blood glucose?
5 mM = 100 mg/100ml
Why can't glucose be stored as is in cells?
Because it's very hydrophilic and individual molecules attain entropy so it would pull water into the cells which would then lyse
Describe glycogen's structure.
α-1,4 glycosidic bonds with branching α-1,6 linkages every 8-10 residues
What is the purpose of glycogen's branching?
To increase the number of non-reducing ends that are accessible for rapid glucose mobilization
How many glucose units does each glycogen granule include?
Up to 120,000
What % of skeletal muscle cell mass is comprised of glycogen?
0.5% on average
1-2% at most
What % of liver cell mass is comprised of glycogen?
4% on average
10% at most
Order of preferred heart metabolism?
1. Aerobic FA metabolism
2. Aerobic ketone bodies metabolism
3. Aerobic glucose bodies metabolism
4. Anaerobic glucose bodies metabolism
What are the 4 main purposes of glycogen?
1. Glucose homeostasis
2. Reduces osmotic pressure in the cell
3. Rapid energy release
4. Massive energy release thanks to many non-reducing ends
How does glycogen reduces osmotic pressure inside the cell?
By reducing the number of molecules of glucose floating around (osmotic pressure directly related to # of solutes)
What are the 2 molecules that starch is made of?
Describe the structure of amylose
Describe the structure of amylopectin
α-1,4 linkages and α-1,6 branching linkages every 24-30 residues
Why do potatoes go bad?
Because they are made of amylose which non-reducing ends are accessible to bacteria
What are 4 advantages of glycogen compared to TAGs?
1. Faster mobilization and metabolism
2. Both aerobic and anaerobic metabolisms are options
3. Ability to maintain glycemic levels
4. Cori cycle
What are 2 disadvantages of glycogen compared to TAGs?
1. Limited storage quantity because glycogen is hygroscopic: it binds 3-4 times its weight in water
2. Limited storage time
After how many hours of fasting are liver stores functionally depleted? Meaning they are not the main contributors to blood glucose levels
Describe the glycogenolysis pathway before the enzyme gets close to the branching. 4 steps
1. Glycogen phosphorylase phosphorylates C1 = G1P
2. Phosphoglucomutase (using Ser) = G6P
3b. IN LIVER: G6P transported to ER lumen by G6P transporter: G6P phosphatase = glucose + Pi
4b. Glucose transported to cytosol by glucose transporter/Pi transported to cytosol by Pi transporter
4c. Glucose exits cell via GLUT2
Why can't glycosidic linkages simply be hydrolyzed to mobilize glucose?
Because the energy in the bond would be released and lost as heat
Which are the only 2 organs that have the enzyme G6P phosphatase? Why?
Liver and kidneys because only organs able to break down glycogen for glucose homeostasis and perform gluconeogenesis
Describe the glycogenolysis pathway when the enzyme gets close to the branching. When does this happen exactly? 2 steps
Once it reaches the 4th glucose from the branching point:
1. Transferase activity of debranching enzyme: transfers 3 glucose from the branch and adds it to the non-reducing end of another chain
2. Glucosidase activity of debranching enzyme: hydrolyzes α-1,6 branching linkage
Which bonds have more energy: α-1,6 branching linkages or α-1,4 glycosidic linkages ?
α-1,4 glycosidic linkages
Which enzyme is faster: glycogen phosphorylase or debranching enzyme?
What is the rate-limiting step of glyceogenolysis?