Lecture 25: Glycogen Metabolism Flashcards
What is glycogen a polymer of?
Glucose residues
How are glucose residues combined to form glycogen?
Linked by:
* a(1->4) glycosidic bonds [primary, linear]
* a(1->6) glycosidic bonds [branch points]
What does glycogen maintain?
Blood glucose levels
How does glycogen maintain blood sugar level?
- When blood sugar is high, the body converts glucose into glycogen to reduce BSL
- When blood sugar is low, glycogen breaks down to release glucose to raise the BSL
Where is gluclose predominately stored as glycogen?
Liver and muscle cells
Glycogen Degradation (catabolism)
What is the enam that removes glucose residues from glycogen?
Glycogen Phosphorylase
Glycogen Degradation (catabolism)
What end can glycogen phosphorylase work at?
reducing or non-reducing
Non-reducing end
can only go linear
Glycogen Degradation (catabolism)
What does glycogen phosphorylase catalyze and what does it release?
- Catalizes: phosphorolytic cleavage of a(1->4)
- Releases: glucose-1-phosphate
cannot cut a(1->6)
Glycogen Degradation (catabolism)
What does glucose-1-phosphate have to undergo to be used for metabolism? What does it become?
Isomerization to become glucose-6-phosphate
this is the active form for glycolosis and pentose phosphate pathway
Glycogen Degeneration (catabolism)
What enzyme is used to isomerize the glucose-1-phospahte?
phosphoglucomutase
is reverable
Glycogen Degradation (catabolism)
What must happen to glucose-6-phosphate so it be released into blood?
it must be dephosphorylated
Glycogen Degradation (catabolism)
What is the phosphate added to glucose and what is that addition called?
Phosphate: orthophosphate
This reaction is called: phosphorolysis
Glycogen Degradation (catabolism)
What is phosphorolysis similar to and how?
Hydrolysis
The main difference is that hydrolysis adds water while phosphorolysis adds a phosphate
Branching Points in Glycogen
Where does glycogen phosphorylase stop on glycogen when coming to branches?
4 residues away from the branch point
called the limit branch
Branching Points in Glycogen
What enzyme is used to cleave the branching points?
Debranching enzyme
Branching Points in Glycogen
What are the two active sites of the Debranching Enzyme?
- Transferase
- a(1->6) glucosidase
Branching Points in Glycogen
What does the trasferase site do?
in debranching enzyme
transfers 3 glucose residues from the 4-residue limit to the end of another branch so the other branch is only a single residue
Branching Points in Glycogen
What does the a(1->6) glucosidase site do?
in debranching enzyme
catalyzes hydrolysis of that linkage to free that glucose
Regulation of Glycogen Degradation
What is glycogen phosphorylase regulated by?
- allosteric effectors that signal the energy state of the cell (if the cell has enough energy storage, ex: ATP, glucose, regulates how much is released based on what is already there)
- reversible phosphorylation
Regulation of Glycogen Degradation
Glycogen phosphorylase exists in what 2 interconvertible forms?
- Phosphorylase a (usually in Relaxed state, R, active)
- Phosphorylase b (usually in Tense state, T, inactive)
Regulation of Glycogen Degradation
Phosphorylase kinase does what for glycogen phosphorylase?
converts phosphorylase b to a by adding a phosphate (activates the glycogen phosphorylase)
Regulation of Glycogen Degradation
What are critical hormones that signal for glycogen breakdown?
- Glucagon
- Epinephrine
Regulation of Glycogen Degradation
What situlation would we need these hormones to be released?
When we need a sudden amount of energy released for use in something like running
Regulation of Glycogen Degradation
What is the general pathway from the hormones to glycogen phosphorylase?
- Activate the adenylase cyclase to produce cAMP
- cAMP activates Protein kinase A
- Protein Kinase A activeates phosphorylase kinase
- phosphorylase kinase adds phosphate to phosphylase b to create phosphorylase a (in glycogen phosphorylase)
Glycogen Synthesis
What is the basic definition of glycogen synthesis?
addition of glucose onto an existing glycogen
Glycogen Synthesis
What is the active form of glucose that is the precursor for glycogen synthesis?
Uridine diphosphate glucose (UDP-glucose)
bonding of UTP to glucose-1-phosphate
Glycogen Synthesis
What initiated glycogen synthesis?
Glycogenin (small protein/enam)
Glycogen Synthesis
What does gycogenin also do?
Acts as the primer
builds a short linear glucose polymer behind it
Glycogen Synthesis
What enam catalizes the elongation of the glycogen chains created by glycogenin?
Glycogen Synthase
Glycogen Synthesis
Which enam controls the rate of glycogen synthesis?
glycogen synthase or glycogenin
Glycogen synthase
Glycogen Synthesis
What enam controls the additions of branches onto glycogen chains?
Branching enam
a(1->6)
Glycogen Synthesis
Why is branching in glycogen chains useful?
It increases both the rates of synthesis and the degredation by increasing potential sites of action
Regulation of Glycogen Synthesis
What enam regulates glycogen synthesis?
Insulin
Regulation of Glycogen Synthesis
What does insulin do?
Produced in response to High blood glucose, triggers the phosphorylation and inactivation of the glycogen synthase kinase. Prevents the phosphorylation of glycogen synthase
Phosphorylation of glycogen synthase stops it from creating glycogen
Glycogen General
What does phosphorylation do to the enzymes for breakdown vs the enam for synthesis?
Breakdown: activates
Synthesis: Inactivates
