Glycogen Metabolism Flashcards
Glucose is obtained from
Dietary Intake, Gluconeogensis, Glycogen
____ is the:
- required to maintain constant levels in the blood
- preferred energy source for the brain and required for the RBCs
- Essential for exercising muscles
- obtained from Diet, GNG, Glycogen stores
Glucose
This source of glucose (and glucose precursors) is often sporadic, dependent on the content of the diet, and not always a reliable source of blood glucose.
Diet/Dietary Intake
This is a mechanism for storing glucose in a quick mobilizable form.
Glycogen
___ can be slow in responding to a falling blood glucose level, but is used to provide sustained synthesis of glucose.
Gluconeogenesis
____ may be contained by virtually any cell in the human body
Glycogen
Glycogen stores are typically found in _____.
Skeletal muscle and the liver
This tissue typically uses glycogen as a source of energy.
Skeletal muscle
This tissue uses its glycogen stores in order to maintain blood glucose levels during early stages of fasting.
Liver
This tissue is composed of ~100 grams of glycogen. This makes up about 10% of the tissues weight within an adult.
Liver
This tissue is composed of ~400 grams of glycogen. This only makes up ~1-2% of the weight of this tissue.
(Skeletal) Muscle
Glycogen storage is associated w/significant amounts of ____ storage. It is about ___x the weight of glycogen.
Water; ~5
True/False: Your weight can vary due to the amount of glycogen you have stored.
True
Why are the branches important on the molecule?
- Because they increase the solubility of glycogen molecules.
- B/c they increasing the number of nonreducing ends that allow for faster synthesis and degradation.
In the absence of a dietary source of glucose, glycogen is degraded to glucose and rapidly released from _______.
liver and kidney glycogen
Muscle glycogen is degraded within exercising muscle in order to
provide the muscle with an important energy source
As glycogen stores are depleted, the synthesis of glucose through ____ takes over.
Gluconeogensis (GNG)
____ serves as the glucose source in the gap b/w the fall of blood glucose hours after a meal and the onset of gluconeogensis which needs time to kick in a few hours after.
Glycogen
____ molecule is a branched chain polysaccharide made exclusively of alpha-D-glucose
Glycogen
The primary glycosidic bond within the glycogen structure is a(n)
alpha(1,4) linkage
True/False: There are 10-40x10^4 glucose molecules within 1 glycogen granule.
False: There are 10-40x10^3 glucose molecules per 1 glycogen granule.
After an average of ___ glucosyl residues, there is a branch containing a(n) _____.
8-10; alpha(1,6) linkage
Large molecules of glycogen exist as ___ that are associated w/enzymes necessary for synthesis and degradation.
discrete cytoplasmic granules (beta-particles)
Steps for the Synthesis of Glycogen (Glycogenesis):
- Synthesis of Uridine disphosphate glucose
- Synthesis of a primer to initiate glycogen synthesis.
- Elongation of glycogen chains
- Formation of branches
Steps for the Degradation of Glycogen (Glycogenolysis):
- Shortening of chains
- Removal of chains
- Conversion of Glucose 1-P to Glucose 6-P
- Dephosphorylation of Glucose 6-P to Glucose.
What occurs during Step 1 of Glycogenesis?
The synthesis of uridine disphosphate glucose via Hexo/Glucokinase, Phosphoglucomutase, and UDP-glucose pyrophosphorylase. This is highly exergonic and ensures UDP-glucose is synthesized.
What occurs during Step 2 of Glycogenesis? Why does this happen?
The synthesis of a primer to initiate glycogen synthesis.. This occurs because glycogen synthase can NOT add UDP-glucose to a single glucose molecule. It can only ELONGATE existing glycogen molecules (primers).
What happens if no glycogen primers are available? (3 things)
- a protein glycogenin serves as a primer
- specific Tyr residue serves as attachment point for Glycogen synthase
- Glycogenin itself catalyzes this attachment reaction and the attachment of next few UDP-glucose molecules via alpha(1,4) glycosidic bonds
What is the enzyme responsible for the 3rd step of Glycogenesis?
Glycogen synthase
What does Glycogen synthase do in the 3rd step in Glycogenesis? (3 things)
- Elongates existing glycogen primers
- Transfers UDP-glucose to non-reducing end of the primer
- Forms alpha(1,4) glycosidic bond ONLY b/w C-1 of UDP-glucose and C-4 from the primer.
Which enzyme is the rate limiting enzyme within Glycogenesis?
Glycogen synthase
What enzyme removes a chain of 6-8 glucosyl residues from the end of the glycogen chain? What type of bond is it breaking?
The branching enzyme; breaks an alpha(1,4) bond
This enzyme attaches a chain of 6-8 glucosyl residues to a non-terminal glucosyl residue by an alpha(1,6) bond.
The branching enzyme
This enzyme functions as a 4:6 trasnferase.
The branching enzyme.
alpha(1,4) –>
alpha(1,6) –>
elongation
branching
Glycogenolysis is the
degradation of glycogen
Name the 4-steps of Glycogenolysis:
- Shortening of chains
- Removal of branches
- Conversion of glucose-1-P to glucose-6-P
- Conversion of glucose-6-P to glucose
True/False: The enzymes for Glycogenesis and Glycogenolysis is the same.
False; a separate set of enzymes are used for Glycogenolysis.
Where does the 4th step in Glycogenolysis occur?
Within the liver ONLY
The enzyme responsible for the first step in Glycogenolysis is ___
Glycogen phosphorylase
Tissue specific isoforms for the first step of Glycogenolysis:
liver, muscle, brain
What type of bonds does the Glycogen phosphorylase cleave in the 1st step of Glycogenolysis?
alpha(1,4) glycosidic bonds
What does Glycogen phosphorylase use to cleave an alpha(1,4) glycosidic bond?
What else occurs during this step?
An inorganic Pi
The inorganic phosphate simultaneously attaches itself to the glucose to yield glucose 1-phosphate
What does Glycogen phosphorylase requires as a coenzyme?
Where is this derived?
pyridoxal phosphate (PLP) derived from Vitamin B6
