Chapter 11 - glycogen synthesis and glycogenolysis Flashcards
What is glucose an energy source for?
Brain Cells lacking a mitochondria like RBCs Cells with few mitochondria Exercising muscles Sudden and strenuous activity
Where can blood glucose be obtained from?
Diet
Gluconeogenesis
Degradation of glycogen (glycogenolysis)
How can glucose be stored for later use?
as glycogen
Where does glycogen storage occur?
Liver and in skeletal muscle
TF: Most cells store small amounts of glycogen for own use?
True
Where does glycogen degradation and synthesis occur in the cell/
Cytosol
From what can glycogen be made?
Excess blood glucose
Recycling glucogenic metabolites (lactate, glycerol, amino acids)
What is the function of glycogen stores in the muscle?
Serve as a fuel reserve for synthesis of ATP
When are muscle glycogen stores replinished?
After depletion by strenuous exercise
What happens when glycogen stores are totally used up in the muscles but you still need more ATP?
Proteins are degraded to Amino Acids that can enter gluconeogenesis
What are the functions of glycogen stores in the liver?
Maintain blood glucose concentration for 10-18 hours at a time (nocturnal fasting)
What happens in the liver during the well fed state and during fasting?
When well fed, glycogen storage increases
When fasting they are depleted
How does glycogen exist in the cytoplasm?
As granules
What does the core complexes of glycogen consist of?
Glycogenin protein
What end is the non reducing end in glucose?
C4
How many non reducing ends are in glycogen?
Around 2000
What is the relationship between branch points and terminal residues for glycogen?
The more branch points, the more terminal residues are available at one time
What makes it possible for glycogen to release several glucose residues at once?
Highly branched structure of glycogen
What does glycogen produce in muscles?
G6P
What joins each alpha (OH below the ring) D glucose in glycogen?
alpha-1,4 glycosidic bonds
How often do branch points occur in glycogen and how are they joined?
Every 8-10 glucosyl residues
Joined by alpha-1,6 glycosidic bonds
What are the three steps of glycogen degradation?
Release of Glucose 1 phosphate from glycogen
Remodeling of glycogen for continued degradation
Conversion of glucose 1 phosphate into glucose 6 phosphate
What does each residue cleaved from glycogen react with?
Phosphate to give G1P
What kind of reaction cleaves glucose of from glycogen?
Phosphorolysis (glycogen phosphorylase)
What does glycogen phosphorylase do?
Cleaves the alpha 1,4 linkages in the non reducing ends of glycogen
Do you need ATP for the cleavage of glycogen?
No
How are branch points removed?
By 2 enzymatic activities in a single enzyme
Debranching enzyme that transfers a block of three residues to the non-reducing end of the chain
Debranching enzyme cleaves the single remaining alpha 1,6 linked glucose (becomes a free glucose)
What does phosphoglucomutase do?
Mutases transfer a side chain group throughout the same molecule
In this case the mutase is moving the phosphate from C1 to C6
What happens to G6P?
It is hydrolyzed by glucose 6 phosphotase (dephosphorylated) to release glucose
Where does Glucose and Pi go after being released?
They are shuttled back to the cytoplasm by transporters
What does liver glycogen phosphorylase do?
Breaks down glycogen into glucose 1 phosphate
What does muscle glycogen phosphorylase do?
Breaks down glycogen to glucose 1 phosphate in the muscle
What do muscle cells lack?
Glucose 6 phosphatase
Where does glucose 6 phosphate go in the muscle cells since it cannot be dephosphorylated there?
Enters the glycolytic pathway directly and this saves 1 ATP
What are the 3 fates of G6P?
Processing by the glycolytic pathway
Conversion into free glucose for release into the blood (only in liver)
Processing by the PPP
Where does glycogen synthesis occur in the cell?
Cytosol
TF: glycogen synthesis requires less energy than glycogen breakdown
False
What must blood glucose be in order to synthesize glycogen?
Phosphorylated
Attached to UDP
Added to glycogen
Why are there multiple steps for glycogen synthesis?
To allow for multiple points of regluation
What is the first step for glycogen synthesis?
Glucose 6P is converted to glucose 1 P by phosphoglucomutase (transfers phosphate group)
Where does the energy needed come from for glycogen synthesis?
Hydrolysis of UTP
What is the second step for glycogen synthesis? (Stage 1)
Glucose 1P reacts with UTP to produce Uridine diphosphate glucose and pyrophosphate
What does UDP glucose pyrophosphorylase do?
Catalyzes the interaction of G1P and UTP
What does purophohatase do?
hydrolyzes PPi to Pi and PI
What happens in stage 2 of glycogen synthesis?
UDPG is added to growing chain of glycogen (creates a new alpha 1,4 glycosidic bond that is catalyzed by glycogen synthase)
When is UDP released from UDP glucose?
When the new alpha 1,4 glycosidic bond is formed on the nonreducing (C4) end of the already existing glycogen
Where is the alpha 1,4 glycosidic bond formed in glycogen synthesis?
On the hydroxyl group of the anomeric carbon of the new glucose and the C4 non reducing end of the already existing glycogen
What regenerates UDP to UTP?
UDP and ATP are used to make UTP by nucleoside diphosphate kinase
What is the primer for glycogen?
Polysaccharide of glucose
How is the primer for glycogen synthase made?
Glycogenin enzyme that is a dimer of two identical subunits
What does each subunit of glycogenin generate?
a polysaccharide of 10-14 glucose residues in length
What extends the primer?
Glycogen synthase
What does a branching enzyme do?
Catalyses the reactions needed to introduce a branch point during the synthesis of glycogen
How does the branching enzyme introduce branch points during synthesis of glycogen?
Transferring a segment (7 residues long) from the end of a growing chain to the branch point where it catalyzes the formation of the alpha 2,6 glycosidic linkage
Where does the alpha 1,6 glycosidic linkage occur in glycogen?
At least 4 residues away from the previous branch point
What extends branch points?
Glycogen synthase
What is a major controlling factor in synthesis and breakdown of glycogen?
Glycogen phosphorylase
What is glycogen phosphorylase subjected to for control?
Allosteric control and covalent modification
What leads to activation of glycogen phosphorylase?
Phosphorylation of enzyme
What leads to inhibition of glycogen phosphorylase?
Dephosphorylation of enzyme
How is phosphorylation of glycogen phosphorylase controlled?
By the hormones Epinephrine and glucagon (activates breakdown of glycogen)
What does phosphorylation do to glycogen synthase?
Inactivate it
What does dephosphorylation of glycogen synthase cause?
Activation of the enzyme
What is phosphorylation of glycogen synthase controlled by?
Stimulated by hormone signals of glucagon and epinephrine via the enzyme cAMP-dependent protein kinase
What allosterically inhibits glycogen phosphorylase?
ATP and glucose 6 phosphate
What inhibits glycogen phosphorylase in the liver?
Glucose
What activates muscle glycogen phosphorylase?
Calcium ion release
What activates glycogen phosphorylase?
AMP increase
What allosterically activates glycogen synthase?
Elevated levels of glucose 6 phosphate
What does the insulin signaling pathway do?
Increases glucose import into the liver
Activates glycogen synthase (dephosphorylation) - increase glucose synthesis
Inactivates glycogen phosphorylase (dephosphorylation)
What does the glucagon-signaling pathway do?
Increases glucose export from liver
Activates glycogen phosphorylase (phosphorylation)- increases glycogen degredation
Deactivates glycogen sythase (phosphorylation)
