Regulation of glycogen metabolism Flashcards
Where’s glycogen stored?
Primarily found in liver (10% of weight) and muscle (1-2% wt)
In what forms is glycogen stored?
Stored in granules:
- α-rosettes containing 20-40 β- particles
- β-particle contains 55k glu with 2k non-reducing ends
How fast is glycogen depleted?
Depleted after 12-24h fasting in liver and 1h of strenuous exercise in muscle
Why is glycogen is stored in far smaller amounts compared to fat?
It has high affinity for water-> bulky
Which tissues/organs store glycogen?
Astrocytes, heart, adipose tissue store glycogen
What does glycogenesis start with?
With UDP-glucose- precursor
It’s a sugar nucleotide
Where does glycogenesis take place?
Can take place in any cells; but predominant in liver
What has to be done to a precursor for glycogen chin to be made?
Why?
For glycogen chain to be synthesized, the precursor sugar has to be charged
Charged with nucleotide UDP
Charge is important for incorporation of glucose into the glycogen chain
What is needed to charge glycogen precursor?
For charging to happen, NDP-sugar pyrophosphorylase is needed. Pro means 2
How’s glycogen precursor charged?
NDP-sugar pyrophosphorylase uses glucose with phosphate on it and NTP (uracil triphosphate)
Condensation reaction occurs between a nucleoside triphosphate (NTP) and a sugar phosphate.
The negatively charged oxygen on the sugar phosphate serves as a nucleophile, attacking the aphosphate of the nucleoside triphosphate and displacing pyrophosphate-> 2 phosphate groups are removed- > UDP glucose is formed
How is the reaction of charging glycogen precursor pulled forward?
The reaction is pulled in the forward direction by the hydrolysis of PPi into individual inorganic phosphate groups
Net formula for charging glycerol precursors
Sugar phosphate + NTP-> NDP-sugar +2Pi
What are the enzymes that are required to make glycogen?
- NDP-glucose pyrophosphorylase charges glycogen precursor
- Glycogenin starts polymerization process of NDP-sugar and also acts as a primer
- Glycogen synthase further extends glycogen chain. BAsically continues the work of glycogenin
- Glycogen branching enzyme (amylo (1->4) to (1->6) transglycosylase): makes α1->6 bonds bonds
found at the branch points of glycogen
What’s glycogenin?
It’s a protein that is both a primer on which new chains are assembled and the enzyme that catalyzes polymerization
What are the steps of making glycogen
- UDP-glucose pyrophosphorylase adds a NTP group to a sugar, charging it and forming an NDP-sugar- activation
- Glycogenin acts as a primer and accepts glucose from UDP-Glc. Glucose residue is transferred from UDP-glucose to the hydroxyl group of Tyr of glycogenin glucosyltransferase activity
- The new chain is extended by the sequential addition of seven more glucose residues, each derived from UDP-glucose. This is catalyzed by chain-extending activity of glycogenin
- Glycogen synthase takes
over, which promotes the transfer of the glucose residue from UDP-glucose to a nonreducing end of a branched glycogen molecule . Forms 1-4 bonds
Glycogenin remains buried within the β particle, covalently attached to the single reducing end of the glycogen molecule
How is glucose residue attached to glycogenin
Through glycosyl transferase activity- the hydroxyl group of Tyr on UDP-glucose attack Tyr of glycogenin resulting in a glycosylated Tyr residue
Then by glycogen synthase
How’s 1->4 glycosidic link formed?
After Tyr residue of glycogenin was glycosylated, the C-1 of another UDP-glucose molecule is now attacked by the C-4 hydroxyl group of the terminal glucose, and this sequence repeats 6 times to form a glycogen molecule of six to eight glucose residues attached by (1->4) glycosidic linkages.
4th carbon of last glucose attacks first carbon of incoming glucose
What bond does glycogen synthase form?
1->4 glycosidic linkages
What are the 4 phases of glycogen synthesis?
Step 1: Formation of UDP-glucose (NDP-sugarpyrophosphorylase)
Step 2: Initial short chain synthesis (Glycogenin)
Step 3: Elongation (Glycogen synthase)
Step 4: Branching (Glycogen-branching enzyme)
What are the enzymes of glycogenolysis?
- Glycogen phosphorylase
- Glycogen debranching enzyme
- Phosphoglucomutase
What does Glycogen phosphorylase depend on?
Glycogen phosphorylase depends on Pyridoxal phosphate cofactor
What does glycogen phosphorylase do?
Glycogen phosphorylase catalyzes the reaction in which an (1->4) glycosidic linkage between two glucose residues at a non-reducing end of glycogen (extreme end) undergoes attack by inorganic phosphate (Pi), removing the terminal glucose residue as -D-glucose 1-phosphate. This phosphorolysis reaction is repetitive; the enzyme removes successive glucose residues until it reaches the fourth glucose unit from a branch point
Breaks 1->4 bond
What’s special about glucose phosphate produced during glycogen phosphorylase catalyzed reaction?
D-glucose 1-phosphate is produced which isn’t very useful- usually should be G6P
As it is phosphorylated it cannot leave the cell and enter the metabolic pathways as the phosphate is on the wrong carbon
How does glycogenolysis occur at a branch?
When glycogen phosphorylase arrives to a branch, each of each contains about 4 glucose units, it reaches its limit
After a debranching enzyme comes in
Has 2 functions:
- Glycosyl transferase function- shifts a block of 3 glucose units out of 4 that are left to one other branch- using 1->4 linkage> Creates new longer glycogen chain with only one glucose sticking out in 1->6 linkage
- Glucosidase function- debranching enzymes releases oen last left-over glucose as free glucose by the debranching enzyme’s (1->6) glucosidase activity. This glucose is not phosphorylated
How is D-glucose 1-phosphate produced by glucogenolysis made useful again?
Phosphoglucomutaseconverts it into G6P
How does glycogen synthase work?
Glycogen synthase requires an existing short chain created by glycogenin- continues work of glycogenin
Requires a UDP glucose; creates 1,4-glycosidic bond
4th carbon of last glucose attacks first carbon of incoming glucose
Which type of bond does glycogen synthase create?
1-> 4 glycosidic bond
What’s glycogen branching enzyme?
An enzyme that creates branches
Breaks a 1->4 bond between 4th and 5th carbon in glycogen core are bring is too a first glucose unit. creating a new non-reducing end and a 1-> 6 linkage
Glycogen branching enzyme can only work from _th glucose onwards only from a core or a branch
Glycogen branching enzyme can only work from 5th glucose onwards only from a core or a branch
What does glycogen branching enzyme increase?
Water solubility
Number of non-reducing ends->more ends, more glucose synthesizing/ glycogen lysing enzymes can attack glycogen to make more glucose
What’s the other name for glycogen branching enzyme?
Amylo (1-4) to (1-6) transglycosylase
What are the 4 step of gluconeogenesis?
vStep 1: Formation of UDP-glucose (NDP-sugarpyrophosphorylase)
vStep 2: Initial short chain synthesis (Glycogenin)
vStep 3: Elongation (Glycogen synthase)
vStep 4: Branching (Glycogen-branching enzyme)
What are the enzymes of glycogenolysis?
- Glycogen phosphorylase
- Glycogen debranching enzyme
- Phosphoglucomutase
What does Glycogen phosphorylase depend on?
Glycogen phosphorylase depends on Pyridoxal phosphate cofactor
What’s the function of Glycogen phosphorylase?
Glycogen phosphorylase catalyzes the reaction in which an (1->4) glycosidic linkage between two glucose residues at a nonreducing end of glycogen (extreme end) undergoes attack by inorganic phosphate (Pi), removing the terminal glucose residue as -D-glucose 1-phosphate. This phosphorolysis reaction is repetitive; the enzyme removes successive glucose residues until it reaches the fourth glucose unit from a branch point
What kind of bond does glycogen phosphorylase break?
1->4
What is the stop point of glycogen phosphorylase?
When it reaches a branch
What’s the downside of glucose phosphate produced by glycogen phosphorylase?
It produces glucose 1-phosphate which As it is phosphorylated it cannot leave the cell and enter the metabolic pathways as it has a phosphate attached to a wrong carbon
How does glycogenolysis occur at a branch?
glycogen phosphorylase stops working
Debranching enzyme comes in
Has 2 functions:
Glycosyl transferase function- shifts a block of 3 glucose units out of 4 that are left to one other branches- using 1->4 linkage> Creates new longer glycogen chain with only one glucose sticking out in 1->6 linkage
Glucosidase function- debranching enzymes releases glucose as free glucose left-over through debranching enzyme’s (1->6) glucosidase activity. This glucose is not phosphorylated
How’s the problem of glucose 1-phosphate resolved?
Phosphoglucomutase transfers phosphate group from carbon 1 to carbon number 6
Doesn’t add phosphate group, just transfers
Is Phosphoglucomutase activity reversible?
Its function is reversible- can convert glucose-1-phosphate back and forth to glucose-6-phosphate
What’s the fate of G6P in muscles and liver after being produced by glycolysis
In muscles this glucose-6-phsophate is ready to do work
In liver- liver is not supposed to use glucose. It is supposed to release it, so when glycogen lysis occurs in the liver, the phosphate group needs to be removed from the glucose- Glucose-6-phosphatase
What’s the function of glucose-6-phosphatase?
Removes a phosphate group from G6P so it can be released from liver into circulation
What are the 3 forms of glucose phosphate that are interconvertible thanks to glucose-6-phosphatase
Glucose 1-phosphate
Glucose 6-phosphate
Glucose 1.6-bisphosphate (extra PO3 group is attached to Ser residue of glucose-6-phosphatase)
Which GLUT transporter is expressed in liver and muscles
Liver -GLUT2
Muscles-GLUT4
Is glucose 6-phosphatase expressed in muscles?
Glucose 6-phosphatase is not expressed in muscles
How’s glucose 6-phosphate treated in the liver?
Glucose-6-phosphatase removes the phosphate group from glucose
from glucose 6- phosphate - it is converted to regular glucose and it is now able to pass through a glucose transporter - GLUT2
What is the last enzyme of glycogenolysis?
Glucose-6-phosphatase
Describe GLUT4 activity
GLUT4 that is present in the muscles s sensitive to insulin (insulin dependent). Insulin brings GLUT4 to the cell membrane- it is only available when insulin is present
GLUT4 doesn’t’ allow glucose to leave muscle cells into circulation. Glucose only enters the muscles
What happens to glucose in regulation of glycogen metabolism when it needs to be stored?
• When glucose needs to be stored it is converted to glucose 6-phosphate by hexokinase
as it can then enter glycogen pathway
Glucose 6-phosphate can always be mutated to __ by _ (irreversible/reversible)
Glucose 6-phosphate can always be mutated to glucose 1-phosphate by Phosphoglucomutase (reversible)
UDP-glucose can be turned into _ using __ and __
UDP-glucose can be turned into glycogen using glycogenin, glycogen synthase and glycogen branching enzyme
Glycogen can then be converted back to glucose 1-phosphate by __, __, and __
Glycogen can then be converted back to glucose 1-phosphate by glycogen phosphorylase, glycogen debranching enzyme, and phosphoglucomutase
glucose 1-phosphate then can be converted to __ by __
glucose 1-phosphate then can be converted to glucose by glucose 6-phosphatase
What is the function of Glycogen phosphorylase?
Glycogen phosphorylase converts glycogen to glucose 1-phosphate
How’s glycogen phosphorylase regulated?
Its activity depends on whether or not its phosphorylated
Phosphorylated= more active. Non-phosphorylated=non-functional
There’s a specific kinase that phosphorylates phosphorylase- phosphorylase b kinase- increases activity of Glycogen phosphorylase
B kinase is activated by various ligands: cAMP, PKA, glucagon (GPCR)
Protein phosphatases (PP1) remove phosphates from target proteins -> inactivates Glycogen phosphorylase PP1 is s
What are the ligands of phosphorylase b kinase?
cAMP, PKA, glucagon (GPCR), epinephrin
How’s glycogen phosphorylase affected in resting and contracting muscles?
In contracting muscles, where there’s a requirement of glucose for glycolysis Glycogen phosphorylase is active and phosphorylated
In resting muscles Glycogen phosphorylase is inactive and dephosphorylated - no glucose is required for glycolysis so it is stored
Describe hormonal regulation of glycogenolysis in mycotes
Muscles
Ligand- epinephrine
Receptor- GPCR
GPCR+ligand-> cAMP->PKA-> phosphorylase kinase-> glucose 6-phosphate is released from glycogen-> glycolysis
Describe hormonal regulation of glycogenolysis in Hepatocytes
Liver
Ligand- Glucagon
Receptor- GPCR
GPCR+ligand-> cAMP->PKA-> phosphorylase kinase-> glucose 6-phosphate is released from glycogen-> free glucose-> circulation
What are the hormones that control glycogenolysis?
Liver- glucagon
Muscles- epinephrine
What does function of glycogen synthase depend on?
Function of glycogen synthase depends on whether or not is phosphorylated
Phosphorylated= inactive
Dephosphorylated= active
How does insulin affect glycogen synthase?
Insulin binding to its receptor activates a tyrosine protein kinase in the receptor, which phosphorylates insulin receptor substrate-1 (IRS-1).
The phosphotyrosine in this protein is then bound by PI-3K, which converts PIP2 in the membrane to PIP3.
A protein kinase (PDK-1) that is activated when bound to PIP3 activates a second protein kinase (PKB), which phosphorylates GSK3 inactivating it.
The inactivation of GSK3 allows phosphoprotein phosphatase 1 (PP1) to dephosphorylate and thus activate glycogen synthase.
In this way, insulin stimulates glycogen synthesis
Which 2 substances activate glycogen synthase
Insulin and glucose/G-6-P stimulates dephosphorylation of glycogen synthase, activating it
Glucagon and epinephrine inhibits dephosphorylation of glyc
Which 2 substances inactivate glycogen synthase
Glucagon and epinephrine inhibits dephosphorylation of glycogen synthase, inactivating it
High blood glucose- events
High blood glucose results in insulin releases
In hepatocytes:
- Insulin inactivates GSK3, acting through a cascade and
activates a protein phosphatase, e.g PP1.
These two actions activate glycogen synthase.
PP1 also inactivates glycogen phosphorylase α and phosphorylase kinase by dephosphorylating both-> glycogen breakdown is stopped.
Glucose enters the hepatocyte through the high-capacity transporter GLUT2, always present in the plasma membrane-> increase in glucose blood concentration-> inreased expression of glycolytic enzymes (hexokinase II, PFK-1, pyruvate kinase). In cells where these enzymes are expressed, glycolysis increases.
Low blood glucose events
The drop in blood glucose triggers the release of glucagon which is a GPCR ligand. It acts through cAMP, which, acting through a cascade activates PKA.
PKA mediates all the effects of glucagon. It phosphorylates phosphorylase
kinase, activating it and leading to the activation of glycogen phosphorylase-> increases glycogen breakdown. PKA phosphorylates glycogen synthase, inactivating it and blocking glycogen synthesis.
It phosphorylates PFK-2/FBPase-2, leading to a drop in the concentration of the regulator fructose
2,6-bisphosphate, which has the effect of inactivating the glycolytic enzyme PFK-1 and activating the gluconeogenic
enzyme FBPase-1-> glycolysis is stopped. PKA also phosphorylates and
inactivates the glycolytic enzyme pyruvate kinase-> glycolysis is stopped
This release of glucose is possible only in liver and kidney, because other tissues lack __
This release of glucose is possible only in liver and kidney, because other tissues lack glucose 6-phosphatase
Difference in the regulation of carbohydrate metabolism in liver and muscle.
In liver, either glucagon (indicating low blood glucose) or epinephrine (signaling the need to fight or flee) has the effect of maximizing the output of glucose into the bloodstream. In muscle, epinephrine increases glycogen breakdown and glycolysis, which
together provide fuel to produce the ATP needed for muscle contraction.
Epinephrine and Glucagon GPCR signaling increases __ and __
Epinephrine and Glucagon GPCR signaling increases glycogenolysis and glucose release.
In case of excess glucose, liver can use it for __
In case of excess glucose, liver can use it for glycolysis
Describe pyruvate kinase specific for the liver
Pyruvate kinase L specific to liver is inactivated by phosphorylation. This doesn’t occur in the muscles-> a lot of glucose is created in the muscles from the pyruvate that is released
How physiology of skeletal muscle differs from
that of liver that i simportant in metabolic regulation?
- Muscles lack glucagon receptor
- Muscle isoform Pyruvate kinase is not phosphorylated by PKA -> doesn’t become inactivated
- Muscle does not produce F26BP
