Lecture 18: Glycogen And Metabolism Flashcards
1
Q
Describe the structure of glycogen
A
- Osmotically inactive, easily mobilized
- 12 branched layers of glucose
- Linked together with alpha 1-4 bonds, branched points at alpha 1-6 bonds.
- Non reducing ends have a terminal hydroxyl at C4.
- Reducing end is connected to glycogenin
- Stored in liver and muscles as granules
2
Q
Describe glycogen metabolism
A
- Regulated storage and release of glucose
- Synthesis and degradation of glycogen involve different pathways
- Both pathways regulated independently
- Regulation
- Allosteric control
- Covalent modification through reversible phosphorylation of key enzymes
- Hormonal control
3
Q
Describe the products of glycogenolysis (the basic version)
A
- Glycogen broken down to release glucose-1phosphate
- Glycogen remnant remodeled to permit further degradation
- Glucose -1-phosphate converted to glucose-6 phosphate
- Glycolysis
- Free glucose for release into blood stream
- Pentose phosphate pathway – NADPH and ribose derivative
4
Q
What are the basic steps in glycogenolysis?
A
- Glycogen + Glycogen Phosphorylase =
- 5: Glucose 1-Phosphate
- G-1-P + Phosphoglucomutase =
- 5: G-6-P
- G-6-P can go one of several ways. Glucose 6-Phosphatase for conversion to Glucose
5
Q
What are the Four Key Enzymes of Glycogenolysis?
A
- one to degrade glycogen (chain shortening) (Glycogen Phosphorylase)
- two to remodel glycogen remnants (Glycogen Transferase and alpha1-6 Glucosidase)
- one to convert glycogen breakdown product suitable for further metabolism (?)
6
Q
Describe Glycogen Chain Shortening
A
- Glycogen phosphorylase (GP) (rate limiting enzyme) catalyzes the cleavage of glycogen.
- Chain shortening occurs at the non-reducing end of the polymer
- GP adds an orthophosphate and releases a glucose residue as glucose-1-phosphate
- Uses pyridoxal phosphate (vitamin B6) as a cofactor
- Phosphorolysis of glucose residues continues till the GP gets within 4 residues of the α-1,6 linkage of a branch point.
7
Q
Describe the branch transfer and the release of glucose
A
- Transferase transfers a block of 3 of the remaining 4 glucose to the non-reducing end of the main chain forming an α-1,4 bond.
- Debranching enzyme or α-1,6 glucosidase cleaves the α1,6 bond of the single remaining glucose residue to release the free glucose. Glucose phosphorylated by hexokinase
- Transferase and α-1,6 glucosidase convert branched glycogen into a linear structure for further action by GP
8
Q
Describe the phosphoglucomutase step
A
- Converts Gluc-1-phosphate to Gluc-6-phosphate
- A phosphoryl group is transferred from the enzyme to the substrate, and a different phosphoryl group is transferred back to restore the enzyme to its initial state.
9
Q
Describe the glucose 6-phosphatase step
A
- Gluc-6-phosphate cannot get out of the cell
- Only the liver has glucose 6-phosphatase
- Converts it to glucose
10
Q
How is glycogen phosphorylase regulated?
A
GP regulated by:
- several allosteric effectors (signal energy state of the cell)
- reversible phosphorylation (responsive to hormones)
11
Q
What are the two states of Glycogen Phosphorylase?
A
Exists in 2 forms:
- Active “a” form (R relaxed state) – in liver
- Inactive “b” form (T tense state) – in muscle
- Both isozymes exist in equilibrium between R and T
12
Q
What inhibits phosphorylase a in the liver?
A
The binding of glucose to Phosphorylase a shifts it into the T state.
13
Q
What can inhibit and activate phosphorylase b in the muscle?
A
- Phosphorylase B switches to it’s R state in the presence of AMP
- It is usually held stable in it’s T state by the presence of ATP and Glucose 6-Phosphorylase
14
Q
Describe the allosteric muscle regulation of Glucose Phosphorylase in Muscle
A
- Default “b” form or inactive form
- Activated by AMP
- Binds to active site and stabilizes conformation of b in the active R state
- During muscle contraction ATP converted to AMP by myosin and adenylate kinase signaling the GP to breakdown glycogen
- ATP and Gluc-6-phosphate are negative allosteric regulators
- Under normal physiological conditions GP inactive because of inhibitory effect of ATP and Gluc-6-phosphate
15
Q
What is the general role of phosphorylation?
A
- Phosphorylation of a single serine residue converts b to a.
- Conversion initiated by hormones
- Phosphorylation carried out by phosphorylase kinase (PK)
16
Q
In rough terms, describe what must occur for Phosphorylase to be activated
A
- Phosphorylase Kinase starts off in it’s inactive state
- PK is partially activated by the presence of Calcium ions.
- Partly activated PK is fully activated by the presence of PKA Hormones.
- Fully activated phosphorylase kinase uses ATP to transfer phosphorylase from phosphorylase B to phosphorylase A
17
Q
Describe the hormonal control of GP
A
- Low blood sugar levels release glucagon (acts on liver)
- Muscle activity releases epinephrine (effects are on muscle)
- Effects of both hormones mediated via G protein coupled receptors (GPCR)
- Epinephrine and glucagon signal glycogen breakdown
18
Q
Describe the GPCR-mediated pathway
A
Glucagon and epinephrine Work through GPCR - do not cross membrane like steroid hormones
- Glucagon and epinephrine signal glycogen breakdown.
19
Q
What biochemical reaction is generally regarded as the “off switch” for glycogen breakdown.
A
- Shuts down when secretion of hormone stops
- PK and GP are dephosphorylated and inactivated
- Breakdown of glycogen stops
- Synthesis of glycogen promoted
20
Q
Compare Phosphorylase in the Muscle and Liver
A
- Liver and muscle forms of GP are products of separate genes. Called isozymes.
- Differ in their sensitivities to regulatory molecules.
- Both forms activated by phosphorylation by PK and inhibited by ATP and G6P.
- Muscle form is allosterically activated by AMP (measure of low energy status of cell)
- Liver enzyme is inactivated by free glucose (indicator of blood sugar levels). Unaffected by AMP.
- Mutation in liver GP causes Hers disease
- Mutation in muscle GP causes McArdle syndrome
21
Q
What do glucagon and epinephrine do in a fasting state, and in a period of exercise?
A
- In the liver, hormones signal glycogen to break down into glucose, and for lactose to be converted back into glucose
- In the muscle cells, Both glucose and glycogen are broken down into G-6-P.
22
Q
How is glucose built back into glycogen?
A
- Glucose
- Add Hexokinase and (ATP -> ADP) =
- Glucose 6-P
- 5: Add Phosphoglucomutase =
- Glucose 1-P
- 5 Add UDP-Glucose pyrophosphorylase (UTP->2Pi) =
- UDP-Glucose
- 5: Add Glycogen Synthase (Rate limiting step) (Primer->UDP) =
- (Doesn’t really have a name. Non-branched glycogen)
- 5 Add Glucosyl 4:6 transferase
- Glycogen
23
Q
What is the first of 3 key events of glycogenesis?
A
- Trapping and Activation of Glucose:
- Glucokinase/hexokinase in cytosol of hepatocytes and muscle cells catalyze phosphorylation of glucose to glucose-6-phosphate
- This traps the glucose in these cells
- Phosphoglucomutase then reversibly isomerizes glucose-6phosphate to glucose-1-phosphate
- Uridine diphosphate(UDP)-glucose pyrophosphorylase then transfers the glucose-1-phosphate to uridine triphosphate (UTP) which generates UDP-glucose (active form of glucose)
- Breakdown of pyrophosphate to Pi generates energy and drives the reaction forward
24
Q
What is the second of the three key steps involved in glycogen synthesis?
A
- Elongation of a glycogen primer:
- Preexisting short glycogen polymer serves as a primer to which glucose units are added
- Primer formation done by Glycogenin, a Mn requiring protein
- Glycogen synthase (rate limiting enzyme). Catalyzes the transfer of glucose from UDPglucose onto the non-reducing end of glycogen chain. Forms α-1,4 glycosidic bonds between glucose molecules
25
What is the third of the three key steps of glycogen synthesis?
- Branching of glycogen chains:
- When glycogen chain reaches 11 residues, a fragment of the chain (about 7 residues long) is broken off at an α -1, 4 link and reattached elsewhere through an α -1, 6 link by glucosyl (4:6) transferase.
- The new branch point must be at least 4 residues away from a preexisting branch
- Branching increases solubility of glycogen and increases number of terminal non-reducing ends.
- Increases rate at which glycogen can be synthesized and degraded
26
Describe the branching enzyme of glycogen synthesis
See Slide 42
- The branching enzyme removes an oligosaccharide of approximately seven residues from the nonreducing end and creates an internal alpha 1-6 linkage.
- Synthase extends non-reducing ends followed by more branching
27
Describe how glycogen synthesis is regulated
- Glycogen synthase: Key enzyme
- Exists in 2 forms, one form present in liver and second in muscle and other tissues
- Active non-phosphorylated “a” form
- Inactive phosphorylated “b” form
- Interconversion mediated by covalent modification (finetuning role)
- Phosphorylated by several kinases most importantly glycogen synthase kinase (GSK)
- GSK under the control of insulin and PKA
- Allosteric regulation – gluc-6-phosphate powerful activator, stabilizes R state
28
Provide an overview for the regulation of glycogen metabolism
- Regulation very important to maintain blood sugar and provide energy to muscles
- Pathways of synthesis and degradation are independent
- Allows for independent regulation
- Two key enzymes: glycogen phosphorylase and glycogen synthase, the rate limiting steps of degradation and synthesis, respectively.
- Both enzymes are regulated by reversible phosphorylation, but effects are in opposite directions
- Big one: Glucagon and Epinephrine are able to control PKA synthesis via PKA
29
Name the cascades that stimulate glycogen breakdown and the cascades that prevent glycogen synthesis.
Stimulate Glycogen Breakdown:
Activated cAMP activates Protein Kinase A, which activates Phosphorylase Kinase which activates Phosphorylase into it's A form.
Prevents Glycogen Synthesis:
Glycogen Synthase Kinase joins with activated Protein Kinase A, to DEactivate Glycogen Synthase into it's B state.
30
How does PPI inhibit and activate other enzymes after a full meal
PPI does pretty much three things.
1. Inhibits Glycogen Breakdown by inactivating Phosphorylase Kinase (directs it towards it's B state)
2. Inhibits Phosphorylase (directs it towards it's B state)
3. Stimulates Glycogen Synthesis by activating Glycogen Synthase
31
Describe in further detail the regulation of glycogen metabolism
- Glycogenesis is favored in fed state (when glucose and insulin high and cellular ATP high - signal of high energy)
- When glycogen synthesis favored, the depospho form glycogen synthase (active) and glycogen phosphorylase (inactive) are predominant
- Glycogenolysis favored in fasting state (when blood glucose levels are low and glucagon levels are high and cellular calcium and AMP are elevated (in exercising muscles)
- When glycogen degradation is favored phosphorylated forms of glycogen synthase (inactive) and glycogen phosphorylase (active) are predominant
32
Describe the mechanism of regulation by insulin
- High blood glucose
- Release of insulin by β cells of pancreas
- Binding of insulin to its receptor tyrosine kinase
- Activation of signaling cascade
- Four key proteins:
–GLUT 4 (glucose transporter)
–Protein kinase B (PKB)
–Protein phosphatase 1 (PP1)
–Glycogen synthase kinase 3 (GSK3)
33
Describe the outcome of insulin regulation
- Formation of the insulin receptor complex
- Activation of PKB
- Translocation of GLUT to membrane
- PKB phosphorylates PP1 (activate) and GSK3 (inactivate)
- Active PP1 dephosphorylates glycogen synthase (activate) and dephosphorylates glycogen phosphorylase (inactivate)
- Net result - activation of glycogen synthase and inactivation of glycogen phosphorylase
34
Describe Type 2 Diabetes
- Reduced sensitivity to insulin
- Called insulin resistance
- Mutations in insulin receptor and/or downstream signaling proteins
- Down-regulation in receptor levels triggered by elevated insulin (leading to endocytosis and degradation of the insulin receptor). Not replaced by translation
35
What acts as a glucose sensor in liver cells?
Glycogen Phosphorylase
- See Slides 54 & 55
36
Again, describe glycogen phosphorylase in both the liver, and in muscle
- Liver and muscle forms of GP are products of separate genes. Called isozymes.
- Differ in their sensitivities to regulatory molecules.
- Both forms activated by phosphorylation by PK and inhibited by ATP and G6P.
- Muscle form is allosterically activated by AMP (measure of low energy status of cell)
- Liver enzyme is inactivated by free glucose (indicator of blood sugar levels). Unaffected by AMP.
- Mutation in liver GP causes Hers disease
- Mutation in muscle GP causes McArdle syndrome
- In McArdle's Syndrome, there is limited ability to perform strenuous exercise because of painful muscle cramps. Can live relatively normal life apart from that.
- In Her's Syndrome, there is enlargement of the liver, moderate to severe hypoglycemia, ketosis, hyperuricemia, and hyperlipemia.
