Glycogen Metabolism and Regulation |
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Major concepts:
1. How is glycogen metabolism connected to glycolysis? How are glycogen synthesis and glycogen degradation similar, and how are they different?
2. How is a phosphorylase different from a phosphatase?
3. What are the advantages of regulating glycogen phosphorylase covalently, and how is it regulated covalently?
4. What is the main advantage of regulating glycogen phosphorylase allosterically, and how is it regulated allosterically?
5. How is glycogen synthase regulated?
6. How is blood glucose level related to regulation of glycogen metabolism? How are insulin, epinephrine, and glucagon related to regulation of glycogen metabolism?
Core knowledge:
1. What is the reaction catalyzed by glycogen phosphorylase, and what type of bond in glycogen is the substrate for glycogen phosphorylase?
2. What is the reaction catalyzed by phosphoglucomutase, and what type of enzyme is it?
3. What do the branching and debranching enzymes do?
4. What are the reactions catalyzed by UDP-glucose pyrophosphorylase and glycogen synthase? What part of glycogen is the substrate for glycogen synthase, and what type of bond does glycogen synthase make?
5. What enzyme covalently modifies glycogen phosphorylase, and what is the effect of the modification? You should use the terms R and T in your answer. How is the regulating enzyme itself regulated?
6. What enzyme covalently modifies glycogen synthase, and what is the effect of the modification?
7. What is the function of protein phosphatase 1 (PP1), and under what conditions is it most active?
Glycogen molecule:
Glycogen degradation = removing glucose from storage (catabolic pathway)
converts glycogen → glucose-6-P for glycolysis, other pathways
requires 3 enzymes, no energy
A. Glycogen phosphorylase adds phosphate to terminal (1→4) glycoside bond
1. Reaction: glycogenn+1 + HPO42− → glucose-1-P + glycogenn
phosphorolysis (like hydrolysis but with phosphate); essentially irreversible
2. Enzyme structure:
a. Large enzyme with a glycogen storage site
b. Requires pyridoxal phosphate as a prosthetic group
c. Active site is in a cleft
d. Two subunits, R and T conformations, cooperative binding of substrate
3. Regulated reaction of glycogen degradation
B. Phosphoglucomutase
1. Reaction: glucose-1-P ↔ glucose-6-P
2. Enzyme structure: phosphorylated serine in the active site participates in the reaction
glucose-1-P ↔ glucose-1,6-bis-P ↔ glucose-6-P
requires glucose-1,6-bisphosphate to activate enzyme initially
C. Debranching enzyme has two activities
1. Transferase: transfers 3 glucose residues of a terminal branch containing 4 residues
to a longer chain, creating an α-(1→4) bond
2. Hydrolase: hydrolyzes the α-(1→6) bond of the remaining residue in the branch
3. Function: providing substrate for glycogen phosphorylase
In liver, glucose-6-phosphatase in the ER (endoplasmic reticulum) removes phosphate so
glucose can be released into the blood.
A. Requires two ER membrane carriers for two forms of glucose + 1 for phosphate:
1. T1 transports glucose-6-P into the ER.
2. T2 transports glucose from ER to cytosol.
3. Phosphate carrier transports Pi from ER to cytosol.
B. GLUT2 transports glucose across the plasma membrane along concentration gradient.
C. Separation of enzymes helps with separating glycolysis from gluconeogenesis.
Glycogen synthesis = putting glucose into storage (anabolic pathway)
requires energy
uses a nucleoside sugar
distinguishes synthesis from degradation,
makes the intermediate more reactive
makes synthesis reactions irreversible
requires four enzymes + 1 glycogenin molecule/glycogen molecule
A. Phosphoglucomutase transfers P from C6 of glucose to C1
B. UDP-glucose pyrophosphorylase converts glucose-1-P to nucleoside-glucose
1. Reaction: glucose-1-P + UTP ↔ UDP-glucose + PPi
2. Effectively an irreversible reaction because PPi is converted → 2 Pi by pyrophosphatase
ΔG°′ for PPi = − 25 kJ/mol
3. The coupled reaction is glucose-1-P + UTP + H2O → UDP-glucose + 2 Pi
4. UDP-glucose pyrophosphorylase is named for the reverse reaction
UDP-glucose + PPi → UTP + glucose-1-P
C. Glycogen synthase adds glucose to the nonreducing end of glycogen molecule:
UDP-glucose + glycogenn → UDP + glycogenn+1
synthesizes an α–(1→4) glycoside bond and releases UDP
D. Branching enzyme = amylo (1 → 4) to (1 → 6) transglycosylase = glycosyl–(4→6)–transferase
1. Removes terminal 6-7 residues of strand that has at least 11 residues
2. Transfers them to C6-OH of more interior residue g branch ΔG°′ = 0
Advantages: branching increases solubility, ability to synthesize, and ability to degrade
E. Glycogenin = protein that initiates synthesis by adding glucose to Tyr on itself.
Uses UDP-glucose as substrate
Adds additional glucoses to the 1st until the chain is long enough to be a substrate for
glycogen synthase.
Regulation – First, a quick review of regulation of enzyme activity inside a cell:
A. Rationale:
1. Cells must respond to changing conditions and maintain homeostasis (steady state).
2. Reactions that are intrinsically very favorable MUST NOT reach equilibrium.
Product concentration would be excessive.
ATP hydrolysis would lose its value.
3. Ability to detect, and respond to, change relies on maintaining low concentrations
of 2nd messengers.
B. Methods
1. Control of synthesis (by control of transcription) and degradation = slow response
2. Allosteric regulation:
a. association with metabolites ↔ change in activity
b. association with regulatory proteins, such as CaM
3. Covalent modification: temporary addition of a functional group → change in activity
Big picture on glycogen metabolism:
High blood glucose concentration → glycogen synthesis (storing energy)
Low blood glucose concentration or sudden need → glycogen degradation
Regulated enzymes of glycogen metabolism: glycogen phosphorylase and glycogen synthase
Glycogen phosphorylase
A. Covalent regulation:
1. Phosphorylase kinase catalyzes activation:
glycogen phosphorylase b + 2 ATP → glycogen phosphorylase a–(phosphate) 2 + 2 ADP
Phosphate is transferred to a Ser side chain → stabilizing the R conformation.
2. Phosphoprotein phosphatase 1 (PP1) catalyzes inactivation:
glycogen phosphorylase a-(phosphate) 2 + 2 H2O → glycogen phosphorylase b + 2 Pi
Removal of phosphate stabilizes the T conformation, which has a protein loop that
blocks access to the active site of glycogen phosphorylase.
3. Phosphorylase kinase is regulated covalently and allosterically:
a. PKA catalyzes covalent activation:
phosphorylase kinase b + ATP → phosphorylase kinase a-phosphate + ADP
b. Ca2+ with calmodulin activates phosphorylase kinase allosterically.
4. PP1 activity is regulated indirectly.
PP1 is more active when glucose concentration is high, less active when it is low.
B. Allosteric regulation varies with cell type.
a. Skeletal muscle: glycogen phosphorylase b is allosterically activated by AMP and
allosterically inhibited by ATP and glucose-6-phosphate
b. Liver: glycogen phosphorylase a is inhibited by glucose.
Glycogen synthase
A. Covalent regulation in response to glucagon/epinephrine
1. PKA and GSK3 catalyze inactivation:
glycogen synthase a + 3 ATP → glycogen synthase b-(phosphate)3 + 3 ADP
2. PP1 catalyzes activation:
glycogen synthase b-(phosphate)3 + 3 H2O → glycogen synthase a + 3 Pi .
B. Covalent regulation in response to insulin
1. Insulin receptor activation → → inhibition of glycogen synthase kinase 3 (GSK3)
by phosphorylation
2. PP1 is then able to remove phosphates, activating glycogen synthase.