Glycogenolysis: Phosphoglucomutase
Type of Enzyme?
- Active site contains___________ residue
- Reaction proceeds via intermediate, ___________
Isomerase
phosphoserine
Glu-1,6-bisP
Glycogenolysis (glycogen breakdown)
•Glycogen phosphorylase
–Cannot approach any closer than _________ residues from a branch point
•Debranching enzyme:
–Transferase: shifts block of 3 ______ units
–α-1,6-glucosidase: hydrolyzes branch point, generating ____
- Glucose converted to Glu6P via ________
- 4th activity: phosphoglucomutase
4
glucose
glucose
hexokinase
Glycogenesis: Other Enzymes Involved
- Branching enzyme catalyzes the formation of a-1,6 glycosidic linkages in glycogen by transferring a block of about 7 residues to a more interior location.
- New branchpoint must be at least ________ residues from preexisting one
- Branching increases the _______ of glycogen and facilitates both its synthesis and its degradation
- Addition of one glucose residue to glycogen uses ___________ ATP equivalents.
4
solubility
two
Glycogen synthesis requires the protein Glycogenin (G)
•Glycogenin
–acts as both an initial primer and an _______
–First, transfers glucose from ________ to hydroxyl of __________ on protein
–Next, extends chain by addition of _______ more residues
- ________(and branching enzyme) then takes over
- Glycogenin remains buried within the b-particle, attached to the single__________ end
enzyme
UDP-glucose;tyrosine
7
Glycogen synthase
reducing
Glycogenolysis: Glycogen Phosphorylase
•Carries out ___________ (cleavage of a bond using ______________)
–Energetically favorable; product already phosphorylated
–Glu1P, being charged, remains inside ________ cell and is used for energy; in liver, Glu1P converted to glucose and secreted
–Requires _______________PLP) as cofactor
phosphorolysis;orthophosphate, Pi
muscle
pyridoxal phosphate
Glycogenesis (Glycogen Synthesis): _________ is the substrate for glycogen synthase
*Note that subsequent hydrolysis of___________- drives the reaction forward
UDP-glucose
pyrophosphate (PPi)
Glycogen
- A readily-mobilizable storage form of __________
- Found in two places in body:
_____________
________________
•Present in granules called ____________
–Contain _________ that degrade, synthesize and regulate glycogen
glucose
–liver
-muscle
“b-particles”
enzymes
Hormonal Control of Glycogen Metabolism
- How would you expect low blood glucose level to affect glycogenesis? How are low blood glucose levels achieved in the liver and muscle?
- How would you expect high blood glucose level to affect glycogenesis?
Low blood glucose level, through the action of glucagon (liver) or epinephrine (liver, muscle) inhibits glycogenesis and stimulates glycogenolysis.
–High blood glucose level, through the action of insulin, stimulates glycogenesis and inhibits glycogenolysis in liver and muscle.
The Epinephrine (or Glucagon)-Triggered Signaling Cascade by which Phosphorylase is Activated
T-to-R Transition of Muscle (and Liver) GPPase Responsive to Cellular Conditions
- MUSCLE: GPPase “b” usually inactive, because levels of allosteric inhibitors (_________ and ________) usually high
- GPPase “b” is only active when level of __________ is high.
–Therefore, GPPase ________ is responsive to energy charge
–Here, AMP is “plugging the gap” until the hormone ________ can take over and convert “b” to “a”
- But GPPase _______ nearly always “on”—or active
- LIVER: default state is ________ form, which is allo-sterically inhibited by ________.
–Insensitive to AMP
ATP, Glu6P
AMP
“b”
epinephrine
“a”
“a”
glucose
Glycogen: A large,___________ polymer of glucose
- residues connected by _________ o-glycosidic bonds with a-1,6 branch points every ~10 residues
- > 20,000 non-reducing ends per b-particle
–Targets of glycogen phosphorylase-catalyzed removal of glucose
•Only one reducing end—it is attached to the protein ____________
branched
a-1,4; a-1,6
glycogenin
Hormonal Control of Glycogen Metabolism: After a Meal or at Rest
Regulation of Muscle Glycogen Phosphorylase Activity
•Highly regulated both allosterically (_______-form) and by reversible covalent modification (___________)
–Allosteric inhibitors: ________ and ________; activator: _______
- Phosphorylation is under ___________ control
- Exists in two interconvertible forms
–GPPase “a”: phosphorylated, usually active, in _______-state
–“b”: unphosphorylated, usually inactive, in ________-state
–GPPase “b” converted to “a” via phosphorylation of single __________ residue on each subunit
•Enzyme: ____________
“b”; phosphorylation
ATP, Glu6P
AMP
hormonal
R
T
serine
Phosphorylase kinase
Fates of Glucose 6-Phosphate derived from Glycogen
Phosphorylase b Kinase is Itself Regulated (in muscle)
•Partially activated by either _______or ________ binding but fully activated when both involved
–Ca2+ levels rise in muscle cell during muscle _________
- Phosphorylation site is on _______subunit
- Ca2+ binds to ________subunit (also known as ______)
phosphorylation; Ca2+
contraction
b
d ; calmodulin
Hormonal Regulation of Phosphoprotein Phosphatase-1 (PP1) in Liver
- High glucose levels activate glycogenesis
- Binding of glucose to GPPase causes an R-to-T shift, dissociation of GPPase, and _________ of PPI, allowing it to _______ GPPase and GS
- “glucose sensor”
activation;dephosphorylate
Overview of Glycogen Metabolism
•Breakdown involves 4 enzymatic activities
–Release of glucose 1-phosphate (from NR ends. Enzyme: _____________
–Remodeling to remove branchpoints. Enzyme (s):_________
–Conversion of Glu1P to Glu6P. Enzyme: __________
•Synthesis involves
–Glycogen synthase: uses ___________ as substrate
–Branching enzyme: creates a-1,6 branchpoints
•The two processes reciprocally regulated; different enzymes, different substrates/products
glycogen phosphorylase
transferase and glucosidase
phosphoglucomutase
UDP-glucose
Hormonal Regulation of Phosphoprotein Phosphatase-1 (PP1) in Muscle
- PP1 is tightly bound to a “glycogen-targeting protein,” GM, which forms a complex with the 3 enzymes GS, GPPase and phosphorylase kinase
- 1) PP1 becomes __________when GM is singly phosphorylated (by ISK), but 2) double-phosphorylation of GM (by PKA) leads to its _________
Activated; inhibition
In liver, insulin activates Glycogen Synthase by inactivating ___________
- Glycogen synthase is major regulatory enzyme of __________
- Exists in two states: active, “a” (unphosphorylated) and inactive, “b” (phosphorylated)
- Can be phosphorylated on various _____residues
- Action of GSK3 opposed by _______and ______
Glycogen Synthase Kinase-3
glycogenesis
serine
Phosphoprotein Phosphatase-1 ; insulin