Glycogen Breakdown Flashcards
What is the composition of glycogen?
Alpha (1-4) linked D-glucose with alpha(1-6) linked branches every 8-14 residues.
What type of shape does a glycogen molecule take?
Spheroidal
In what types of cells is glycogen most prominent?
Muscle and liver
Where do the enzymes that catalyze glycogen synthesis and breakdown come from?
They are contained in glycogen.
From which ends of glycogen is glucose removed from?
Non-reducing ends
True or false: Glycogen contains many reducing ends, but only one non-reducing end.
False; glycogen contains many non-reducing ends, but only one reducing end.
What is glycogenolysis?
The breakdown of glycogen into glucose
The breakdown of glucose
glycogenolysis
What pathway converts glucose to pyruvate?
glycolysis
What pathway converts pyruvate to glucose?
gluconeogenesis
What is the end of a glycogen molecule with a free carbon #1 called? How many reducing ends does each molecule of glycogen have?
A reducing end; one
What does glycogen phosphorylase do?
Cleaves glycogen bonds by substituting a Pi to form glycogen and glucose-1-phosphate (G1P).
Which enzyme breaks glycogen branch bonds in the second step of glycogenolysis?
Glycogen debranching enzyme
Which enzyme converts G1P to G6P?
Phosphoglucomutase
What is the RDS of glycogenolysis?
The first step, glycogen phosphorylase
What controls glycogen phosphorylase?
1) Allosteric interactions (inhibitors=ATP, G6P, and glucose, activators=AMP)
2) Covalent modification (phosphorylation and dephosphorylation)
What is the phosphorylated form of glycogen phosphorylase?
Phosphorylase a
What is the dephosphorylated form of glycogen phosphorylase?
Phosphorylase b
Why can glycogen phosphorylase only get within 5 units from a branch?
The crevice on its surface that connects the glycogen binding site to the active site is too small.
True or false: Several glucose units can be cleaved by glycogen phosphorylase without unbinding/rebinding glycogen between catalytic cycles?
True
What is the cofactor for glycogen phosphorylase? What is it derived from?
pyridoxal-5-phosphate (PLP); from vitamin B6
True or false: Glycogen phosporylase undergoes conformational changes from T to R and vice versa.
True
What activator promotes a conformation change in glycogen phosphorylase from T to R?
AMP
True or false: The T site is buried in glycogen phosphorylase and has low substrate affinity.
True
What substrate binds at the T state of glycogen phosphorylase, inhibiting the shift from the T to the R state?
ATP
Which form of glycogen phosphorylase requires AMP for activity?
Phosphorylase b
Which form of glycogen phosphorylase is active without AMP?
Phosphorylase a
Which glycolysis enzyme works to remove branches?
Glycogen debranching enzyme
How does the glycogen debranching enzyme work?
It removes a trisaccharide from a branch to a non-reducing end, then hydrolyzes the remaining alpha (1-6) bond, yielding glucose and debranched glycogen.
True or false: The debranching enzyme phosphorylizes the alpha(1-6) glucose bond.
False; it hydrolyzes it
True or false: The glucose that is being removed by a debranching enzyme is G1P.
False; it is glucose
True or false: The debranching enzyme works much faster than glycogen phosphorylase.
False; glycogen phosphorylase is much faster.
Which branches of a glycogen molecule are degraded most quickly by a debranching enzyme?
The outer banches (in just a few seconds); the slower debranching occurs.
Why does maximum muscle contraction last only a few seconds?
In part, because the debranching of inner branches of glycogen is slow.
True or false: The debranching enzyme has only one active site.
False; it has two
Which enzyme converts G1P to G6P in glycogenolysis?
Phosphoglucomutase
True or false: Near equilibrium, the reaction of G1P to G1,6P to G6P by phosphoglucomutase is irreversible.
False
What can happen to the G6P from glycogenolysis?
It can enter glycolysis or enter the Hexose Monophosphate Pathway (Pentose Phosphate Pathway).
In the liver, what must happen to G6P for it to be put back into circulation from the cell?
It must be de-phosphorylated to glucose.
What enzyme transports G6P to the ER to be be de-phosphorylated?
G6P translocase
What enzyme dephosphorylates G6P to glucose in the ER?
G6Pase (Glucose-6-phosphatase)
What transporter transports glucose out of the liver?
GLUT2
Why do muscles and other tissues besides the liver not release glucose back into circulation?
They lack GTPase to break G16 back down to glucose.
What is the result of any defect in the G6P hydrolysis system in liver/
Type 1 Glycogen Storage Disease
What are the symptoms of Type 1 Glycogen Storage Disease?
1) Increased [G6P] and liver and kidney[glucose]
2) Inability to increase blood glucose concentration
3) Hepatomegaly and hypoglycemia
4) General failure to thrive
