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Chapter 18

BIOC 3300 Chapter Notes - Chapter 18: Glycogen Phosphorylase, Pyridoxal Phosphate, Phosphorolysis


Department
Biochem & Molecular Biology
Course Code
BIOC 3300
Professor
Mc Leod Roger
Chapter
18

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1. Where does
glycogen store?
In the liver and skeletal muscle.
(a) Glycogen is major energy source for contraction of skeletal muscle
(b) in liver glycogen is stored to maintain blood glucose. About 2-8% of liver weight is glycogen.
2. Bond involved in
mobilization of
glycogen?
Phosphorolysis of α-1,4-glycosidic
linkages of the glycogen polymer
3. Reactions involved in mobilization of glycogen?
Phosphorolysis; Hydrolysis.
4. Enzymes in mobilization of glycogen?
5.
6. Glycogen phosphorylase
Catalyses the reaction releasing glucose- 1-P from the non-
reducing end of glycogen.
- cannot break down α1,6 linkages at branch points.
- phosphorylase stops at a point 4 glucose residues away from
the branchpoint.
- pyridoxal phosphate is a cofactor.
- Reaction is near equilibrium but high concentration of Pi
makes forward reaction favoured
8. Debranching
enzyme
Required to mobilize at branchpoints.
4 - Glycogen Metabolism and Gluconeogenesis
Enzymes in mobilization of glycogen
Glycogen phosphorylase; Debranching enzyme;
Phosphoglucomutase (muscle); Glucose-6-phosphatase (liver).

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15. Glycogen
16. Nonreducing
end of
glycogen
17.
glycosidic
linkages
18. 1,6-
glycosidic
linkages
19. What is the
building
units for
glycogen
biosynthesis?
UDP-glucose
24. What are enzymes used in glycogen
biosynthesis?
Glycogen Synthase. Branching Enzyme.
12. Why
phosphoglucom
utase only exists
in muscle?
To keep glucose-6-P in muscle
and offer energy by glycolysis
for muscle activity.
13. Glucose-6-
phosphatase
Only in the liver.
Generate glucose from
glucose-6-P for transport in the
blood
14. Why Glucose-6-
phosphatase only
exists in muscle?
To release glucose-6-P to the
blood and offer energy for the
brain.
9. What two activities
does debranching
enzyme have?
α1,4- α1,4-glucantransferase.
α1,6-glucosidase.
-glucantransferase activity transfers triglucose from the branchpoint
chain to another outer branch.
- α1,6-glucosidase activity releases glucose from the branchpoint.
10.
Products of glycogen breakdown are predominantly glucose-1-P with
some glucose (amount depends on number of branchpoints)
11. Phosphoglucomutase Only in muscle. Converts glucose-1-P to
glucose-6-P for entry to glycolysis.
What are the products in mobilization of glycogen?
α-1,4-

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20.
UDP-glucose is synthesized
from glucose- 1-P by UDP-
glucose pyrophosphorylase.
Released PPi is hydrolysed
to 2Pi by abundant
pyrophosphatase, driving
reaction to ompletion.
The reaction is a
phosphoanhydride exchange in
which the phosphoryl oxygen of
G1P attacks the phosphorus
atom of UTP to form UDPG and
release PPi. The PPi is rapidly
hydrolyzed by inorganic
pyrophosphatase.
21. How is glycogen
assembly initiated?
Glycogen assembly is initiated by addition of a short chain (up to 8) of glucose
residues onto the protein glycogenin.
glycogenin-glucose chains serve as "primers" for glycogen synthase to extend with
additional UDPglucose units.
22. glycogenin
Serve as "primers" for
glycogen synthase to
extend
23. glycogenesis
UDP-glucose pyrophosphorylase
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