CARBO-4-glycogen 2

Glycogen

Glycogen, a branched polymer of glucose, represents a storage form of glucose. Glycogen synthesis and degradation occur primarily in liver and skeletal muscle, although other tissues, including cardiac muscle and the kidney, store smaller quantities.

A glycogen granule

Glycogen is stored in the cytoplasm as either single granules (skeletal muscle) or
as clusters of granules (liver) . The granule has a central protein core with polyglucose chains radiating outward to form a sphere. Glycogen stored in the liver is a source of glucose mobilized during hypoglycemia.
Muscle glycogen is stored as an energy reserve for muscle contraction. In white (fast-twitch) muscle fibers, the glucose is converted primarily to lactate, whereas in red (slow-twitch) muscle fibers, the glucose is completely oxidized.

GLYCOGEN SYNTHESIS

Synthesis of glycogen granules begins with a core protein glycogenin. Glucose addition to a granule, begins with glucose 6-phosphate, which is converted to glucose I-phosphate and activated to UDP-glucose for addition to the glycogen chain by glycogen synthase. Glycogen synthase is the rate limiting enzyme of glycogen synthesis.

Branching Enzyme (Glycosyl α1 ,4:α1 ,6 Transferase)

GLYCOGENOLYSIS

The rate-limiting enzyme of glycogenolysis is glycogen phosphorylase (in contrast to a hydrolase, a phosphorylase breaks bonds using Pi rather than H20). The glucose I-phosphate formed is converted to glucose 6-phosphate by the mutase used in glycogen synthesis .


Glycogen Phosphorylase
Glycogen phosphorylase breaks α-l,4 glycosidic bonds, releasing glucose I –phosphate from the periphery of the granule.
Glycogen phosphorylase cannot break α-l,6 bonds and therefore stops when it
nears the outermost branch points.

Debranching Enzyme (Glucosyl α-l ,4: α-l,4 Transferase and α-l,6 Glucosidase)

Debranching enzyme deconstructs the branches in glycogen that have been exposed by glycogen phosphorylase. Debranching enzyme:

Breaks an α- l ,4 bond adjacent to the branch point and moves the small oligoglucose chain released to the exposed end of the other chain
Forms a new α -l ,4 bond
Hydrolyzes the α- l ,6 bond, releasing the single residue at the branch point as free glucose. This represents the only free glucose produced directly in glycogenolysis.

GENETIC DEFICIENCIES OF ENZYMES IN GLYCOGEN METABOLISM

Important genetic deficiencies, listed in The table below are classed as glycogen storage diseases because all are characterized by accumulation of glycogen in one or more tissues.

HOME WORK :Answer the following questions

1) Which of the following is false about glycogen molecules?
a) Glycogen is polysaccharide
b) Glycogen is polymer of beta-D-Glucose*
c) Glycogen consist of α(1-4) and α (1-6) glycosidic linkage
d) Glycogen have are helical structure with branching.


2) Which of the following organs do not have glycogen storage
a) Liver
b) Muscle
c) Intestine
d) Erythrocytes*

3) Which of the following enzyme is responsible for glycogen breakdown?

a) Glycogen phosphorylase*
b) Glycogen phosphatase
c) Glycogen hydrolase
d) Glycogen phosphoglycosidase

4) Liver glycogen contributes to maintenance of glucose but not muscle glycogen. Which of the following enzyme is absent on muscle?
a) Glycogen phosphorylase
b) Hexokinase
c) Glucose-6-phosphatase*
d) Debranching enzyme

5) Which of the enzyme is responsible for hydrolysis of α (1-6) glycosidic bond present at branching point of glycogen molecules?
a) β-Glucosidase
b) α- Glucosidase*
c) Glycosidase
d) Phosphorylase


6) Glycogen phosphorylase is responsible for breakdown of glycogen to
a) Glucose
b) Glucose-1-phosphate
c) Glucose-6-phosphate*
d) Maltose

7) Which of the following is false about enzyme glycogen phosphorylase of glycogen breakdown?
a) Glycogen phosphorylase are active as homodimer.
b) Glycogen phosphorylase are present in two conformation state
c) Glycogen phophorylase possess high affinity binding to glycogen in T conformational state*
d) Glycogen phosphorylase high affinity binding to glycogen in R conformation state

8) Which of the following metabolite allosterically activate glycogen phosphorylase?

a) ATP
b) AMP*
b) Glucose-6-P
d) Glucose-1-P

9) Which of the following protein is required for de novo synthesis of glycogen?

a) Glycoprotein
b) Glycogenin*
c) Proteoglycan
d) Glucogenin


10) Which of the following enzyme is responsible for addition of UDP-Glucose to existing chain?
a) Glycogen synthase*
b) Glycogen polymerase
c) Glycogen synthetase
d) Glyocogen lyase