The synthesis of glycogen from glucose (glycogenesis) involves multiple enzymatic steps that takes place in the cytosol. Glucose is first converted to glucose-6-phosphate by hexokinase or glucokinase in muscle and liver respectively. It is then converted to UDP-glucose which acts as the donor of glucose units. A glycogen primer, initiated by the enzyme glycogen synthase, is required to begin glycogen synthesis. Glycogen is then elongated through alpha-1,4 glycosidic linkages by glycogen synthase and branched through alpha-1,6 linkages by the branching enzyme glucosyltransferase. This leads to the formation of the branched glyc

1. Gandham. Rajeev

2. The synthesis of glycogen from glucose is
called as glycogenesis.
Glycogenesis takes place in the cytosol &
requires ATP and UTP, besides glucose.
Steps:
Synthesis of UDP-glucose:
The enzymes hexokinase (in muscle) &
glucokinase (in liver) convert glucose to
glucose 6-phosphate

3. Phosphoglucomutase catalyses the conversion
of glucose 6-phosphate to glucose 1-
phosphate.
Uridine diphosphate glucose (UDP-glucose) is
synthesized from glucose 1-phosphate & UTP
by UDP-glucose pyrophosphorylase.

4. A small fragment of pre-existing glycogen
must act as a ‘primer’ to initiate glycogen
synthesis.
A specific protein ‘glycogenin’ can accept
glucose from UDP Glucose.
The hydroxyl group (OH) of the amino acid
tyrosine of glycogenin is the site at which the
initial glucose unit is attached.

5. The enzyme glycogen initiator synthase
transfers the first molecule of glucose to
glycogenin.
Then glycogenin itself takes up a few glucose
residues to form a fragment of primer which
serves as an acceptor for the rest of the
glucose molecules.

6. Glycogen synthase is responsible for the
formation of 1,4-glycosidic linkages.
This enzyme transfers the glucose from UDP-
glucose to the non-reducing end of glycogen to
form ɑ- 1,4 linkages.

7. Glycogen synthase can catalyse the synthesis
of a linear unbranched molecule with ɑ-1,4
glycosidic linkages.
Glycogen is a branched tree-like structure.
The formation of branches is brought about by
the action of a branching enzyme, namely
glucosyl -4-6 transferase.

8. This enzyme transfers a small fragment of 5 to 8
glucose residues from the non-reducing end of
glycogen chain (by breaking ɑ-1,4 linkages) to
another glucose residue where it is linked by ɑ-
1,6 bond.
This leads to the formation of a new non-
reducing end, besides the existing one.
Glycogen is further elongated & branched, by
the enzymes glycogen synthase & glucosyl 4-6
transferase.

9. Glucose
Glucose-6P
ADP
ATP
Glucokinase
Glucose-1P
Phosphoglucomutase
UDP-Glucose
UDP
UDP glucose
pyrophosphorylase
PPi
UTP

10. UDP-Glucose
UDP
Glycogen initiator
synthase
OH
UDP
O
Glycogen Primer
Glycogenin
13UDP
13UDP
Glycogen synthase
O
10 1
6
1

11. O
Glucosyl transferase
10
1
6
1
O
101
16
7
11
Elongation by glycogen
synthase (ɑ1,4 bonds
Branching by glucosyl 4,6
transferase (ɑ1,6 bonds)
Glycogen

12. Glycogen

13. Textbook of Biochemistry – U Satyanarayana
Textbook of Biochemistry – DM Vasudevan