GLYCOGENESIS
- 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.
- 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