1. بسم الله الرحمن الرحيم

2. Protein Biosynthesis -2By
Amr S. Moustafa, M.D.; Ph.D.

5. Polyribosome (Polysomes)
Several ribosomes and one mRNA

6. Protein Targeting
Free Vs attached ribosomes
Localization signals

7. Regulation of Translation
The dephosphorylated eIF: Active form
Heme prevents eIF phosphorylation
Heme stimulates translation
Proteins/mRNA interactions:
e.g., Ferritin and transferrin receptor synthesis

8. Posttranslational Modifications
Trimming
Covalent modifications:
Phosphorylation Biotinylation
Glycosylation Farnesylation
Hydroxylation Acetylation
Carboxylation
Ubiquitination

9. Posttranslational Modifications: Trimming
Removal of a part of the translated sequence
Proteases protein activation
Cellular site of cleavage:
ER and Golgi apparatus
Secretory vesicles: e.g., insulin
After secretion: e.g., collagen
At sites of action: zymogen e.g., trypsinogen

10. Posttranslational Modifications: Phosphorylation
Addition of phosphate to hydroxyl group of
Serine and threonine
Tyrosine
Reversible:
Addition by kinases
Removal by phosphatases
Increase or decrease the activity of a specific protein
e.g., Glycogen synthase and glycogen phosphorylase

11. Posttranslational Modifications: Phosphorylation

12. Posttranslational Modifications: Glycosylation
Modifications of :
proteins of plasma membrane
proteins of lysosome
secretory proteins
Addition of carbohydrate chains to:
Hydroxyl group of serine or threonine (O-linked)
Amide group of asparagine (N-linked)
Site: Golgi apparatus and ER
Protein targeting:
e.g., mannose-6-phosphate to lysosomal enzymes

13. Posttranslational Modifications: Glycosylation

14. Posttranslational Modifications: Hydroxylation
Proline and lysine
Endoplasmic reticulum
Prolyl or lysyl hydroxylases
e.g., Collagen

15. Posttranslational Modifications: Other Covalent Alterations - 1
Carboxylation:
γ-carboxylation of glutamic acid residue for
maturation of vitamin K-dependent clotting factors
II, VII, IX and X

16. Carboxylation

17. Posttranslational Modifications: Other Covalent Alterations - 2
Biotinylation
e.g., Acetyl Co-A
Carboxylase
Farnesylation
e.g., Protein anchoring
to membranes

18. Posttranslational Modifications: Ubiquitination
Proteins:
Defective
Rapid turnover
Ubiquitin:
Small, highly conserved protein
Proteasome:
Complex proteolytic system
ATP-dependent
Cytosol