1. Post Translational Modifications of Proteins

2. Types of Post Translational Modifications.
Phosphorylation
Glycosylation
Ubiquitination
S-Nitrosylation
Methylation
N-Acetylation
Lipidation
Proteolysis

4. Phosphorylation Addition of phosphate group to a protein.
Principally on serine, threonine or tyrosine residues.
Also known as Phospho regulation.
Critical role in cell cycle, growth, apoptosis and signal transduction pathways.
Protein kinases
ATP + protein ———————> phosphoprotein + ADP

5. Phosphorylation

6. Example

7. Glycosylation The covalent attachment of oligosaccharides
Addition of glycosyl group or carbohydrate group to a protein.
Principally on Asparagine, hydroxylysine, serine or threonine.
Significant effect on protein folding, conformation, distribution, stability and activity.

8. Example

9. Classes of Glycans N-Linked glycans O-Linked glycans Phospho glycans
attached to nitrogen of Asparagine or arginine side chains.
O-Linked glycans
attached to hydroxy oxygen of serine,threonine
Phospho glycans
linked through the phosphate of serine.
C-Linked glycans
Rare form, Sugar is added to a carbon on tryptophan side chain.

10. Ubiquitination
Ubiquitin is a small regulatory protein that can be attached to the proteins and label them for destruction.
 Effects in cell cycle regulation, control of proliferation and differentiation, programmed cell death (apoptosis), DNA repair, immune and inflammatory processes and organelle biogenesis.

11. Ubiquitin cycle

13. S-Nitrosylation Nitrosyl (NO) group is added to the protein.
NO a chemical messanger that reacts with free cysteine residues to form S-nitrothiols.
Used by cells to stabilize proteins, regulate gene expression.

15. Alkylation/Methylation
Addition of methyl group to a protein.
Usually at lysine or arginine residues.
Binds on nitrogen and oxygen of proteins
Methyl donor is S-adenosylmethionine (SAM)
Enzyme for this is methyltransferase
Methylation of lysine residues in histones in DNA is important regulator of chromatin structure

16. Example
Where SAM (S-adenosyl methionine) is converted into SAH(S-adenosyl homocysteine)

17. N-Acetylation Addition of acetyl group to the nitrogen.
Histones are acetylated on lysine residues in the N-terminal tail as a part of gene regulation.
Involved in regulation of transcription factors, effector proteins, molecular chaperons and cytoskeletal proteins.
Methionine aminopeptidase (MAP) is an enzyme responsible for N-terminal acetylation

18. Example

19. Where,
HDACs = Histone deactyllase ,
KATs = N-acetyltransferase.

20. Lipidation
Lipidation attachment of a lipid group, such as a fatty acid, covalently to a protein.
In general, lipidation helps in cellular localization and targeting signals, membrane tethering and as mediator of protein-protein interactions.

21. Types of lipidation
C-terminal glycosyl phosphatidylinositol (GPI) anchor
N-terminal myristoylation
S-palmitoylation
S-prenylation

22. C-terminal glycosyl phosphatidylinositol (GPI) anchor
GPI anchors tether cell surface proteins to the plasma membrane
GPI-anchored proteins are often localized to cholesterol- and sphingolipid-rich lipids, which act as signaling platforms on the plasma membrane.

23. N-myristoylation
It is the attachment of myristoyl group a 14-carbon saturated fatty acid (C14) to a protein.
It is facilitated by N-myristoyltransferase (NMT) and uses myristoyl-CoA as the substrate.

24. S-palmitoylation
It is addition of C16 palmitoyl group from palmitoyl-CoA
Palmitoyl acyl transferases (PATs) enzyme favors this step.
Reversed by thioesterases

26. S-prenylation
Addition of a farnesyl (C15) or geranylgeranyl (C20) group to proteins.
Enzyme involved in this reaction is farnesyl transferase (FT) or geranylgeranyl transferases (GGT I and II).

27. Disulfide Bonding
Disulfide bonds are covalent bonds formed between two cysteine residues (R-S-S-R).
These bonds contribute to the correct folding of proteins as other elements of secondary structure

28. Disulfide Bonding

29. Proteolysis Cleavage of peptide bonds by proteases.
Examples of Proteases- Serine Proteases, Cysteine Proteases, Aspartic acid Proteases.
Involved in Antigen processing, Apoptosis, Cell signalling

31. Protein modifications can affect the protein involved in several different ways:
Change protein conformation to activate of inhibit activity
Alter association of the protein with other proteins in the cell
Induce relocation of the protein between compartments of the cell
All of the above at once.