II. Post-transcription | 27. Post-translational modifications of proteins Flashcards
- Posttranslational modifications (PTMs)
a/ Definition of Posttranslational modifications (PTMs)?
It refers to the covalent and enzymatic modification of proteins after protein synthesis. PTMs occur at distinct amino acid side chains or peptide linkages.
-> There are more than 200 different types of PTMs and they will eventually determine the function of the polypeptides.
- Posttranslational modifications (PTMs)
b/ What are the 3 main types of Post-translational modifications of proteins?
1/ N-terminal modifications
2/ C-terminal modifcations
3/ Internal amino acid modifications
- N-terminal modifications
a/ What are the 3 types of N-terminal modifications
- Deformylation of formylmethionine
- Acetylation
- Myristoylation
- N-terminal modifications
a/ The role of and steps of Deformylation of formylmethionine
- Formylmethionine is important in initiation of translation in bacteria
- Formylmethionine deformylase cleaves the formylmethionine into methionine and the formyl group
- N-terminal modifications
b1/ Characteristics of Acetylation
- N-acetylation is the transfer of an acetyl group (CH3CO) onto the N-atom of an amino acid.
- The first amino acid in the polypeptide chain, methionine, is cut off and replaced by an acetyl group.
- One of the most common co-translational (during synthesis) covalent modifications of proteins in eukaryotes
- N-terminal modifications
b2/ Examples of acetylation
Eg: acetylation of histone proteins, which play a role in activation of genes in chromatin
- Histone acetyl transferase (HAT) will add acetyl groups
-> reduce chromosomal condensation
-> transcription
- Histone deacetylase (HDAC) will remove acetyl groups
-> increase chromosomal condensation
-> transcription repression
- N-terminal modifications
c/ Characteristics of Myristoylation
- Lipid modification, where a myristoyl group is covalently attached (by an amide bond = peptide bonds) to the amino group of N-terminal glycine
- Plays an essential role in membrane targeting
- Myristoyl arises from myristic acid
- E.g, PKA
- C-terminal modifications
a/ What are the 3 types of C-terminal modifications?
- Amidylation
- Cholesterylation
- Glycosylphosphatidylinositol coupling
- C-terminal modifications
b/ Characteristics of Amidylation?
- Common posttranslational modification in peptide hormones (ACTH, oxytocin)
- Addition of an amide group to the C-terminal of a polypeptide chain
- Amide group is provided by a glycine residue
- The amide group neutralizes negative charges on the C-terminal
- C-terminal modifications
c/ Characteristics of Cholesterylation?
- Cholesterylation is a post-translational attachment of sterol to proteins.
- Occurs at C-terminal carboxyl-group
- e.g. Hedgehog
- C-terminal modifications
d/ What happen in Glycosylphosphatidylinositol coupling?
C terminal, hydrophobic peptide signals GPI coupling.
-> GPI tethers proteins on the cell surface.
- Internal amino acid modifications
a/ Which amino acids are not modified?
All amino acids except Ile, Leu, Val, Ala and Phe are modified
- Internal amino acid modifications
c/ What are the 7 types of modifications?
1) Hydroxylation
2) Lysine deamination
3) Carboxylation
4) Methylation (heterochromatin formation)
5) Acetylation
6) Phosphorylation
7) Glycosylation
- Internal amino acid modifications
b/ Characteristics of Hydroxylation?
- posttranslational hydroxylation involves the oxidative conversion of a C-H bond to a C-OH group on an amino acid side chain
- From lecture: if collagen is not hydroxylated (lysine & proline), the triple helix will be much weaker due to less strong H-bonds; Reduction of Fe3+ is vitamin C dependent
- Internal amino acid modifications
c/ Characteristics of Lysine deamination?
- Amino group (-NH3) is turned into an aldehyde group (-CHO)
- Lysyl oxidase performs the reaction with its co-factor (copper -> norleucin)
- Since the aldehyde group is very reactive, it can react with lysine/proline (amino group) to cross-linkthis is a covalent bond, so it makes collagen triple helix stronger (decreased flexibility)
- Internal amino acid modifications
d/ Characteristics of Carboxylation
- When carboxylic acid (-COOH) is added to an amino acid
- Occurs in glutamate residues to produce γ-carboxyglutamate
- Important modification in the activation of blood coagulation cascades - γ- carboxyglutamate binds Ca2+, which is essential for blood clotting (Vitamin K as cofactor, produced in ER)
- Internal amino acid modifications
e/ Characteristics of Methylation?
- Covalent attachment of a methyl group (-CH3) to the amino acid by an enzyme
- Methylation is most common in arginine and lysine
- The catalysator is methyltransferase
- Ex: arginine or lysine methylation (mono-, di- or trimethylation of lysine residues)
- Ex: methylation of histone proteins, which helps to create distinct regions in
chromatin. Methylated histones act epigenetically to repress or activate gene expression. Histone methyltransferase adds methyl groups to histone proteins
- Internal amino acid modifications
f/ Characteristics of phosphorylation
- addition of a phosphate group (-PO42-) to amino acids
- Usually happens to serine, threonine and tyrosine found on the polypeptide chain
- Protein kinases phosphorylate proteins by catalyzing the transfer of the phosphoryl group of an ATP molecule (phosphorylate OH-groups)
- Phosphatases dephosphorylate proteins by hydrolytic removal of phosphoryl group (Pi)
- This type of modification is important for protein function as it activates/deactivates many enzymes and controls the
activity/structure/localization of many other proteins - Effects: PO42- carries 2 (-) charges
-> causes conformational
change in the protein
-> attracts (+) charged side chains
-> can affect binding of ligands (activate/inactivate enzymes)
- Internal amino acid modifications
g1/ What is Glycosylation?
- Addition of carbohydrate (a sugar component) onto the polypeptide chain
- Internal amino acid modifications
g2/ What are the 2 types of Glycosylation?
1/ O-glycosylation
2/ N-glycosylation
- Internal amino acid modifications
g3/ What is an example of Glycosylation?
glycosylated proteins can usually be found as membrane proteins that act as receptors for important biological molecules
- Internal amino acid modifications
g4/ What are the 4 common functions of Glycosylation?
- Protecting proteins from degradation
- Selective labelling of proteins
- Determines cell-cell connections
- Essential in protein folding quality control
- Internal amino acid modifications
g5/ Characteristics of O-glycosylation
1/ Adds carbohydrate on OH-group of serine, threonine, tyrosine
2/ In Golgi
3/ Added by O-GlcNAc transferase
(O-linked N-acetylglucosaminyltransferase )
- Internal amino acid modifications
g6/ Characteristics of N-glycosylation
1/ Adds carbohydrate to amide group of asparagine
2/ Starts in ER and finish at Golgi
3/ Added by N-acetyltransferase
- Post-translational modifications related to tethering membrane proteins
a/ What are the 6 types of post-translational modifications related to tethering membrane proteins?
- Myristoylation:
- Prenylation
- Palmitoylation
- Phosphatidylethanolamidation
- Cholesterylation
- GPI coupling
- Post-translational modifications related to tethering membrane proteins
b/ Characteristics of Prenylation
1/ Protein prenylation involves the transfer of either a farnesyl or a geranylgeranyl moiety to C-terminal cysteine(s) of the target protein.
2/ Farnesyl-OPP, geranylgeranyl-OPP
3/ e.g. Ras
- Post-translational modifications related to tethering membrane proteins
c/ Characteristics of Phosphatidylethanolamidation
Serine
LC3 protein is the only protein (autophagy)
- Post-translational modifications related to tethering membrane proteins
d/ Characteristics of Palmitoylation
1/ Internal Cys or Ser
2/ Palmitoylation involves the addition of a Palmitol-CoA to the cysteine residue of proteins by thioester bond
3/ e.g. Ras
- Characteristic modifications in the endoplasmic reticulum
a/ What are the types of modifications in ER?
1/ N-glycosylation
2/ Ca2+ binding by carboxylation
- Characteristic modifications in the endoplasmic reticulum
b1/ What is N-glycosylation?
It is the attachment of an oligosaccharide, a carbohydrate consisting of several sugar molecules, sometimes also referred to as glycan, to a nitrogen atom
- Characteristic modifications in the endoplasmic reticulum
b2/ What happen during N-glycosylationin ER? (2 steps)
It starts in the ER and finish in the Golgi
(1) A lipid-linked oligosaccharide (N-acetyl glucose, mannose and glucose) is anchored to the ER lumen thanks to dolichol
(2) The lipid-linked oligosaccharide binds to a growing polypeptide chain by oligosaccharide transferase through the recognition of an asparagine signal on the chain
- Characteristic modifications in the endoplasmic reticulum
c/ Characteristics of Ca2+ binding by carboxylation
- Indispensable for blood clotting
- Vitamin K dependent, in ER
- Modification results in increased calcium binding capacity
