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MCB 2 > II. Post-transcription | 27. Post-translational modifications of proteins > Flashcards

II. Post-transcription | 27. Post-translational modifications of proteins Flashcards

1
Q
  1. Posttranslational modifications (PTMs)
    a/ Definition of Posttranslational modifications (PTMs)?
A

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.

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2
Q
  1. Posttranslational modifications (PTMs)
    b/ What are the 3 main types of Post-translational modifications of proteins?
A

1/ N-terminal modifications
2/ C-terminal modifcations
3/ Internal amino acid modifications

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3
Q
  1. N-terminal modifications
    a/ What are the 3 types of N-terminal modifications
A
  1. Deformylation of formylmethionine
  2. Acetylation
  3. Myristoylation
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4
Q
  1. N-terminal modifications
    a/ The role of and steps of Deformylation of formylmethionine
A
  • Formylmethionine is important in initiation of translation in bacteria
  • Formylmethionine deformylase cleaves the formylmethionine into methionine and the formyl group
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5
Q
  1. N-terminal modifications
    b1/ Characteristics of Acetylation
A
  • 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
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6
Q
  1. N-terminal modifications
    b2/ Examples of acetylation
A

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
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7
Q
  1. N-terminal modifications
    c/ Characteristics of Myristoylation
A
  • 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
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8
Q
  1. C-terminal modifications
    a/ What are the 3 types of C-terminal modifications?
A
  1. Amidylation
  2. Cholesterylation
  3. Glycosylphosphatidylinositol coupling
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9
Q
  1. C-terminal modifications
    b/ Characteristics of Amidylation?
A
  • 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
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10
Q
  1. C-terminal modifications
    c/ Characteristics of Cholesterylation?
A
  • Cholesterylation is a post-translational attachment of sterol to proteins.
  • Occurs at C-terminal carboxyl-group
  • e.g. Hedgehog
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11
Q
  1. C-terminal modifications
    d/ What happen in Glycosylphosphatidylinositol coupling?
A

C terminal, hydrophobic peptide signals GPI coupling.
-> GPI tethers proteins on the cell surface.

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12
Q
  1. Internal amino acid modifications
    a/ Which amino acids are not modified?
A

All amino acids except Ile, Leu, Val, Ala and Phe are modified

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13
Q
  1. Internal amino acid modifications
    c/ What are the 7 types of modifications?
A

1) Hydroxylation
2) Lysine deamination
3) Carboxylation
4) Methylation (heterochromatin formation)
5) Acetylation
6) Phosphorylation
7) Glycosylation

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14
Q
  1. Internal amino acid modifications
    b/ Characteristics of Hydroxylation?
A
  • 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
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15
Q
  1. Internal amino acid modifications
    c/ Characteristics of Lysine deamination?
A
  • 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-linkthis is a covalent bond, so it makes collagen triple helix stronger (decreased flexibility)
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16
Q
  1. Internal amino acid modifications
    d/ Characteristics of Carboxylation
A
  • 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)
17
Q
  1. Internal amino acid modifications
    e/ Characteristics of Methylation?
A
  • 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
18
Q
  1. Internal amino acid modifications
    f/ Characteristics of phosphorylation
A
  • 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)
19
Q
  1. Internal amino acid modifications
    g1/ What is Glycosylation?
A
  • Addition of carbohydrate (a sugar component) onto the polypeptide chain
20
Q
  1. Internal amino acid modifications
    g2/ What are the 2 types of Glycosylation?
A

1/ O-glycosylation
2/ N-glycosylation

21
Q
  1. Internal amino acid modifications
    g3/ What is an example of Glycosylation?
A

glycosylated proteins can usually be found as membrane proteins that act as receptors for important biological molecules

22
Q
  1. Internal amino acid modifications
    g4/ What are the 4 common functions of Glycosylation?
A
  • Protecting proteins from degradation
  • Selective labelling of proteins
  • Determines cell-cell connections
  • Essential in protein folding quality control
23
Q
  1. Internal amino acid modifications
    g5/ Characteristics of O-glycosylation
A

1/ Adds carbohydrate on OH-group of serine, threonine, tyrosine
2/ In Golgi
3/ Added by O-GlcNAc transferase
(O-linked N-acetylglucosaminyltransferase )

24
Q
  1. Internal amino acid modifications
    g6/ Characteristics of N-glycosylation
A

1/ Adds carbohydrate to amide group of asparagine
2/ Starts in ER and finish at Golgi
3/ Added by N-acetyltransferase

25
Q
  1. Post-translational modifications related to tethering membrane proteins
    a/ What are the 6 types of post-translational modifications related to tethering membrane proteins?
A
  1. Myristoylation:
  2. Prenylation
  3. Palmitoylation
  4. Phosphatidylethanolamidation
  5. Cholesterylation
  6. GPI coupling
26
Q
  1. Post-translational modifications related to tethering membrane proteins
    b/ Characteristics of Prenylation
A

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

27
Q
  1. Post-translational modifications related to tethering membrane proteins
    c/ Characteristics of Phosphatidylethanolamidation
A

Serine
LC3 protein is the only protein (autophagy)

28
Q
  1. Post-translational modifications related to tethering membrane proteins
    d/ Characteristics of Palmitoylation
A

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

29
Q
  1. Characteristic modifications in the endoplasmic reticulum
    a/ What are the types of modifications in ER?
A

1/ N-glycosylation
2/ Ca2+ binding by carboxylation

30
Q
  1. Characteristic modifications in the endoplasmic reticulum
    b1/ What is N-glycosylation?
A

It is the attachment of an oligosaccharide, a carbohydrate consisting of several sugar molecules, sometimes also referred to as glycan, to a nitrogen atom

31
Q
  1. Characteristic modifications in the endoplasmic reticulum
    b2/ What happen during N-glycosylationin ER? (2 steps)
A

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

32
Q
  1. Characteristic modifications in the endoplasmic reticulum
    c/ Characteristics of Ca2+ binding by carboxylation
A
  1. Indispensable for blood clotting
  2. Vitamin K dependent, in ER
  3. Modification results in increased calcium binding capacity

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