VL 5 (Silke Leimkühler)

Question 1

What is the role of posttranslational modifications?

Answer 1

• Regulation of the activity
  – Turns an activity on or off
  – Generates a different function
• Protein-Protein Interactions
  – Modified site interferes with a binding site
• Subcellular localization
  – Momdified site is a Targettingsignal
  – Provides a Membrane-Ancor
• aging
  – Protein is labelled for degradation
• Introduction of chemical groups
  (e.g changes they hydrophobicity of a protein)
• Energy-metabolism
• oxidative phosphorylation of the respiratory chain
• Phosphorylation in protein synthesis
• Signal transduction
• Protein degredation
• Immune System

Question 2

List of posttranslational modifications?

Answer 2

• N-Glykosylation (in Golgi and ER)
• formation of disulfide bonds in ER (Protein-Disulfid Isomerase)
• Folding old proteins (chaperson, Peptidyl-Disulfid Isomerases)
• proteolytic cleavage (ER, Golgi, Lysossome)
• modifications of aa resdiues
  –> Phosphorylation (Ser, Thr, Tyr)
  –> Hydroxylation (Pro)
  –> Acetylation (Lys, N-Terminus)
  –> Methylation (His)
  –> Carboxylation (Glu)

Question 3

What is proteolitic cleavage of proteins?

Answer 3

Proteolytic cleavage is basically the process of breaking the peptide bonds between amino acids in proteins. This process is carried out by enzymes called peptidases, proteases or proteolytic cleavage enzymes.

Protease by active site machinery
* Serine protease
* Cystein protease
* Aspartic acid protease
* Zinc protease

Question 4

What is phosphorylation?

Answer 4

the attachment of a phosphate group to a molecule or an ion

Phosphat–> Ser ( Thr, Thy, Asp, His)
* very often 1P (1UE)
* insertion: by Protein kinase (phosphorylation)
* cleavage: by Protein-Phosphatase (dephosphorylation)

Role of Phosphorylation and dephosphorylation:
* reulation of enzyme activity; signal transduction
* driving teh regulated assembly and disassembly of protein complexes

• addition of phosphate group (2- charges) to a residue ca attract + charges side chains
  –> causing major conformation changes
• attached PO4– groups can form structures that ca be recognized as binding sites by other proteins

Question 5

What is Glykosylation?

Answer 5

• Addition of Polysaccheride
• Glycosyl grops are added in ER and Golgi
• main form of protein modification

Role of Glycosylation:
* information about the matuiration of a protein
* confirmational change
* interactions with other proteins
* protection ageainst protease

Question 6

What is Protein-Disulfid-Isomerase?

Answer 6

Protein disulfide isomerase (PDI) is a folding assistant in the endoplasmic reticulum (ER) of eukaryotic cells.
Roles of PDI:
* acting as a chaperone
* a binding partner of other proteins
* hormone reservoir
* a disulfide isomerase in the formation of disulfide bonds.

• oxidase activity: the introduction of disulfide bonds bewteen cystein residues
• Isomerase activity: the rearrengement of incorrect sisulfide bonds to form correcty folded, active proteins

Example: modification of cysteins and methionines

Question 7

What is Protein Lipidation?

Answer 7

Lipidgroups added posttranslational to protein

proteins can be attached by at least seven types of lipids:
*(fatty acids, lipoic acids, isoprenoids, sterols, phospholipids, glycosylphosphatidylinositol anchors, and lipid-derived electrophiles.)

Question 8

Lipid-linked proteins

Answer 8

GPI-Anker: Glycophosphadity-Inositol (GPI) addition to the C-terminus

• Is composed of Oligosaccharides and Inositol-Phospholipides
• Anchors surface-proteins with the membrane-
• Is added in the ER
• 1 GPI-Ancor per protein
• Localized at the outer cytoplasmic membrane
• Can be cleaved by PI-Phospholipase C (PI-PLC)
• Often found at the surface of parasitic protozoas (z.B. Trypanosomas und Leishmania)

Question 9

Name covalent protein modifications

Answer 9