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Semester 1 - MCBG > 13. Post Translation Modification Of Proteins > Flashcards

13. Post Translation Modification Of Proteins Flashcards

1
Q

What are the two type of post translation modification?

A
  1. Proteolytic cleavage

2. Chemical modification

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2
Q

What is Proteolytic cleavage?

A

breaking peptide bonds to remove part of the protein

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3
Q

What is Chemical modification?

A

addition of functional groups to amino acid residues

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4
Q

Which proteins are synthesised on free ribosomes?

A

Protein destined for cytosol, or posttranslational import into organelles

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5
Q

Which proteins are synthesises by ribosomes on the RER?

A

Protein destined for membrane or secretory pathway via co- translational insertion

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6
Q

What is required for protein sorting?

A
  • a signal (address), intrinsic to the protein
  • a receptor that recognizes the signal and which directs it to the correct membrane
  • a translocation machinery
  • energy to transfer the protein to its new place
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7
Q

Which proteins re targeted for secretion?

A

• Extracellular proteins
• Membrane proteins
• Vesicular proteins (lysosomes,
endosomes etc)

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8
Q

What are the two types of secretion from cells?

A
  • Constitutive secretion

* Regulated secretion

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9
Q

Which secretion type is not regulated?

A

Constitutive secretion - happens all the time

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10
Q

Give examples of cell which use regulated secretion?

A
  1. Endocrine cells – secreting hormones
  2. Exocrine cells – secreting digestive juices
  3. Neurocrine cells – secreting neurotransmitters
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11
Q

What is a signal sequence?

A

A signal is a short peptide present at the N-terminus of the majority of newly synthesized proteins that are destined towards the secretory pathway.

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12
Q

Describe the structure of a signal sequence

A
  • N-terminal aa sequence • 5- 30 amino acids in length
  • Central region rich in hydrophobic residues
  • Able to form a-helix
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13
Q

What does the pre part of preproalbumin mean?

A

“Pre” part of preproalbumin defines the signal sequence which is removed during processing

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14
Q

What is the signal recognition particle?

A

The signal recognition particle (SRP) is an abundant, cytosolic, ribonucleoprotein (protein-RNA complex) that recognizes and targets specific proteins to the endoplasmic reticulum

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15
Q

What is the SRP composed of?

A

Composed of 6 proteins and a short piece of RNA

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16
Q

What does the SRP recognise and bind to?

A

Recognises the signal peptide, the ribosome and the receptor that is resent in the ER membrane

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17
Q

Describe the synthesis of secretory proteins and their translocations across the ER membrane

A
  1. Signal sequence on nascent polypeptide is recognised by the SRP
  2. SRP binds to the polypeptide and ribosome
  3. Translation halted by binding of SRP
  4. SRP recognised by SRP receptor/docking protein on ER membrane
  5. In making the interaction with the docking protein, the SRP is released from the signal sequence
  6. This removes the inhibitory constraint on translation so translation continues
  7. The signal sequence then interacts with a signal sequence receptor (SSR) within a protein translocator complex
  8. The ribosome becomes anchored to this pore complex, through which the growing polypeptide chain is extruded.
  9. The signal sequence is cut by the enzyme signal peptidase
  10. Once translation is finished, the ribosome is detached and goes back into the cytoplasm to find more mRNA
18
Q

What are the functions of the endoplasmic reticulum?

A
  • Insertion of proteins into membranes
  • Specific proteolytic cleavage
  • Glycosylation
  • Formation of S-S bonds
  • Proper folding of proteins
  • Assembly of multisubunit proteins
  • Hydroxylation of selected Lys and Pro residues
19
Q

What is N-linked glycosylation?

A

Addition of sugars to the amino group on an asparagine residue

20
Q

Why is glycosylation of proteins important?

A
  • needed for Correct protein folding
  • needed or Protein stability
  • Facilitates interactions with other molecules
21
Q

What do deficiencies in N-linked glycosylation lead to?

A

Deficiencies in N-linked glycosylation lead to severe

inherited human disease: Congenital disorders of glycosylation (CDG)

22
Q

What type of bond formation occurs n the ER lumen?

A

Disulphide bonds

23
Q

Between which groups do disulphide bonds form?

A

Disulphide bonds four between sulfhydryl groups in cysteine residues

24
Q

What is the role of protein disulphide isomerase (PDI) in the formation of disulphide bonds in the ER lumen?

A
  • PDI is important in allowing the proteins to form the correct disulphide bonds
  • If there are many cysteine residues, there are multiple possible combinations of disulphide bonds, the PDI ensures the correct pairs of disulphide bonds form so the protein fold correctly
25
Q

What happens if there are folding problems?

A
  • Protein may be trapped in mis-folded conformation
  • Protein contains mutation resulting in mis-folding
  • Protein may be incorrectly associated with other sub-units
26
Q

Which protein in the ER attempt to correct the folding problems?

A

chaperone proteins

27
Q

What do chaperone proteins in the ER do?

A

They get misfolded proteins back to the correct folding state:

• retain unfolded proteins in the ER

  • act as sensors to “monitor” extent of protein mis-folding
    * mediate increased transcription of chaperones
    * mediate reduction in translation
28
Q

What happens if mis-folding cannot be corrected?

A

• Protein may be returned to cytosol for degradation

• Protein may accumulate to toxic levels in the ER resulting
in disease
> This may arise due to single mutation

29
Q

Which 2 organelles carry out post translation modification?

A

ER and golgi apparatus

30
Q

What is the basic unit of collagen?

A

Tropocollagen

31
Q

Describe the structure of collagen fibres

A

• 300nm rod-shaped protein
• 3 polypeptides (a chains), each ~1000aa long • glycine in every 3rd position along each a chain
ie (Gly-X-Y)
n
repeat
• H-bonds between a chains stabilise structure
• characteristic triple helix (right-handed) • mostly proline or hydroxyproline in X and
some Y positions

32
Q

What are 3 physical properties of collagen?

A
  • non-extensible
  • non-compressible
  • high tensile strength
33
Q

Why is glycine the only amino acid found in the middle of the helix?

A

Glycine is the only amino acid with a side chain small enough to fit in the middle of the helix

34
Q

How is collagen synthesised?

A
  • Collagen alpha chains have a signal sequence that will target it to the ER for secretion
  • As each of the alpha chains of collagen are produced, they are entered into the lumen of the RER
  • The signal sequence is cleaved by signal peptidase - from preprocollagen a chains to procollagen a chains
  • Hydroxylation also occurs in the ER - addition of hydroxyl groups to proline and lysine residues
  • N linked glycosylation also occurs - addition of N linked oligosaccharides
  • Then there is addition of galactose to hydroxylysine residues
  • The 3 alpha chains come together due to disulphide bonds between cysteine groups of the 3 procollagen chains - forming the triple helix
  • Then winding of the helix occurs
  • the procollagen is then transferred to Golgi apparatus where it is modified some more and packaged not a vesicle
  • the procollagen is then secreted out of the cell by exocytosis
  • in the extracellular space, N and C terminal pro peptides are removed by procollagen peptidases, forming tropocollagen
  • the tropocollagen molecules come together to form collagen fibrils which come together to form collagen fibres.

•

35
Q

What is the function of prolyl hydroxylase?

A

To add hydroxyl groups to proline residues. Hydroxylation allows increased H-bonding to stabilise triple helix

36
Q

What is Prolyl hydroxylase associated with?

A

associated with PDI in the ER

37
Q

What does Prolyl hydroxylase require for activity?

A

requires vitamin C and Fe++ ions for activity

38
Q

What causes scurvy?

A

Deficiency of vitamin C.

Scurvy is due to weak tropocollagen triple helices

39
Q

Why is tropocollagen secreted before final processing occurs?

A

Assembly of the mature collagen fibres would cause the cell to explode!

40
Q

Describe how cross-linking among tropocollagen forms?

A

Strong covalent cross links between lysine residues in individual fibrils form due to Lysol oxidase.

41
Q

What causes Ehlers-Danlos syndrome (EDS) ?

A

Mutation in collagen type V or lysyl oxidase deficiency

Semester 1 - MCBG (18 decks)

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