Post Transitional Protein Processes

Question 1

Name 2 types of post-translational modifications and describe them.

Answer 1

Proteolytic Cleavage-breaking peptide bonds to remove part of the protein

Chemical modification-addition of functional groups to amino acid residues

Question 2

What is protein sorting?

Answer 2

Proteins destined for the cytosol or importation into organelles are synthesised on free ribosomes

Proteins destined for the membrane or secretary ppathway via co-translation insertion are synthesised by ribosomes on the RER

Question 3

What is required for protein sorting?

Answer 3

A signal, intrinsic to the protein (address)

A receptor that can recognise the signal and direst it to the correct membrane

A translocation machinery (get protein into organelle)

Energy to transfer the protein to its new place

Question 4

What 3 types of proteins are targeted for secretion?

Answer 4

Extracellular proteins

Membrane proteins (go through secretion pathway but stop in membrane)

Vesicular proteins (lysosomes etc (through secretory ER pathway)

Question 5

Name the 2 types of secretion from cells (of protien)

Answer 5

1) Constitutive secretion

2) Regulated secretion

Question 6

What is Consitiutive secretion?

Give an example.

Answer 6

Constant secretion

Eg, Albumin (a plasma protein) is secreted constantly

Question 7

What is Regulated secretion?

Answer 7

Controlled secretion (can be up or down regulated as needed)

Endocrine cells-secrete hormones
Exocrine cells-secrete digestive juices
Neuroscience cells-secrete neurotransmitters

Question 8

What is a signal sequence (referring to the signal sequence on the Nterminus)

Answer 8

N-terminal AA sequence
5-30 amino acids in length
Central region is rich in hydrophobic residues
Able to form an a-helix

“Pre”proalbumin - the pre defines the signal sequence, this is removed during processing

Question 9

What is the Signal Recognition Particle? SNP

Answer 9

Receptor needed to bind the signal peptide onto proteins destined for the ER.
Composed of 6 proteins and short piece of RNA
Recognise the signal peptide on the protein and the ribosome and the destination.

Question 10

Explain the synthesis of proteins and their translocation across the ER membrane.

Answer 10

1) Ribosome in cytosol starts synthesising the protein, starting with its N-terminal signal sequence.
2) The SRP binds to the signal sequence on the protein and halts transcription. The SRP binds to the SRP receptor on a protein in the membrane called a translocon.
3) GTP is hydrolosyed to GDP, this releases the SNP and opens the translocon. Synthesis of the protein now continues but it’s synthesised through the translocon into the ER lumen.
4) A signal peptidase cleaves the signal sequence form the N-terminal of the protein.
5) The protein synth continues through the membrane into the ER lumen.
6) The ribosome detaches and the synthesised protein folds inside the ER lumen. The translocon closes again

Question 11

What are the function of the ER?

Answer 11

Insertion of proteins into membranes 
Specific proteolytic cleavage 
Glycosylation 
Formation of S-S bonds
Proper folding of proteins 
Assembly of multi-subunit proteins 
Hydroxylation of selected Lys and Pro residues

Question 12

What is N-linked Glycosylation?

Answer 12

Sugars are added on an Aparagine side chain.

N-linked chain just refers to the fact the sugar is added to n amino group

Question 13

Why is Glycosylation of proteins important?

Answer 13

Correct protein folding
Protein stability (longer lived)
Facilitate interactions with other molecules (io proteins on cell surface to react, immune recognition sites)

Question 14

What happens when disulphide bonds form in the ER lumen?

Answer 14

Cytosines on AA groups can form disulphide bonds together. A PDI ensures the correct combo of bonds is formed and so the protein has the correct shape for being active. The protein is oxidised in this process

Question 15

What can happen if there are folding problems in a Protein?

Answer 15

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

Question 16

What are Chaperone proteins?

Answer 16

They attempt to correct mis-folding of proteins in the ER.
They retain unfolded proteins in the ER until they are fixed
They act as sensors to monitor the extent of mis-folding-this mediates increased transcription of chaperones or reduces translation

Question 17

What happens if mis-folding of proteins in ER lumen can be corrected?

Answer 17

Protein may be returned o cytosol for degradation (parts then re-used)

Protein may accumulate to toxic levels in the ER resulting in disease-may arise due to single mutation

Question 18

Why is collagen a triple helix?

Answer 18

Non-extensible
Non-compressible
High tensile strength

Question 19

Why does collagen use glycine in every 3rd position of the a-chain?

Answer 19

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

Question 20

What is the basic unit of collagen?

Answer 20

Troop collagen

Question 21

What are the main steps in collagen synth and modification?

Answer 21

1) Syntheis and entry of chain into lumen of RER
2) Cleavage of signal peptide (remove the pre from preprochain)
3) Hydroxylation of selected proline and lysine residues
4) Addition of N-linked oligosaccharides
5) Addtiion of galactose to hydroxylysine residues (requires vitamin C, adds OH to Prolyne using an enzyme)

Question 22

What is Prolyl Hydroxylase?

Answer 22

Associated with PDI (disulphide makes in ER) in the ER

Requires vitamin C and FE2+ ions for activity

Allows increased h bonding to stabilise triple helix

Scurvy is due to weak troop collagen triple helixes

Question 23

Why don’t C-terminal amino acids join the helix?

Answer 23

They are in the premature protein to stop the helixes joining up inside the ER (where they’d be too big to leave). They are removed after exotosis by the transport vesicle so the Procollagen is now called Collagen and can now form fibres and cross links.

Question 24

Give an example of a connective tissue disease.

Answer 24

Ehlers-Danlos syndrome

Mutation in collagen 5 of Lyly oxidase deficency