Proteins - Post translational Modification Flashcards Preview

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Flashcards in Proteins - Post translational Modification Deck (33)
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1
Q

Where are proteins made on free ribosomes in the cytosol targeted for?

A

Cytosol

Organelles - e.g. nucleus, mitochondria, peroxisomes

2
Q

Where are proteins made on RER bound ribosomes targeted for?

A

For ER, Secretion/Lysosomes/Plasma membrane

3
Q

Protein targeting: Which out of ER, Lysosome, Mitochondria, Nucleus, Peroxisome

1) Doesn’t require direct energy
2) Has a C terminal signal
3) Doesn’t require receptor
4) Is kept unfolded on entry to organelle

A

1) ER - indirect by H+ pump to keep lysosome pH5
2) ER - KDEL is C terminal, Peroxisome SKL is C terminal.
3) Nucleus - importin through pores
4) Mitochondria - chaperones keep unfolded

4
Q

What is constitutive vs regulated secretion? What 3 cell types tend to do regulated secretion?

A

Constitutive constant - collagen, albumin

Regulated - at specific times e.g. insulin

Endocrine/exocrine/neurocrine

5
Q

Where are signal sequences on secretory proteins e.g. collagen/albumin, how many amino acids, are they hydrophillic/phobic, and what do they form to help get across membrane?

A

N terminus
5-30 amino acids
Central region hydrophobic
Can form alpha helix

6
Q

What does the ‘pre’ part denote in preproalbumin etc

A

The signal sequence

7
Q

What are the 5 basic stages to synthesis of secretory proteins and their translocation across the ER membrane?

A

1) Signal - N terminus
2) SRP - binds signal and ribosome - halts translation
3) SRPreceptor translocon binds all of above
4) Peptide enters - signal sequence cleaved by signal peptidase
5) Translation continues - fully folded proteins

8
Q

What are the basic steps of preproinsulin processing in ER/Golgi? (4)

A

1) In ER- signal peptidases cleaves signal sequence
2) Folding of proinsulin with 3 x cysteine-cysteine disulphide bonds
3) Endopeptidases cleave B chain
4) To golgi - mature insulin 2 peptides joined with disulphide bonds packages in zymogen granules for secretion

9
Q

What are the differences in membrane protein biosynthesis compared to secreted after the signal sequence has been cleaved by endopeptidases (3)?

A

1) Membrane proteins have a stop transfer sequence which halts protein in membrane
2) Moves out of translocon = transmembrane domain
3) Translation continues and more transmembrane domains are made if needed

10
Q

What 7 functions of ER in protein procesing?

A
Proteolytic cleavage
Proper folding
Insertion into membrane
Assembly of multi subunits
Glycosylation
Hydroxylation of Lys Pro
Dilsuphide bonds
11
Q

3 reasons glycosylation of proteins is so important - what disease can occur if there is a deficiency in N-Linked glycosylation

A

1) Correct protein folding
2) Protein stability
3) Facilitates interactions with other molecules

Congenital disorders of glyosylation - very severe

12
Q

Where is the sugar added to in N-Linked glycosylation?

A

Asparagine side chain

13
Q

Which enzyme plays a role in formation of disulphide bonds in ER?

A

Protein Disulphide Isomerase

14
Q

What do chaperone proteins in ER do if proteins are misfolded? Give examples of chaperones that do this

A

Can actively fold/restrict folding to allow for misfiled proteins to be refolded correctly.

E.g. BiP - a Heat Shock Protein 70, Calreticulin, Calnexin

15
Q

What if misfolding cannot be corrected (2)

A

1) Back to cytosol for degradation

2) May accumulate in ER - toxic

16
Q

Can a single mutation lead to aggregation of misfiled proteins in ER?

A

Yes

17
Q

Apart from recruiting chaperones to help refold misfolded proteins, what other thing can the cell do to help prevent this happening?

A

Stop translation

18
Q

How and where does O linked glycosylation occur?

A

Golgi

Adds sugar to OH group of Ser Thr

19
Q

What kind of glycoslyation do you see in EC matrix/mucus secretions? What do these two things have in common relevant to this kind of glycosylation?

A

O linked - important in Proteoglycans.

20
Q

What are the 4 ‘Threes’ of tropocollagen structure? Which are the most common AA to be in the X and Y positions?

A

triple helix
300nm rod
Gly XY (Gly every 3rd AA)
3 polypeptide chains

Commonly proline and hydroxyproline in XY positions

21
Q

Is tropocollagen non extensible?

A

Yes-

22
Q

Is tropocollagen non compressible?

A

Yes

23
Q

Where does Gly sit in the triple helix? Why?

A

In the middle - small

24
Q

Where are the H+ bonds formed between the helices in tropocollagen? Which attribute of collagen do the H+ bonds give rise to? What other bonds mean collagen is very structurally stable?

A

Proline gets hydroxylated post translation - forms H+ bonds

High tensile strength

Cross link covalent bonds between collagen molecules. Disulphide bonds between cysteine residues.

25
Q

What alpha chains is the most common collegen (type I) made up of

A

2 x alpha 1 (I)

1 x alpha 2(I)

26
Q

What is the relevance of prolyl hydroxylase in Scurvy?

A

It is Ca2+ dependent so lack of Ca2+ means less prolines hydroxylated so less H+ bonds, weaker collagen

27
Q

Why is procollagen/tropocollagen not triple helical at N and C terminus in cell?

A

To prevent it attempting to cross-link into fibres in cell. Needs to be outside cell due to size

28
Q

With what bonds are collagen molecules laterally aggregated and in what formation?

A

Covalent cross links - staggered array.

29
Q

What is the relevance of lysyl oxidase in collagen synthesis? What two things does it require to work?

A

Catalyses formation of covalent cross-links in lateral aggregation of collagen fibres. Vit B6 cofactor and Cu++ ions

30
Q

What disease can happen with Lysyl oxidase deficiency or collagen type V deficiency?

A

Ehlers - Danlos Syndrome (Stretchy skin)

31
Q

Name 3 enzymes important in collagen synthesis from the lecture

A

Signal peptidase
Prolyl Hydroxylase
Lysyl Hydroxylase

32
Q

What is a common motif of collagen?

A

Gly XY repeat

33
Q

How is collagen synthesised and secreted?

A

CHAASPOGRL