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Flashcards in Protein Targetting Deck (57)
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1

How do most proteins achieve their final conformation?

Themselves

2

State 2 additional post-modifications of proteins after translation, and what they entail.

- proteolytic cleavage - breaking peptide bonds to remove part of the protein
- chemical modification - addition of functional groups to amino acid residues

3

What is the peroxisome targeting sequence? What does addition of this mean?

SKL (serine - lysine - leucine) - addition of this will target a protein to the peroxisome

4

What is the peroxisome targeting sequence? Which end of a protein is this sequence added?

SKL (serine - lysine - leucine) - this sequence is added to the C-terminus end of a protein (towards the end)

5

Once tagged, how does a protein transverse the peroxisome membrane?

Through a transport channel comprised of 13 PEX proteins - this transfer requires ATP hydrolysis

6

What must the protein-targeting sequence bind in order to move the protein through the PEX transport channel in the peroxisome membrane?

A PEX5-cargo complex

7

Failure to target proteins to peroxisomes can lead to abnormal peroxisomes - what biological affects may this have? List 1 disease associated with abnormal peroxisomes.

Peroxisomes break down fatty acids - if this function is disturbed fatty acids may accumulate which can impair neuronal function - long bone shortening may occur as a result of abnormal protein targeting to peroxisomes - Zellweger syndrome is associated with abnormal protein targeting to peroxisomes

8

What is constitutive secretion?

Secretion that is continual (occurs all the time)

9

Describe the conformation and composition of the sequence that is added to molecules that require secretion from the cell.

The sequence is added to the N-terminus of the protein, and is around 5-30 amino acids in length - the central part of this signalling sequence is composed mainly of hydrophobic residues, which form an alpha helix in solution

10

What is required to target a protein to the ER for secretion? How is this molecule composed?

The signal recognition particle recognises the targeting sequence and moves the targeted molecule to the ER - it is composed of 6 subunits and a small stretch of RNA

11

What organelle does the signal recognition particle direct a protein with a target sequence and an associated ribosome towards?

The ER

12

Does binding of the signal recognition peptide instigate or inhibit translation of the RNA molecule associated to a ribosome?

The signal recognition particle prevents translation, while it directs the free ribosome to the surface of the ER

13

How does a type I membrane protein, designed to reside in the membrane of the ER, stop itself from being completely excluded into the lumen?

It contains a stop-transfer anchor sequence - an alpha-helical membrane-spanning sequence

14

List 7 functions of the endoplasmic reticulum.

- proteolytic cleavage
- insertion of proteins into membranes
- glycosylation
- formation of disulphide (s-s) bonds
- proper folding of proteins
- assembly of multi-subunit proteins
- hydroxylation of selected lysine and proline residues

15

What is N-linked glycosylation?

The addition of sugars (a glycan) to the amide nitrogen of an asparagine residue of a protein

16

Give 3 reasons why glycosylation is important.

- correct folding of proteins
- give a protein stability
- facilitates interactions with other molecules

17

Between what 2 residues does a disulphide bond form?

Between 2 cysteine residues

18

What is the role of protein disulphide isomerase?

Protein disulphide isomerase ensures that the correct disulphide bonds are formed - it can remove wrong ones and correct them

19

What is O-linked glycosylation? Where specifically does this occur?

O-linked glycosylation is the attachment of a sugar molecule to the O of an OH- (hydroxyl) group of a serine or threonine residue - this occurs specifically in the Golgi apparatus

20

Describe the processing of preproinsulin.

The N-terminal signal sequence is cleaved as the protein enters the ER lumen - a pair of disulphide bonds form between the a & c chain within the ER lumen, causing the b chain to bend - the b chain is then cleaved in the Golgi apparatus, to form insulin

21

In which molecules is O-linked glycosylation an important characteristic?

Glycoaminoglycans (GAG's)

22

What is the opposite of constitutive secretion? Give 3 examples.

Regulated secretion:
- endocrine secretion
- exocrine secretion
- paracrine secretion

23

What cell type secretes collagen?

Fibroblasts

24

What is the basic unit of collagen? Describe its structure.

The basic unit of collagen is procollagen - it is a 300nm rod-shaped protein, consisting of 3 alpha chains, which are each around 1000 amino acids long - a glycine residue is present every 3rd amino acid along each alpha chain

25

What is special about the properties of glycine that it features so prominently in procollagen/collagen?

Glycine is the amino acid with a side chain small enough to fit into the middle of the helix form by the 3 alpha subunits

26

What other amino acids are associated with the structure of procollagen/collagen? How are these important?

Proline and/or hydroxyproline - these allow the formation of hydrogen bonds between the alpha subunits, which give procollagen structural stability

27

What molecule brings procollagen to the ER? Where is the signal peptide then cleaved?

The signal recognition particle binds the signal sequence of pre procollagen and brings it (and its associated ribosome) to the ER - the signal sequence is then cleaved off within the lumen of the ER

28

Where does the hydroxylation of certain glycine and proline residues occur on a procollagen polypeptide?

In the lumen of the RER

29

What type of glycosylation does procollagen undergo in the ER?

Procollagen undergoes N-linked glycosylation in the RER

30

What is scurvy the result of?

Scurvy is the result of unstable connections between the alpha subunits the comprise a collagen helix - this is due to poor prolyl hydroxylase activity, which catalyses increased hydrogen bonding between the 3 alpha subunits