Protein Targeting And Modification

Question 1

What happens to ribosomes in a protein designated for a secretory pathway?

Answer 1

Attach to the er membrane

Question 2

In which signals are the protein signalling sequences retained?

Answer 2

Perioxsome and nucleus.

Question 3

In what signals are the protein signalling sequences removed? T

Answer 3

Mitochondrial matrix and the ER lumen.

Question 4

How are proteins imported into the mitochondrial matrix?

Answer 4

Protein fed through a pore in the outer membrane and then in am other adjacent pore in the inner membrane.

Question 5

What causes pyruvate dehydrgenase deficiency?

Answer 5

Reduced protein uptake into the mitochondrial membrane.

Question 6

How are proteins imported to the nucleus?

Answer 6

Importin binds cargo containing a nuclear localisation signal in the cytosol, and inside the nucleus ron-GTP binds to importin which causes a conformational change and removes protein, and ron-GTP is recycled into the cytoplasm.

Question 7

What is the PTS1 directed import of peroxisomal matrix proteins?

Answer 7

Peroxisomal input receptor binds with PTS, and the peroxos,al protein remains folded and the receptor integrates with and opens the translocon, the targeting sequence dissociates from receptor where it is returned to the cytoplasm.

Question 8

How are proteins targeted to the ER Lumen?

Answer 8

SRP attaches to the Ribosome and Protein, which attaches to a SRP receptor on the ER membrane, GTP is hydrolysed opening the translocon and the protein is synthesised through the translocon, and a signal peptidase cleaves the signal, and once it has finished the SRP dissociates.

Question 9

How are proteins inserted into the ER membrane?

Answer 9

Protein continues to be synthesised through the open translocon (starts in much the same way as a secretory particle), and a signal peptidase cleaves signal as usual, until a hydrophobic stop transfer sequence is reaches, sticking the protein in the cell membrane and synthesis of protein continues on the other side.

Question 10

What is the importance of peptidyl-proyl isomerases?

Answer 10

Accelerate the interconversion of cis and trans forms of proline residues, which needs to occur to facilitate the folding reactions for some proteins.

Question 11

What is the enzyme involved in disulphide bond formation?

Answer 11

Protein disulphide isomerase

Question 12

How is it ensured valuable enzymes stay in the ER lumen?

Answer 12

Retrograde transport.

Question 13

What is the receptor that ensure retrograde transport?

Answer 13

KDEL receptors

Question 14

What is the importance of chaperone proteins?

Answer 14

Attempt to correct folding, such as BiP which binds to exposed amino acids sequences normally buried and calnexin and calreticulin binding to olgisacchrides on incompletely folded protein.

Question 15

How are lysomal proteins delivered to lysosomes?

Answer 15

By the use of mannose 6phosphate, which is added in Golgi by the enzyme n-acetyl glucosamine and binds to a m6p receptor, which pinches off in a vesicle in a clathrin coat to go to a lysosome.

Question 16

What is the difference between O and N linked glycoslation?

Answer 16

Both occur in the ER, but sugar is bonded to an NH2 in asparagine side chain in n linked glycoslation, whereas the sugar is bonded to an-OH of a serine or theronine residue in o-linked glycoslation.

Question 17

What is the importance of glycoslation?

Answer 17

Allows correct protein folding, and protein stability as well as cell signalling.

Question 18

What is required for protein sorting?

Answer 18

A signal, intrinsic to the protein, a translocation machinery and energy to transfer the protein to its new place.

Question 19

What are the two two types of protein secretion?

Answer 19

Regulated and constitutive secretion.

Question 20

Where is type II collagen commonly found?

Answer 20

Cartilage

Question 21

What are the features of tropocollagen?

Answer 21

3 polypeptides each about 1000 amino acids long, right handed triple helix (which is non compressible, non extensible and has a high tensile strength), glycine in every third position and also regularly contains proline or hydroxyproline residues.

Question 22

What modifications does collagen undergo in the endoplasmic reticulum?

Answer 22

Synthesis and entry of chain through endoplasmic reticulum, cleavage of signal peptide, hydroxylation of selected proline and lysine residues, addition of n-linked oligosaccharides, addition of galactose to hydroxylysine residues, formation of disulphide bonds, formation of triple helix with extra lengths of protein at the end.

Question 23

What modifications does collagen undergo in the cytosol?

Answer 23

Removal on N and C terminal polypeptides, lateral association of collagen molecules by covalent crosslinking and aggregation of fibrils.

Question 24

What enzyme is used to hydroxylate the proline residues in collagen?

Answer 24

Prolyl hydroxylase

Question 25

What is the importance of the hydroxylation of the proline residues?

Answer 25

Produces more hydrogen bonds which stabilises the triple helix.

Question 26

What post translational modifications does insulin undergo?

Answer 26

Specific folding which is stabilised by disulphide bonds, and connecting peptides being removed to leave two chains

Question 27

What are the three enzymes required in the post translational modifications of insulin?

Answer 27

pC3 endoportease, PC2 endoprotease, carboxy peptidase

Question 28

Why is proteolytic processing so common in the secretory pathway?

Answer 28

Some secreted proteins, such as hydrolytic enzymes, would be destructive inside the cell, can give rise to very small proteins that would be too short to enter the endoplasmic reticulum, and multiple bioactive products can be produced from the same proteins.