Post-Translocational Modifications

Question 1

What is a proteolytic cleavage?

Answer 1

Breaking peptide bonds to remove part of a protein - to make the protein inactive or to unsure it fold properly.

Question 2

What is chemical modification?

Answer 2

The addition of a functional group to amino acid residues.

Question 3

Where are proteins originally synthesised?

Answer 3

On the ribosomes of the endoplasmic reticulum.

Question 4

After synthesis on the ribosomes, where are some locations proteins travel to?

Answer 4

Cytoplasm
Nucleus
Peroxisomes
Mitochrondria

Question 5

What four components are needed for protein sorting?

Answer 5

A signal - ‘take me to the…’ - like an address
A receptor - recognises the signal
A translocation machinery
Energy to transfer

Question 6

What signal is used to send the proteins to the peroxisomes?

Answer 6

A peroxisomal targeting sequence (PTS)
This is serine - lysine - leucine (SKL)
Present on the C terminus

Question 7

What receptor is used when sending proteins to the peroxisomes?

Answer 7

The receptor Pex5

This binds to the protein cargo in the cytoplasm

Question 8

What translocational machinery to used to take proteins to the peroxisomes?

Answer 8

13 PEX proteins that make up the transport channel

They bind the Pex5-cargo complex (Receptor + protein + translocational machinery)

Question 9

What energy is used when taking the proteins to the peroxisomes and what is it needed for?

Answer 9

ATP hydrolysis

Not needed for protein to get across receptor, but to recycle receptor.

Question 10

The the various steps from ER to peroxisomes of a protein.

Answer 10

Peroxisomal targeting sequence (PTS) of serine - lysine - leucine (SKL) present on the C terminus signals to be taken to the peroxisomes.
The translocational machinery of 13 pex proteins bind with the protein ‘cargo’
The receptor Pex5 pulls away from the peroxisomal membrane and binds to the cargo.
Brings cargo to the peroxisomal surface, releasing cargo across the membrane.
ATP hydrolysis is sued to recycle the receptor, so it can bind to another cargo in the cytoplasm.

Question 11

What is Zellweger syndrome?

Answer 11

Reduction is functioning peroxisomes, unable to break down fatty acids.
This accumulations of fats causes neural impairment.

Question 12

What is Rhizomelic chondrodysplasia punctata?

Answer 12

A mutation in the peroxisomes means they cant break down plasmalogens (phospholipid).
Results in skeletal abnormalities and respiratory problems.

Question 13

What three types of proteins are sent for secretion?

Answer 13

Extracellular proteins, membrane proteins, vesicular proteins.

Question 14

What are two types of secretions in the cell? Example?

Answer 14

Constitutive - going all the time e.g. Collagen.

Regulated - when needed e.g. Endocrine, exocrine, neurocrine cells.

Question 15

What signal is needed to send a protein for secretion?

Answer 15

An N terminal amino acid sequence 5-30 amino acids in length.
Unable to form an alpha helix which is removed after synthesis e.g. Preproinsulin and proinsulin

Question 16

What receptor is used to send proteins to be secreted?

Answer 16

Signal particle Receptor

Question 17

What translocational machinery to used to send proteins for secretion?

Answer 17

Signal recognition particle (SRP).

Composed of 6 proteins and short piece of RNA.

Question 18

What is the mechanism of protein from ribsomes to the ER to be used as a secretion?

Answer 18

The signal recognition particle binds to the protein, recognising its amino acid signal.
Once attached to the protein, it brings it to the ER surface, binds to the SRP receptor and releases the protein inside.
The SRP is then released and the signal sequence on the protein is cleaved by a signal peptide.

Question 19

Name some functions of the ER?

Answer 19

Insertion of proteins into membranes
Specific proteolytic cleavage
Glycosylation 
Formation of S-S bonds
Hydroxylation
Assembly of multisubunit proteins 
Proper folding of proteins

Question 20

What is N-linked glycosylation and why is it important?

Answer 20

A sugar is added on an asparagine side group (reactions involves an amino group NH2)
This is key for correct protein folding and stability.

Question 21

What is a disulphide / SS bond?

Answer 21

A bond between two cysteine residues

This determines the tertiary structure of the protein

Question 22

Is misfolding cannot be correcting what two things will happen to the protein? Give a disease example for each.

Answer 22

Protein is returned to the cytosine for degradation e.g. Cystic Fibrosis, Alzheimars
Proteins accumulate to toxic levels in the ER e.g. Parkinson’s, Charcot-Marie-Tooth Syndrome

Question 23

What are the five layers of the Golgi complex and their position?

Answer 23

From the ER going away from:
Cis-Golgi network
Cis Cisterna
Medial Cisterna
Trans Cisterna
Trans Golgi network

Question 24

What is O-linked glycosylation?

Answer 24

The attachment of a sugar to an -OH group - serine and theomine.
Occurs in the Golgi and is important in proteoglycans to produce extracellular matrix and mucus secretions.

Question 25

How does preproinsulin get to its ‘insulin’ form?

Answer 25

Before entry into the ER, the signal peptide of preproinsulin is cleaved to create proinsulin.
In the ER, disulphide (S-S) bonds are formed and the B chain is cleaved off to create insulin, which then enters the Golgi.

Question 26

Why is proteolytic processing complex?

Answer 26

Because there are different amounts of processing enzymes in different cell locations.