Protein Processing and Targeting

Question 1

Name post-translational protein modifications

Answer 1

• Proteolytic cleavage

- Chemical modification

Question 2

What 4 elements are needed in protein sorting?

Answer 2

• Signal
• Receptor
• Translocation machinery
• Energy

Question 3

What are the 2 types of cellular secretion? Describe each

Answer 3

Constitutive - continuous and slow (saliva)

Regulated - endocrine, exocrine and neurocrine

Question 4

Describe 2 characteristics of a protein signal sequence

Answer 4

• Starts at N-terminal
• 5-30 amino acids long
• Central region has lots of hydrophobic residues
• Able to form alpha helix
• Removed during processing after use

Question 5

What is a protein signal recognition particle?

Answer 5

• Binds Signal peptides on proteins destined for ER

- Recognises signal peptide and ribosome

Question 6

How does the signal recognition particle stop and re-start protein synthesis?

Answer 6

• Stops protein synthesis by binding to the ribosome

- Re-starts when it is released after binding to SRPB receptor

Question 7

Which enzyme cleaves proteins to release them into the ER lumen?

Answer 7

Signal peptidase

Question 8

State 4 functions of the endoplasmic reticulum in relation to protein synthesis

Answer 8

• Insertion of membrane proteins
• Proteolytic cleavage to make mature proteins
• Glycosylation
• Formation of disulphide bonds
• Proper folding of proteins
• Assembly of multi-subunit proteins
• Hydroxylation of certain residues

Question 9

What is N-linked glycosylation? State 2 reasons why it is important

Answer 9

• Addition of sugar/carbohydrate to an amino acid
• Ensures correct protein folding
• Stabilises proteins
• Facilitates molecular interaction

Question 10

How can protein folding problems be rectified?

Answer 10

• Corrected by ER chaperons proteins
• Calnexun and Calreticulin enable and check protein folding
• ER monitors extent of protein misfiring

Question 11

Where are disulphide bonds formed and which enzyme enables this?

Answer 11

• ER lumen

- Protein Disulphide Isomerase

Question 12

What 3 protein modifications occur in the Golgi apparatus?

Answer 12

• Phosphorylation
• Glycosylation
• Sulfation

Question 13

What is the basic unit of a collagen fibre?

Answer 13

Tropocollagen

Question 14

Describe the structure of a collagen unit

Answer 14

• 3 polypeptide alpha chain in a triple helix
• Glycine in every third position on each alpha chain
• Glycine sits in the middle of the collagen fibre

Question 15

Name 3 characteristics of collagen fibres

Answer 15

• Non-extendable
• Non-compressible
• High tensile strength
• Contains large amounts of proline and hydroxyproline
• H-bonds between alpha chains

Question 16

Which enzyme hydroxylates proline and lysine residues? What substances does it require to work?

Answer 16

• Prolyl Hydroxylase

- Vitamin C and Fe2+

Question 17

What is the effect of hydroxylation on collagen in terms of bonds?

Answer 17

Increases H bonds and so strength

Question 18

What kind of bond joins collagen chains?

Answer 18

Disulphide bonds

Question 19

What occurs in the Golgi apparatus prior to vesicular secretion?

Answer 19

• Further glycosylation

- Chains completed by addition of glucose

Question 20

What is the name and effect of the enzyme that converts procollagen to tropocollagen?

Answer 20

• Procollagen peptidase
• Cleaves N and C terminal peptides
• Extracellular process

Question 21

What interaction allows collagen fibre formation?

Answer 21

Lateral association with the formation of covalent cross-linking