Post translational modification of proteins and protein targeting

Question 1

What is constitutive secretion?

Answer 1

constant maintain basal level

e.g albumin

Question 2

What is regulated secretion?

Answer 2

only when stimulated

endo/ exo and neuro crine

Question 3

How are proteins targeted?

Answer 3

contain intrinsic signals that govern their transport and localisation in the cell

Question 4

Where are proteins destined for cytosol and post translational import to organelles translated?

Answer 4

free ribsosomes

Question 5

Where are proteins destined for secretion or membrane translated?

Answer 5

ribosomes on rough endoplasmic reticulum

Question 6

What are the requirements for protein sorting? explain each one.

Answer 6

signal - n terminal sequence
receptor to recognise signal - signal recognition particle SRP bind ribosome and protein stopping protein synthesis
translocation machinery- SRP bind SRP receptor on ER membrane pore open into ER
energy to transfer protein to new place- GTP hydrolysed and SRP released protein synthesis resumed

Question 7

What enzyme may remove the signal sequence of the n terminal?

Answer 7

signal peptidase

Question 8

Name the three main functions of the RER

Answer 8

n-linked glycosylation
formation of disulphide bonds
correct protein folding mediated by chaperones

Question 9

Where aa side chain is n-linked glycosylated?

Answer 9

asparagine

Question 10

What is glycosylation needed for?

Answer 10

folding
stability
interactions

Question 11

What enzyme is responsible for formation of S-S bonds? Where do S-S bonds form?

Answer 11

protein disulphide isomerase

between cysteine residues

Question 12

What diseases occur if there is a a deficiency in glycosylation?

Answer 12

congenital disorders of glycosylation

Question 13

What happens if protein mis folds?

Answer 13

may degrade in cytosol

may accumulate in ER–> toxic level e.g. Fabry disease

Question 14

Outline the structure of collagen

Answer 14

right handed triple helix made from three polypeptide chains - h bonds stabilise this formation
glycine every third residue- glycine R group small enough to fit helix formation
Other two residues likely to be proline or hydroxyproline
collagen triple gelix crosslink with each other to form fibrils

Question 15

When does pre pro alpha chain become a pro alpha chain?

Answer 15

once signal sequence has been cleaved by signal peptidase

Question 16

Which residues are hydroxylated and by which enzyme?

Answer 16

proline and lysine

prolyl hydroxylase

Question 17

When are the N and C terminals removed form the propeptides? what is produced? which enzyme is involved?

Answer 17

extracellularly
tropcollagen
procollagen peptidases

Question 18

Why are the terminals not cleaved within the cell?

Answer 18

After cleavage fibrils form if this occurred within the cell then the cell would explode

Question 19

Which enzyme is responsible for the formation of crosslinks between the triple helices to form fibrils?

Answer 19

lysyl oxidase

Question 20

Why is weakened connective tissue seen in scurvy?

Answer 20

weakened collagen structure due to reduced H bond formation
this is because H bond formation reliant on hydroxylation of alpha chains by proylyl hydroxylase
the enzyme proylyl hydroxylase requires vitamin C to function

Question 21

What does Ehlers-Danlos syndrome result from?

Answer 21

lysyl oxidase deficiency so covalent bonds cannot form between triple helices