Protein Quality Control- Lecture 32

Question 1

The final structures of proteins are a consequence of the _____ adopting the _____.

Answer 1

primary amino acid sequence

secondary structures

Question 2

What interactions help to arrange the secondary structures into the most stable form of tertiary structures?

Answer 2

H bonds
salt bridges
Van der Waals

Question 3

Proteins are easily damaged by forces that are _____ or _____.

Answer 3

physical (eg. heat)

chemical (eg oxidation)

Question 4

Damage to proteins can result in ______.

Answer 4

degradation, aggregation, or misfolding

Question 5

What is proteostasis?

Answer 5

the process of cellular quality control that continually monitors the proteome for misfolded or aggregated proteins and target them for degradation and disposal

Question 6

Proteins fold by the principle of _____.

Answer 6

self-assembly

Question 7

Why were the results of Anfisen’s experiment not suitable in physiological settings?

Answer 7

higher temperatures and crowding result in aggregations rather than folding

Question 8

What are the phases of protein folding?

Answer 8

Burst (0-5 us)
Intermediate (5-100 us)
Rate limiting step (100 us- several minutes)

Question 9

What occurs in the bursting step of protein folding?

Answer 9

hydrophobic collapse and secondary structure formation

Question 10

What occurs in the intermediate step of protein folding?

Answer 10

tertiary organization (molten globuole)

Question 11

What occurs in the rate limiting step of protein folding?

Answer 11

final few salt bridges and amino acids re-arrange to lowest free energy state

Question 12

What do molecular chaperone proteins do?

Answer 12

bind weakly to hydrophobic amino acids of proteins, prevent proteins from aggregating, promote protein folding
dependent on ATP binding, hydrolysis, and nucleotide exchange
DON’T increase rate of folding, just yield

Question 13

What are molecular chaperone proteins?

Answer 13

proteins that bind to and stabilize an otherwise unstable conformer of another protein and facilitate its correct fate in vivo

Question 14

What are the families of molecular chaperones?

Answer 14

Hsp70, Hsp60 (chaperonin), and Hsp90

Question 15

Describe the structure of chaperone proteins using bacterial Chaperonin as an example.

Answer 15

homo-oligomer of 14 subunits arranged into 2 stacked rings each of 7 subunits structured like a donut with 7-fold axis and a chamber as well as a smaller lid structure

Question 16

Describe the process of protein folding using chaperone proteins (and the example of Chaperonin).

Answer 16

unfolded proteins bind to rim of barrel –> displaced into cavity by lid structure –> protein folds in sequestered and protected space of chamber –> lid dissociates due to changes in conformation of the large subunit as ATP is hydrolyzed –> folded protein released into cytosol

Question 17

Molecular chaperone gene transcription is controlled by _____ which responds to the presence of _____.

Answer 17

Hsf (heat shock transcription factor)

unfolded protein or heat shock or other types of proteotoxic stress

Question 18

Describe the pathway of Hsf.

Answer 18

stress –> activation of Hsf –> stimulation of expression of molecular chaperones

Question 19

What enzymes are required for the ubiquitin proteasome pathway?

Answer 19

E1 (activating enzyme)
E2 (conjugate enzyme)
E3 (ubiquitin ligases)

Question 20

What are the components of a proteasome (26S complex)?

Answer 20

central catalytic core (20S) and a regulatory cap (19S)

Question 21

What are the activities of eukaryotic proteasomes?

Answer 21

chrmotrypsin-like (cleaves after hydrophobic amino acids)
trypsin-like (cleaves after basic amino acids)
peptidyl-glutamyl peptide hydrolyzing activity (cleaves after acidic amino acids)

Question 22

What do the subunits of the 19S complex do?

Answer 22

recognize ubiqitinylated substrates, deubiquitinylate the substrates, and prepare them for proteolysis via protein unfolding if necessary

Question 23

What is the pathway for the attachment of ubiquitin?

Answer 23

E1 activates the C-terminal glycine by forming ubiquitin-adenylate and transferring activated ubiquitin to a thiol site in E1 –> E2 enzymes accept ubiquitin from E1 and transfer it to the protein substrate in a reaction requiring E3
this is done until four ubiquitins are present on the substrate

Question 24

___ occur when misfolded proteins overwhelm the ubiquitin /proteasome pathway.

Answer 24

Aggregates

Question 25

____ are proteins that form toxic amyloids that are transmissible.

Answer 25

Prions