Protein Translation and Post Translational Modifications

Question 1

How many subunits in the ribosome?

Answer 1

2

Question 2

What is next to the 5’ cap and polyA tail?

Answer 2

A UTR (untranslated region)

Question 3

What are the stop codons?

Answer 3

UAA, UAG and UGA

Question 4

What is special about methionine?

Answer 4

Where translation starts

Question 5

What is methionine’s codon?

Answer 5

AUG

Question 6

How are ribosome sizes measured?

Answer 6

Svedberg units

Question 7

What enzyme attaches amino acids to tRNA?

Answer 7

Aminoacyl tRNA synthetase

Question 8

Describe the mechanism of aminoacyl tRNA synthetase

Answer 8

Uses ATP to form an intermediate known as adenylated amino acid, which has the enzyme bound to AMP which is bound to the correct amino acid. This then binds to the tRNA. The AMP then comes off and the enzyme dissociates. The amino acid is transferred to the 3’ hydroxyl of the tRNA.

Question 9

Explain initiation of transcription

Answer 9

eIF-2 binds to Met-tRNA and a molecule of GTP. This forms a complex in the cytoplasm with the 40S subunit. eIFE and eIF4G binds to the 5’ cap of mRNA and the preinitiation complex this. Once this occurs the preinitiation complex travels down the mRNA looking for AUG (methionine’s codon). When this is the GTP on eIF2 is hydrolysed to GDP and Pi which provides energy to ensure correct base pairing. Once hydrolysed, the conformational change allows 60S subunit to bind. The IF’s and bound GDP then dissociates

Question 10

What is the preinitiation complex?

Answer 10

Met-tRNA, eIF-2 and 40S subunit.

Question 11

Which site does the first tRNA bind to, P or A?

Answer 11

P

Question 12

Which site does the second tRNA bind to, P or A?

Answer 12

A

Question 13

Describe elongation

Answer 13

A new tRNA binds to the A site, peptidyl transferase catalyses the peptide bond formation between the two amino acids. The second tRNA translocates to the P site and the first tRNA dissociates.

Question 14

Describe termination

Answer 14

When the ribosome moves over a stop codon, a group of proteins known as release factors bind to the empty A site. Once they have bound, peptidyl transferase hydrolyses the link, (transfers to polypeptide to water) and releases the ribosome. Everything dissociates

Question 15

What do antibiotics target a lot?

Answer 15

Protein synthesis in prokaryotes

Question 16

What type of proteins are translated on the rER? How are they singled out?

Answer 16

Those destined to be secreted or transmembrane. Have a special signal sequence 20-24 amino acids long at the start.

Question 17

What type of amino acids are in the first part of the sequence of transmembrane proteins? Why?

Answer 17

Hydrophobic, so they have an affinity for lipids

Question 18

Explain how proteins translated in the rER are moved there

Answer 18

Signal sequence on mRNA is bound to by a signal recognition particle (SRP) halting translation. The SRP then binds to a SRP receptor on the rER which then resumes translation. The bound SRP then triggers the assembly of a protein channel within the membrane of the rER which threads the polypeptide through. Once the amino acid sequence is in the rER the signal sequence is chopped off by signal peptidase and is degraded.

Question 19

What happens in the rER for proteins?

Answer 19

They’re folded

Question 20

Where are signal peptides in proteins (rER and non-rER)?

Answer 20

Anywhere in non-rER and only at the start for rER

Question 21

Describe the post translational modifications of insulin

Answer 21

Signal sequence is removed and degraded, 3 disulphide bonds form between cysteine residues to form proinsulin, then is cleaved in two positions to release the C chain. Leaves you with an A and B chain held together by disulphide bonds.