Protein translation and post-translational modification

Question 1

Draw the typical structure of mRNA

Answer 1

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Question 2

What is the sequence for a start codon and what amino acid does it code for?

Answer 2

AUG and Methionine

Question 3

Draw a diagram showing how an amino acid becomes attached to tRNA.? Include the name of the enzyme

Answer 3

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Question 4

List the steps for Initiation

Answer 4

Step 1: dissociation of ribosome subunits (eukaryotic 40S + 60S)

Step 2: assembly of preinitiation complex containing Met-tRNA + eIFs + 40S subunit

Step 3: binding of mRNA to preinitiation complex

Step 4: binding of 60S subunit

Question 5

Draw a diagram illustrating the steps for initiation

Answer 5

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Question 6

List the steps for Elongation?

Answer 6

Step 1: binding of new tRNA carrying second amino acid to “amino acyl” (A) site

Step 2: catalysis of peptide bond formation between the two amino acids by peptidyl transferase (PT) on the large ribosomal subunit

Step 3: translocation of tRNA to P site and dissociation of first tRNA from E site

Question 7

Name the 3 sites on a ribosome

Answer 7

E P A add pic

Question 8

Draw a diagram illustrating the steps for Elongation

Answer 8

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Question 9

List the steps for Termination

Answer 9

Step 1: recognition of stop codon Step 2: release of peptide chain Step 3: dissociation of release factors and ribosomes add pic

Question 10

Explain why some antibiotics inhibit protein synthesis in prokaryotes but not in eukaryotes?

Answer 10

Translational machinery is complex, easily disrupted – common target for antibiotics Antibiotics exploit differences between prokaryotic and eukaryotic ribosomes and translation factors

Question 11

What is a signal sequence?

Answer 11

First 20-24 amino acids = “signal sequence” (enriched in hydrophobic amino acids, e.g. Leu, Ile, Phe, Trp, Tyr, Ala)

Question 12

Synthesis of proteins destined for the secretory pathway to the cell surface occurs on RER in the following stages: List these stages.

Answer 12

Step 1: Recognition of hydrophobic signal sequence by signal recognition particals (SRP)

Step 2: Binding of SRP to a receptor on the RER surface, translation resumes

Step 3: Translocation of growing peptide into the lumen of RER

Step 4: cleavage of signal sequence and protein folding

Transmembrance proteins have additional hydrophobic sequences that stick in the membrane of the RER

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Question 13

Summarise the ways in which newly-synthesised proteins can be post-translationally modified

Answer 13

• Disulphide bond formation (e.g. insulin) • Proteolytic cleavage (e.g. insulin -> A and B chains) • Addition of carbohydrate (Glycosylation) • Addition of phosphate (Phosphorylation) • Addition of lipid groups (Prenylation, Acylation) • Hydroxylation (e.g. Collagen; Leitinger lecture)