Updated: 5/1/2020

Protein Synthesis

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Ribosome
  •  Structure
    • eukaryotes
      • small subunit (40S)
        • synthesized in the nucleus
      • large subunit (60S)
        • synthesized in the nucleolus
      • total = 80S
        • S = sedimentation
    • prokaryotes
      • small subunit (30S) + large subunit (50S) → 70S
  • Catalytic sites 
    • A (acceptor) site
      • accepts charged aa-tRNA 
    • P (peptidyl) site 
      • holds the growing peptide chain and transfers to the aa-tRNA in the A site 
    • E (exit) site
      • allows tRNA to exit the ribosome after releasing its amino acid to the aa-peptide chain
  • Forms
    • polysomes
      • single mRNA being translated simultaneously by several ribosomes
      • can be
        • free
          • proteins for nucleus or mitochondria
        • membrane bound
          • found on rough ER 
          • proteins for secretion or membrane insertion
Steps
  • Initiation 
    • initiation complex generated from
      • small ribosomal subunit
      • mRNA
        • 5'-cap (eukaryotes)
        • Shine-Dalgarno sequence (prokaryotes) 
      • initiation aa-tRNA
        • met-tRNA (eukaryotes)
        • fmet-tRNA (prokaryotes)
      • initiation factors (IFs)
        • help in assembly
        • released when mRNA joins the ribosomal subunit
    • once assembled, recruits the large ribosomal subunit
      • initiation aa-tRNA occupies the P-site
        • all other aa-tRNAs bind the A-site
      • the A-site is empty
  • Elongation
    • aminoacyl-tRNA binds to A site
      • consumes 1 GTP
    • peptidyl-bond formation
      • peptidyl transferase reaction catalyzed by ribosomal rRNA ("ribozyme")
      • transfers growing polypeptide to amino acid in the A site
    • translocation 
      • ribosome moves 3 nucleotides at a time in a 5' to 3' direction
      • moves peptidyl RNA from A site to P site
      • moves empty tRNA from P site to E site for exit
      • A site now empty and ready to accept next aa-tRNA
      • catalyzed by elongation factor-2 (eEF-2)
        • inactivated via ADP-ribosylation by bacterial toxins (Pseudomonas and Diphtheria)
      • consumes 1 GTP
    • cycle is repeated for each amino acid addition to the polypeptide
    • Wobble phenomenon 
      • The anticodon of certain tRNA molecules can bind to multiple codons
      • The first two nucleotides of the codon adequately specify which tRNA (and amino acid) binds the codon, thereby permitting multiple nucleotides in the third position to bind the same tRNA. 
  • Termination
    • stop codon reached in mRNA
    • release factor binds mRNA
    • peptide bond is hydrolyzed and completed protein is released
    • ribosomal complex dissociates
  • Overall energy requirement
    • 4 high energy bonds
      • ATP to AMP for tRNA charging (2)
      • 2 GTP to GDP for elongation (2)
Pharmacologic Inhibition of Prokaryotic Protein Synthesis
  • Bind 30S
    • aminoglycosides
      • inhibit formation of the initiation complex
      • promotes misreading of mRNA
  • Bind 50S
    • chloramphenicol
      • inhibits peptidyl transferase
    • macrolides
      • inhibits translocation
    • clindamycin
      • inhibits translocation

References

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(M1.BC.14.70) The ribosome is an essential organelle that plays an important role in the synthesis of new proteins. The location of the ribosome within the cell can vary and provides a clue to the function of the particular cell. What ribosome location is essential for protein-secreting cells?

QID: 106487
1

Rough endoplasmic reticulum

100%

(28/28)

2

Smooth endoplasmic reticulum

0%

(0/28)

3

Cytosol

0%

(0/28)

4

Cell membrane

0%

(0/28)

5

Nucleus

0%

(0/28)

M 1 E

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(M1.BC.13.4) In translation, the wobble phenomenon is best illustrated by the fact that:

QID: 100069
1

A tRNA with the UUU anticodon can bind to either AAA or AAG codons

59%

(185/316)

2

There are more amino acids than possible codons

13%

(40/316)

3

The last nucleotide provides specificity for the given amino acid

16%

(51/316)

4

The genetic code is preserved without mutations

7%

(22/316)

5

Charged tRNA contains energy needed for peptide bonds to form

3%

(10/316)

M 1 E

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