Harrold Review of Bacterial Protein Synthesis

Stage 1 of Bacterial Protein Synthesis

Formation of an initiation complex

• Involves the combination of ribosomal, messenger, and transfer RNA molecules

Structure of a bacterial ribosome

Catalytic 70S unit

• 50S subunit

  • Contains a key 23S subunit

• 30S subunit

  • Contains a key 16S subunit

The first amino acid in most bacterial proteins is _____

Formyl methionine

The 50S subunit of a bacterial ribosome contains two sites for tRNA molecules:

P Site → the site of the growing peptide chain and the site t which the initial amino acid binds

A Site → the ribosomal acceptor site for the tRNA corresponding to the next amino acid to be added to the protein

Stage 2 of Bacterial Protein Synthesis

Elongation of the peptide chain in a three-step process:

• The tRNA for the next amino acid to be added binds to the A site

• Peptidyl transferase catalyzes the formation of a peptide bond between the amino group of the AA in the A site with the carbonyl group of the AA in the P site

• Movement of the ribosomes occurs in order for protein synthesis to continue → known as translocation

  • The empty tRNA is released from the P site

  • The growing peptide chain moves from the A site to the P site

  • The mRNA/rRNA interface moves to the next codon

Stage 3 of Bacterial Protein Synthesis

The termination codons UAA, UGA, and UAG signal the end of protein synthesis → when one of these codons appears in the A site, tRNA does not bind → the ribosome binds a termination release factor, which recognizes termination codons → “everything dissociates:”

• Peptidyl transferase releases the protein

• The empty tRNA is released from the P site

• THe mRNA detaches from the rRNA

• The 70S unit dissociates into 30S and 50S subunits

How do inhibitors of bacterial protein synthesis function?

Bind to the 30S and/or 50S rRNA subunits and affect one or more steps of protein synthesis

Can be classified as bacteriostatic (most) or bacteriocidal (aminoglycosides)

• Some are bacteriostatic at lower doses and bacteriocidal at higher doses

Why are most inhibitors of bacterial protein synthesis selectively toxic to bacteria?

Different ribosomes

• Bacteria use a 30S + 50S to form a 70S ribosome complex

• Human cells use a 40S + 60S to form a 80S ribosome complex