Protein synthesis (translation) is the process by which ribosomes decode mRNA into a functional polypeptide chain. This fundamental cellular process occurs in the cytoplasm, involving initiation, elongation, and termination phases — each tightly regulated and clinically relevant.
Initiation of Translation
- Eukaryotic initiation factors (eIFs) identify the 5′ cap of mRNA.
- eIFs help assemble the 40S ribosomal subunit with the initiator tRNA (Met-tRNA).
- When the mRNA and 60S subunit join, eIFs are released, forming the 80S ribosome complex (requires GTP).
| Organism | Ribosomal Subunits | Complete Ribosome |
|---|---|---|
| Eukaryotes | 40S + 60S | 80S (Even) |
| Prokaryotes | 30S + 50S | 70S (Prime) |
Elongation
- Aminoacyl-tRNA enters the A site (except for initiator Met-tRNA, which starts at the P site).
- rRNA (ribozyme) catalyzes peptide bond formation, transferring the growing peptide to the tRNA in the A site.
- Translocation: Ribosome moves 3 nucleotides forward toward the 3′ end; peptidyl-tRNA shifts to the P site (requires GTP).
| Site | Function | Mnemonic |
|---|---|---|
| A site | Incoming Aminoacyl-tRNA | A → Arrival |
| P site | Holds a Peptide chain | P → Peptide |
| E site | Exit site for Empty tRNA | E → Exit |
Termination
- Eukaryotic release factors (eRFs) recognize stop codons (UAA, UAG, UGA).
- GTP hydrolysis releases the newly synthesized polypeptide from the ribosome.
Prokaryotic Specifics
- Shine-Dalgarno sequence: Ribosomal binding site in prokaryotic mRNA, recognized by 16S rRNA in the 30S subunit.
- Aligns the ribosome with the start codon (AUG) for accurate initiation.
| Concept | Eukaryotes | Prokaryotes |
|---|---|---|
| Ribosome | 80S | 70S |
| Initiation site | 5′ Cap | Shine-Dalgarno sequence |
| Start codon | AUG (Met) | AUG (fMet) |
Clinical Correlations
- Diphtheria toxin and Pseudomonas exotoxin A inhibit elongation factor-2 (EF-2) → halts translation, causing cell death.
- Antibiotics target bacterial ribosomes selectively (e.g., aminoglycosides, tetracyclines).
Key Points to Remember
- ATP → “tRNA Activation”
- GTP → “tRNA Going places (Translocation & Termination)”
- Translation proceeds N-terminus → C-terminus
- Different subunit sizes allow antibiotics to selectively target bacterial ribosomes.
Learning Objective
By the end of this lesson, the student should be able to:
Describe the steps of eukaryotic and prokaryotic protein synthesis, identify key enzymes and energy sources involved, and explain how bacterial toxins interfere with translation.
