Protein Synthesis (Translation)
Protein synthesis is the process of translating the genetic code (mRNA) into a specific sequence of amino acids. It occurs at ribosomes and requires mRNA, tRNA, amino acids, ribosomes, initiation/elongation/termination factors, and GTP.
The Genetic Code
- Triplet codons on mRNA specify amino acids. 64 codons total: 61 sense codons + 3 stop codons (UAA, UAG, UGA).
- Degenerate (redundant): Multiple codons for the same amino acid (especially at 3rd position — wobble)
- Non-overlapping, commaless, read 5'→3'
- AUG = Start codon (Met in eukaryotes, fMet in prokaryotes)
tRNA Structure
Cloverleaf secondary structure with 4 arms. The anticodon loop base-pairs with mRNA codon (antiparallel). The 3' CCA end (acceptor stem) carries the amino acid. Aminoacyl-tRNA Synthetases charge specific tRNAs with their correct amino acid (uses ATP; hydrolysis to AMP + PPi — equivalent to 2 ATP); this is the fidelity step.
Phases of Translation
Initiation (GTP used):
- 43S preinitiation complex forms (small 40S subunit + Met-tRNAi + initiation factors eIF2·GTP)
- Recruited to mRNA 5' cap by eIF4F cap-binding complex; scans 5'→3' for AUG
- Large 60S joins (eIF5 hydrolyzes GTP) → 80S ribosome
Elongation (GTP used for each amino acid added):
- A-site: Aminoacyl-tRNA binds (EF-1α·GTP in eukaryotes)
- P-site: Peptide chain grows; Peptidyl transferase (rRNA ribozyme) forms peptide bond
- E-site: Discharged tRNA exits
- Translocation: Ribosome moves 3 nt in 5'→3' direction (EF-2·GTP)
Termination: Stop codon (UAA/UAG/UGA) recognized by Release Factors (eRF1+eRF3) → peptide released → ribosome dissociates.
Post-Translational Modifications
- Glycosylation: Addition of carbohydrates (N-linked in ER; O-linked in Golgi)
- Hydroxylation: Pro and Lys in collagen (requires Vitamin C)
- Phosphorylation: Ser, Thr, Tyr by kinases — regulation
- Ubiquitination: Tags proteins for proteasomal degradation
- Acetylation, Methylation: Histones — gene regulation
- Signal sequences target proteins to ER, mitochondria, peroxisomes (SRP recognition)
Antibiotics & Protein Synthesis Inhibitors
- Prokaryotic 30S inhibitors: Aminoglycosides (Gentamicin — misreading), Tetracyclines (block A-site)
- Prokaryotic 50S inhibitors: Chloramphenicol (peptidyl transferase — aplastic anemia), Macrolides (translocation — erythromycin), Linezolid, Clindamycin
- Eukaryotic 80S inhibitors: Diphtheria toxin (inhibits EF-2 by ADP-ribosylation), Cycloheximide, Ricin (depurinates 28S rRNA)
Protein Folding & Chaperones
Chaperones (Hsp70, Hsp90, GroEL/GroES) assist proper protein folding by preventing aggregation. Misfolded proteins → unfolded protein response (UPR) in ER → if unresolvable → apoptosis. Prion diseases involve misfolded PrPc → PrPsc (β-sheet conformation) that templates misfolding of normal proteins — fatal neurodegenerative disorders.