10.9 Translation Inhibitors & Post-Translational Processing

10.9 Translation Inhibitors & Post-Translational Processing

4. Translation Inhibitors and Post-Translational Processing

Pharmacological Translation Inhibitors (Antibiotics & Toxins)

Because bacterial ribosomes (70S = 50S + 30S) differ structurally from eukaryotic cytosolic ribosomes (80S = 60S + 40S), antimicrobial agents selectively inhibit prokaryotic protein synthesis without halting host translation. (Note: the human mitochondrial ribosome is a 55S particle built from a 28S small subunit and a 39S large subunit — a bacteria-like ancestry that explains certain drug toxicities, such as chloramphenicol-induced bone marrow suppression and linezolid-associated cytopenias).

Bacterial Translation Targets:
   [30S Subunit Target]             [50S Subunit Target]
   - Aminoglycosides (Misreading)   - Chloramphenicol (Peptidyl transferase)
   - Tetracyclines (Block A-site)   - Macrolides (Block Translocation/Tunnel)
                                    - Clindamycin (Block Translocation)
                                    - Oxazolidinones (Block Initiation)

• Aminoglycosides (Gentamicin, Streptomycin, Tobramycin): Bind irreversibly to the bacterial 30S subunit; distort ribosomal decoding, causing codon misreading, incorporation of incorrect amino acids, and inhibition of initiation complex assembly.
• Tetracyclines (Doxycycline, Minocycline): Reversibly bind to the 30S subunit; sterically obstruct aminoacyl-tRNA binding at the A site, arresting peptide elongation.
• Chloramphenicol: Binds to the 50S subunit and directly inhibits peptidyl transferase ribozyme activity, preventing peptide bond formation.
• Macrolides (Erythromycin, Azithromycin, Clarithromycin): Bind reversibly to the 50S subunit near the peptide exit tunnel; block ribosomal translocation and arrest nascent chain egress.
• Lincosamides (Clindamycin): Bind an overlapping 50S site in the peptidyl transferase region and block translocation. Because macrolides and lincosamides share this footprint, a single bacterial resistance enzyme — the erm-encoded methyltransferase that methylates an adenine in 23S ribosomal RNA — confers cross-resistance to both classes at once. Passages often use this shared binding pocket to explain why resistance to one drug predicts resistance to the other.
• Oxazolidinones (Linezolid): Bind domain V of the 23S ribosomal RNA in the 50S peptidyl transferase centre, distorting transfer RNA positioning in the A and P sites and preventing assembly of a functional 70S initiation complex. Linezolid is the classic example of an inhibitor acting at initiation rather than elongation, and its resistance mutations map to the same 23S nucleotides that its footprint covers.
• Puromycin: Structural analog of the 3' end of tyrosyl-tRNA. Enters the ribosomal A site, accepts the nascent peptide chain forming a covalent peptidyl-puromycin bond, and causes premature chain termination in both prokaryotes and eukaryotes (used experimentally, not clinically).
• Diphtheria Toxin & Pseudomonas Exotoxin A: Bacterial AB toxins whose catalytic domain executes ADP-ribosylation of eukaryotic eEF-2 on a post-translationally modified histidine residue (diphthamide). Inactivating eEF-2 completely shuts down translocation in eukaryotic cells, causing cellular necrosis and pseudomembrane pharyngitis.
• Ricin & Shiga Toxin (Ribosome-Inactivating Proteins): These AB toxins take a different route to the same endpoint. Their catalytic A chain is an RNA N-glycosidase that removes a single adenine base from the conserved sarcin–ricin loop of the 28S ribosomal RNA in the eukaryotic 60S subunit. That one depurination destroys the elongation-factor binding surface, so eEF-1 and eEF-2 can no longer dock and elongation halts. Contrast the two toxin strategies on the exam: diphtheria toxin chemically modifies the factor (eEF-2), whereas ricin chemically modifies the ribosome (28S rRNA).

Post-Translational Modifications (PTMs) and Protein Trafficking

Following ribosomal synthesis, nascent polypeptide chains undergo covalent modifications and targeted folding to attain functional activity:

1. Signal Peptides and Translocation: Proteins destined for secretion, lysosomes, or the plasma membrane possess an N-terminal hydrophobic signal peptide (15–30 amino acids). The Signal Recognition Particle (SRP) binds the nascent signal peptide as it emerges from the ribosome, pauses translation, and docks the ribosome onto the translocon channel of the rough endoplasmic reticulum (RER). Translation resumes with the peptide threading into the ER lumen, where signal peptidase cleaves off the signal sequence.
2. Molecular Chaperones: Chaperones (such as Hsp70 and large cage-like chaperonins GroEL/ES and TRiC) bind exposed hydrophobic patches of unfolded nascent chains, preventing toxic aggregation and guiding folding into proper tertiary conformations via cycles of ATP hydrolysis.
3. Covalent Enzymatic Modifications:
   • Phosphorylation: Addition of phosphate to serine, threonine, or tyrosine hydroxyl groups by protein kinases (reversed by phosphatases), acting as a reversible regulatory on/off switch.
   • Glycosylation: Attachment of carbohydrate moieties. N-linked glycosylation initiates in the ER lumen (attached to the amide nitrogen of asparagine in Asn-X-Ser/Thr motifs); O-linked glycosylation occurs in the Golgi apparatus (attached to the hydroxyl oxygen of serine or threonine).
   • Ubiquitination and Proteasomal Degradation: Polyubiquitin chains are covalently linked to lysine residues of target proteins via a three-enzyme cascade (E1 activating, E2 conjugating, and E3 ligase enzymes). Polyubiquitinated proteins are recognized, unfolded, and degraded into oligopeptides by the barrel-shaped 26S proteasome.
   • Zymogen Cleavage: Inactive proenzymes (zymogens) are irreversibly activated by proteolytic cleavage of specific inhibitory pro-domains (e.g., conversion of trypsinogen to active trypsin by enteropeptidase in the duodenum, or activation of clotting factors in the coagulation cascade).
   • Lipidation: Covalent attachment of a lipid anchor tethers a soluble protein to a membrane leaflet — prenylation (farnesyl or geranylgeranyl groups added to C-terminal cysteines, as on Ras), N-myristoylation of an N-terminal glycine, and glycosylphosphatidylinositol (GPI) anchors added in the ER lumen. Anchoring is what converts a cytosolic signalling protein into a membrane-localised one, so a mutation that blocks lipidation mislocalises the protein without changing its amino acid sequence.

Two Worked Maturation Pathways the MCAT Reuses

Proinsulin to insulin. Insulin is the exam's canonical proprotein. Translation produces preproinsulin; signal peptidase removes the signal sequence in the ER lumen to give proinsulin (86 residues), which folds and forms its disulfide bonds. In the secretory granule, prohormone convertases (PC1/3, with PC2 contributing in rodent islets) cut at the two pairs of basic residues flanking the connecting segment, and carboxypeptidase E trims the leftover basic residues. The products are mature insulin — a 21-residue A chain disulfide-bonded to a 30-residue B chain — and the excised 31-residue C-peptide. Because C-peptide is released in equimolar amounts with insulin but is absent from injected insulin, a low C-peptide level alongside high circulating insulin is the classic passage signature of exogenous insulin administration rather than endogenous secretion.

Procollagen to collagen fibril. Collagen is the case where post-translational modification drives structure rather than activity. Inside the ER, prolyl-4-hydroxylase and lysyl hydroxylase hydroxylate proline and lysine residues; both are Fe(II)- and alpha-ketoglutarate-dependent dioxygenases that require ascorbate (vitamin C) to keep the active-site iron reduced. Hydroxyproline supplies the interchain hydrogen bonds that stabilise the triple helix at body temperature, which is why vitamin C deficiency (scurvy) produces fragile vessels, poor wound healing, and bleeding gums. Hydroxylysines are then glycosylated, three chains wind into procollagen, and the molecule is secreted. Extracellularly, procollagen peptidase clips the terminal propeptides to yield tropocollagen, and the copper-dependent enzyme lysyl oxidase oxidatively deaminates lysine and hydroxylysine side chains to form the covalent cross-links that give a fibril its tensile strength. A defect at any single step — hydroxylation, propeptide cleavage, or cross-linking — yields a connective-tissue phenotype, which is the reasoning pattern behind Ehlers-Danlos and Menkes passages.

Comparison of Translational Inhibitors & Antibiotics