Post-Translational Modifications
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Direct answer
Ribosomes release most proteins unfinished: chemistry then tailors them through post-translational modification — phosphorylation by kinases (the fastest regulatory switch), N- and O-linked glycosylation (folding, trafficking, recognition), proline and lysine hydroxylation (collagen stability and oxygen sensing), vitamin K-dependent glutamate carboxylation (clotting factors), acetylation and methylation (the histone code), ubiquitination (turnover and signalling) and ADP-ribosylation (both a toxin weapon and an endogenous DNA-repair language). The clinical pay-off is direct: scurvy is failed hydroxylation, warfarin is failed carboxylation, I-cell disease is a missed mannose-6-phosphate tag, HbA1c is a non-enzymatic glycation read-out, and cholera and diphtheria toxins kill by ADP-ribosylating a G-protein and an elongation factor.
What you must remember
- Phosphorylation: the commonest reversible modification, on serine, threonine and tyrosine hydroxyls (tyrosine a small minority of sites); several hundred kinases in the human kinome oppose phosphatases; phosphorylation activates glycogen phosphorylase while inactivating glycogen synthase — reciprocal control in one pathway.
- N-linked glycosylation: the Asn-X-Ser/Thr sequon (X not proline); a preassembled oligosaccharide on dolichol transfers en bloc in the endoplasmic reticulum, followed by the calnexin quality-control cycle; O-linked chains start with GalNAc in the Golgi and build mucins and blood-group antigens; cytosolic O-GlcNAc is a nutrient sensor rivalling phosphorylation.
- I-cell disease: N-acetylglucosamine phosphotransferase fails to add the mannose-6-phosphate tag, so lysosomal enzymes are secreted instead of targeted — coarse facies, restricted joint mobility, plasma enzyme levels high and inclusions everywhere; the post-translational disease with a whole face.
- Hydroxylation: prolyl and lysyl hydroxylases (vitamin C, ferrous iron, α-ketoglutarate) stabilise the collagen triple helix; prolyl hydroxylation of HIF-1α is the cell's oxygen gauge — the von Hippel-Lindau ubiquitin ligase clears only the hydroxylated form, so hypoxia spares HIF-1α and switches on angiogenesis.
- Gamma-carboxylation: the vitamin K-dependent carboxylase makes Gla residues on prothrombin, factors VII, IX and X, proteins C and S, and osteocalcin; warfarin blocks vitamin K epoxide recycling and leaves these factors under-carboxylated.
- Acetylation: histone lysine acetylation by HATs neutralises charge and opens chromatin (HDAC inhibitors treat cutaneous T-cell lymphoma); aspirin permanently acetylates a serine of cyclooxygenase-1 — the antiplatelet mechanism at low dose.
- ADP-ribosylation: cholera toxin locks Gsα in its active state (constitutive cAMP drives secretory diarrhoea — the biochemistry behind oral rehydration salts); diphtheria toxin inactivates elongation factor-2 at diphthamide; pertussis toxin locks Gi; endogenous PARP enzymes repair DNA, and PARP inhibitors exploit BRCA-mutated homologous-recombination deficiency — synthetic lethality in ovarian and breast cancer.
- Also examinable: ubiquitination and SUMOylation, zymogen cleavage (trypsinogen by enteropeptidase), protein disulfide isomerase in the endoplasmic reticulum, copper-dependent lysyl oxidase crosslinking, and Ras prenylation from the mevalonate pathway.
Worked example: five diseases, five modifications
Run one ward round through the list. The elderly man with bleeding gums has scurvy: no ascorbate, no hydroxyproline, a helix that melts at body temperature. The woman on warfarin has an international normalised ratio of 4.5: her prothrombin is being made but not carboxylated, so it cannot anchor on phospholipid. The child with I-cell disease carries a face made by a missing phosphotransferase — every lysosome in the body starved because the address label was never printed. The diabetic man's HbA1c of 8.9 per cent is glycation without any enzyme at all, accumulating by concentration and time. And the outbreak of cholera in the flood-relief camp is ADP-ribosylation of Gsα at scale: cyclic AMP opens chloride channels, water follows, and the sugar-salt sachet works precisely because glucose-coupled sodium absorption survives the toxin. Five beds, five chemistries, one principle — the finished protein is only half the story.
How the viva frames it
One-liners arrive in a predictable pattern: which modification aspirin performs (acetylation), which vitamin Gla formation needs (K), why HbA1c is not an enzymatic modification, and which tag I-cell disease omits. Indian convention adds the public-health bridge: cholera toxin biochemistry ending at oral rehydration salts is a favourite crossing of biochemistry with national programme teaching. Keep the enzyme names paired with their diseases — hydroxylase-scurvy, carboxylase-warfarin, phosphotransferase-I-cell — and the section is bankable marks.
Frequently asked questions
Which post-translational modification dominates rapid regulation?
Serine, threonine and tyrosine phosphorylation by protein kinases, reversed by phosphatases.
What is the N-linked glycosylation sequon?
Asparagine-X-serine/threonine, where X is any amino acid except proline.
Which enzyme tag is missing in I-cell disease?
N-acetylglucosamine phosphotransferase, which adds the mannose-6-phosphate sorting signal to lysosomal enzymes.
How does warfarin affect post-translational modification?
It inhibits vitamin K epoxide reductase, starving the γ-carboxylation of glutamate residues in clotting factors.
Why does cholera toxin cause secretory diarrhoea?
Its ADP-ribosylation of Gsα locks adenylate cyclase on, and cAMP-driven chloride secretion floods the lumen — rehydration sustains the patient while the toxin is cleared.