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Post-translational modifications (PTMs) are covalent modifications of proteins after synthesis, altering protein properties, localization, activity, and lifespan. Common PTMs include phosphorylation (on Ser, Thr, Tyr, adding negative charge and enabling signal transduction), acetylation (on Lys, neutralizing charge, affecting DNA binding), ubiquitination (marking proteins for degradation or signaling), glycosylation (addition of sugars, modifying protein folding and recognition), and proteolytic cleavage (removing signal peptides, pro-domain removal). PTMs are reversible or irreversible and tightly regulated.
You know from studying translation that the ribosome assembles a polypeptide chain by reading mRNA codons and linking amino acids together. But the protein that rolls off the ribosome is often just a rough draft — it may be inactive, unlocalized, or unstable until the cell edits it through post-translational modifications (PTMs). These covalent chemical changes happen after (and sometimes during) translation, and they vastly expand the functional repertoire of the proteome far beyond what the ~20,000 human genes alone could encode.
Phosphorylation is the most common regulatory PTM. Kinases attach a phosphate group (from ATP) to the hydroxyl side chains of serine, threonine, or tyrosine residues, introducing a bulky negative charge that can flip a protein's conformation — and therefore its activity — like a molecular switch. Phosphatases reverse the modification. This kinase/phosphatase toggle is the backbone of nearly every signal transduction cascade: when a growth factor binds a receptor, a cascade of phosphorylation events relays the signal from membrane to nucleus in milliseconds. The speed and reversibility of phosphorylation make it ideal for rapid, dynamic regulation.
Acetylation neutralizes the positive charge on lysine residues by capping the amino group with an acetyl group. The most studied context is histone acetylation: histone tails are rich in positively charged lysines that grip the negatively charged DNA backbone tightly. Acetylation loosens that grip, opening chromatin and promoting gene transcription. Histone acetyltransferases (HATs) add acetyl groups; histone deacetylases (HDACs) remove them. But acetylation is not limited to histones — thousands of non-histone proteins, including metabolic enzymes and transcription factors, are regulated by acetylation as well.
Ubiquitination attaches the small protein ubiquitin (76 amino acids) to a target protein's lysine residues through a three-enzyme cascade (E1, E2, E3). A chain of four or more ubiquitin molecules linked through lysine-48 tags the protein for destruction by the proteasome, the cell's protein-recycling machine. But ubiquitin is more versatile than a simple death tag — monoubiquitination and alternative chain linkages (e.g., lysine-63) regulate endocytosis, DNA repair, and signaling without triggering degradation. Glycosylation adds sugar chains to asparagine (N-linked) or serine/threonine (O-linked) residues, which is critical for proper protein folding in the ER, cell-surface recognition, and protection from proteolysis. Finally, proteolytic cleavage is an irreversible PTM: signal peptides are cut to direct proteins to their destinations, and inactive zymogens (like trypsinogen) are activated by removing an inhibitory pro-domain. Together, these modifications give the cell a rich, combinatorial language for controlling when, where, and how every protein functions.