Post-translational Modification
A post-translational modification (PTM) is any covalent alteration introduced on a peptide or protein after its ribosomal synthesis. PTMs multiply proteome diversity far beyond the number of coding genes: a single polypeptide chain can coexist in dozens of distinct forms depending on the modifications it carries, each endowed with specific subcellular localization, half-life, and activity.
Major types include: phosphorylation (reversible addition of a phosphate group on serine, threonine, or tyrosine by kinases, removed by phosphatases, a pillar of signal transduction), glycosylation (N-linked on asparagine or O-linked on serine/threonine, critical for folding, stability, and cell recognition), acetylation (transfer of acetyl-CoA to lysine or N-terminus, extensively studied on histones), methylation (arginine, lysine, amino acids neutralizing or modulating protein interactions), ubiquitination (ubiquitin chain linked to a lysine, signaling proteasomal degradation or trafficking), SUMOylation (structurally similar to ubiquitination but functionally distinct, affecting localization and interactions), and lipid acylation (myristoylation, palmitoylation, prenylation, which anchor a protein to membranes).
Biological roles span the entirety of cellular physiology: enzyme activation or inhibition, cell-cycle regulation, transcription control (histone code), subcellular sorting, stress response, hormonal signaling, immunity, apoptosis, and maturation of peptide hormones (cleavage by convertases, C-terminal amidation, tyrosine sulfation on peptides like CCK or gastrin).
Detection and quantification rely primarily on high-resolution mass spectrometry, often preceded by specific enrichment (IMAC for phosphopeptides, lectins for glycopeptides, anti-PTM antibodies for acetyl-lysine or methyl-lysine). Modern analytical techniques allow PTM maps at whole-proteome scale (phosphoproteomics, glycoproteomics) and quantification of their dynamics during cellular processes.
In research peptide design, faithful reproduction of native PTMs (C-terminal amidation, native disulfide bridges, N-terminal acetylation) is often essential to achieve biological activity comparable to the endogenous peptide. Modified analogs (C-18 lipid acylation of semaglutide, O-linked glycosylation of mucin mimetics, phosphomimetic Asp/Glu replacing a phosphosite) illustrate how PTMs can be repurposed to optimize stability and activity.