Detailed Explanation
Phosphorylation is the covalent attachment of a phosphate group (PO₄³⁻) from ATP to the hydroxyl group of serine (Ser), threonine (Thr), or tyrosine (Tyr) residues in a peptide or protein. This reaction is catalyzed by kinase enzymes and reversed by phosphatase enzymes. Phosphorylation is the single most prevalent post-translational modification in eukaryotic cells — at any given moment, roughly 30% of all proteins in a human cell are phosphorylated on at least one residue.
The addition of a phosphate group introduces a bulky, negatively charged moiety (−2 charge at physiological pH) that can dramatically alter a protein's conformation, activity, binding partners, and subcellular localization. This makes phosphorylation a molecular on/off switch that controls virtually every cellular process: signal transduction, cell division, metabolism, gene expression, apoptosis, and immune responses. The human genome encodes approximately 518 kinases (the 'kinome') and ~200 phosphatases, reflecting the enormous complexity of phosphorylation-based regulation.
Dysregulated phosphorylation is a hallmark of cancer and other diseases. Many oncogenes encode overactive kinases (e.g., BCR-ABL in chronic myeloid leukemia). Kinase inhibitors — small molecules that block specific kinases — are one of the most successful classes of cancer drugs: imatinib (Gleevec) for CML, erlotinib (Tarceva) for lung cancer, trastuzumab targets HER2 signaling. In peptide science, phosphorylation of peptide substrates is used to study kinase activity, and phosphopeptides are used as tools in signal transduction research.
Key Facts
- Most common post-translational modification in eukaryotic cells
- ~30% of human proteins are phosphorylated at any time
- Catalyzed by kinases (~518 in human genome), reversed by phosphatases (~200)
- Targets: Ser, Thr, Tyr hydroxyl groups
- Molecular on/off switch for signal transduction, metabolism, gene expression
- Kinase inhibitors are a major class of cancer drugs (imatinib, erlotinib)
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