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    Protein phosphorylation: adding a phosphate can change activity without changing the sequence

    Serine, threonine and tyrosine are common phosphorylation sites in cellular proteins. Kinases add phosphate groups and phosphatases remove them; the consequence depends on the protein and the site.

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    Supplement Science Insights Editorial Team

    Published September 23, 2026

    A side chain is modified after the protein is made

    Protein kinases commonly transfer phosphate from ATP to a hydroxyl-containing side chain, particularly serine, threonine or tyrosine. This changes the chemical state of a residue without replacing its encoded identity with a different amino acid. [1]

    Protein phosphatases reverse many of these modifications. The balance between adding and removing phosphate allows cells to regulate proteins in response to signals. This is one form of post-translational modification, meaning a change after translation. [1]

    Phosphorylated does not always mean switched on

    Phosphorylation can activate or inhibit a protein, or change interactions within a signaling pathway. The result depends on the target and position. A measurement showing more phosphorylation therefore needs the identity of the site and protein before its biological meaning can be interpreted. [1]

    The common serine/threonine/tyrosine classification is not a claim that these are the only possible phosphorylated residues in all organisms or chemical settings. Nor does eating more of these amino acids instruct a particular kinase to modify a chosen target. [1]

    A phosphate signal needs a named target

    Imagine two proteins are phosphorylated but one becomes more active and the other less active. A report that phosphorylation increased would conceal the difference. Naming the protein, site and measured functional effect makes the result interpretable.

    Hypothetical example — keeping the scientific comparison precise
    DescriptionInterpretation
    Kinase actionAdds a phosphate group
    Phosphatase actionRemoves a phosphate group
    Functional consequenceDepends on the target and site
    Protein phosphorylation: adding a phosphate can change activity without changing the sequence — original article outline diagram
    Original editorial diagram of this article’s sections; no product photograph or clinical finding is depicted.

    Sources for this article

    1. The Cell, 2nd edition: Regulation of Protein Function
      Source consulted: 2026-09-17. This is a reference resource, not a claim that we conducted the studies or tested a product.