phosphorylationsite

A phosphorylation site is a specific amino acid residue in a protein that can be covalently modified by the addition of a phosphate group. In most eukaryotic proteins, this modification occurs on serine, threonine, or tyrosine residues, carried out by kinases. In many bacteria and archaea, histidine or aspartate phosphorylation is common in two-component signaling systems. The addition of a phosphate introduces a negative charge and often induces conformational changes, altering enzyme activity, protein interactions, subcellular localization, or stability. Phosphorylation is typically reversible, with phosphatases removing phosphate groups, allowing dynamic control of signaling states.

Identification and mapping of phosphorylation sites rely on experimental techniques such as tandem mass spectrometry, phospho-specific antibodies, or mutational analysis to confirm the functional impact of specific residues. Sites are annotated by the residue identity and its position in the protein sequence, and sometimes by cell type, stimulus, or condition. Kinase recognition motifs and the local structural environment influence site selection, and many sites exist in clusters that coordinate signaling networks.

Functionally, phosphorylation serves as a molecular switch in signaling pathways, creating binding surfaces for phospho-recognition domains, modulating catalytic activity, or regulating protein stability. Cross-talk with other post-translational modifications can fine-tune the outcome. Well-studied examples include serine/threonine phosphorylation in metabolic and cell cycle control, and tyrosine phosphorylation in receptor tyrosine kinase signaling. Histidine/aspartate phosphorylation underlies bacterial two-component signaling.

Curated databases such as PhosphoSitePlus, UniProt, and dbPTM collect experimentally observed phosphorylation sites, along with evidence and functional annotations. Limitations include incomplete coverage, condition-specific occupancy, and varying confidence in site localization, underscoring the need for experimental validation for functional interpretation.