Practice question

Protein phosphorylation adds HPO3 group to serine, threonine, or tyrosine hydroxyl residues through kinase activity, fundamentally regulating signaling pathways. In mass spectrometry, attachment of phosphate adds monoisotopic mass of 79.9663 Da, commonly rounded to +80 Da compared to unmodified peptide. This signature shift enables phosphopeptide enrichment identification using titanium dioxide and tandem mass spectrometry. Neutral loss of 98 Da corresponding to H3PO4 may also occur under collision-induced dissociation, providing diagnostic confirmation. Recognizing +80 Da allows mapping of regulatory sites in cell biology.