Practice question

Protein phosphorylation attaches phospho group HPO3 to serine, threonine or tyrosine hydroxyls via kinase action, adding monoisotopic mass 79.966 Da, approximated as 80 Da shift on intact peptide. Tandem mass spectra retain this increment on modified b- and y-ions, enabling site localization. Under low-energy collision-induced dissociation, phosphopeptides often undergo neutral loss of phosphoric acid H3PO4, 98 Da, creating dehydroalanine marker. In contrast, oxidation adds 16 Da and acetylation adds 42 Da. Detecting +80 Da precursor and characteristic fragment ions reliably identifies regulatory phosphorylation controlling cell signaling networks.