Extended data Figure 5. MS2 fragmentation spectra of peptides with oxidized tyrosine.

a) Annotated MS2 fragmentation spectra and respective theoretical fragment ion tables of the doubly charged precursor ion 661.8458 m/z corresponding to peptide VAVHARSY(+15.995)GSIF, and c) the doubly charged precursor ion 458.7279 m/z corresponding to peptide ARSY(+15.995)GSIF, both with the oxidized (+16) Y126 residue. The peptides shown in a) and c) were obtained by proteolytic digestion of the active form of the MfR2 protein with chymotrypsin and pepsin, respectively. The mass error is typically less than 0.01 m/z, in accordance with the high resolution used (15 000). Errors in ppm are indicated for the corresponding fragment ions when detected. Among the fragment ions observed, the most relevant are the b7 and b8 ions for peptide a). The experimental m/z values, the annotation, theoretical m/z values and ppm errors are shown in b), including the peaks for the corresponding isotope envelope. In d) the Y(+O) immonium ion for c) is shown, which demonstrate that Tyr126 is modified by a mass of +15.995 and the absence of the corresponding immonium ion for the unmodified Y. Four independent experiments per peptidase treatment were performed, confirming the modified peptide sequences shown. Figures are taken from one representative experiment.