Figure 2. C119 of Akt3 is the unique HNE-sensing residue.

(a) Domain composition of Akt isoforms: the linker region displays the highest divergence in amino acid sequence (shown) among the three isozymes. C124 of Akt2 (in red) is sensitive to H₂O₂¹⁹. We identified C119 (underlined) of Akt3 to be the site of HNE(alkyne)-modification on the tryptic peptide shown in red (also see Supplementary Fig. 3). (b) MS/MS Spectra of quadruply-charged ions at m/z 737.3197⁴⁺ (Panel-1) and m/z 737.0671⁴⁺ (Panel-2) identify an HNE-modified peptide at C119 residue from Akt3 protein. Although y-ion series in the spectra do not cover the C119 modification, the b-ion series in each MS/MS spectrum along with the high accurate mass (<5 ppm) of the precursor ion (see inset for the MS spectrum) provide a confident identification of C119 modification with reduced HNE-alkyne (+154.1 Da). An additional oxidation on M1 residue and a deamidation on N11 residue (indicated by lower-case m and n) were identified in the peptide (Panel-1). See Supplementary Table 2–3.