Extended Data Figure 4. Amino acids in MEK1 identified to be oxidized by the MCO reaction followed by MS/MS.

Representative annotated MS/MS fragmentation spectra for five indicated MEK1-derived peptides containing oxidized residues a, MEK1^H87 and MEK1^M94 b, MEK1^H100 c,MEK1^H188 d, MEK1^M230 and e, MEK1^H239 highlighted in red. The peak heights are the relative abundances of the corresponding fragmentation ions, with the annotation of the identified matched amino terminus-containing ions (b ions) in blue and the carboxyl terminus-containing ions (y ions) in red. For clarity, only the major identified peaks are labeled. f, Amino acid sequence of human MEK1 with the peptides identified by MS/MS underlined (red: trypsin digest and blue: chymotrypsin digest). Amino acids oxidized only in the presence of H₂O₂ in one to three independent MCO reactions are denoted in red. Amino acids, that when mutated to alanine reduced both binding of MEK1 to a Cu-charged resin and phosphorylation of cellular ERK2, are boxed (from Extended Data Fig. 3c).