FIGURE 2.

Oxidation of Met¹⁴⁴/Met¹⁴⁵ results in the preferential degradation of CaM. ESI-CID MS/MS spectra before (top) or after (bottom) exposure to the proteasome obtained from the [M+14H]¹⁴⁺ ion (m/z = 1195.5) for the intact protein highlighting the y₅ (M¹⁴⁴–Lys¹⁴⁸) and y₆ (Q¹⁴³–Lys¹⁴⁸) ions that include Met¹⁴⁴ and Met¹⁴⁵ and associated oxiforms (i.e., y_n+16 and y_n+32) in which one (i.e., y₅′ or y₆′) or both (i.e., or ) methionines are oxidized to their corresponding methionine sulfoxides. Individual ions correspond to y₅ (581.3 Da), y₅′ (597.3 Da), and