Figure 8.

Modifications of C165A StAhpC by 1 mM hydrogen peroxide. (a-d) Mass spectra of purified C165A StAhpC starting material and after treatment with 1 mM peroxide for 1, 3, and 20 min, respectively (see Experimental Procedures). The theoretical weight of C165A is 20584.32 Da (C₉₂₈H₁₄₂₇N₂₄₁O₂₈₃S₃), and for modifications: 20600.32 Da (+1 Oxygen) 20616.32 Da (+2 Oxygens), 20660.44 Da (BME adduct, + C₂H₄OS). Experimental peaks have values of 20584 Da (grey), 20599 Da (cyan), 20615 Da (blue), and 20659 Da (green). Inferred identities of the observed species assume that additions are associated with the C_P residue. The y-axes are normalized so that the summed areas of the four colored peaks is a constant. (e) Integrated normalized populations of four prominent species as a function of time. We suspect that the time=0 populations attributed to SOH (cyan) and SO₂- (blue) are overestimates, because the peak centers are not at the expected mass and they may be satellite peaks caused by some non-covalent ligand binding to the fully reduced protein. Fortunately, these areas are small and their inclusion does not substantially impact the conclusions.