Figure 4: Thiyl radicals and sulfenic acid species intervene during sulfheme formation.

(a–d) High-resolution mass spectra of the BCN (a,b) and dimedone (C) adducts (ESI+) as well as the HCys derivatives (D; ESI−) formed during the reactivity of catalase (3 μM) with HCys (2 mM). (e,f) Comparative kinetic studies of CAT–Fe(IV)=O formation (e) and CAT–Fe(II) sulfonium generation (f) when catalase (1.75 μM) is exposed to 2 mM HCys in the absence or the presence of various additives. (g) Time course of catalase inactivation observed under the same experimental conditions as those described in (e,f). The kinact values extracted from the analysis of the plots are reported in Table 2 (n=2±s.d.).