FIGURE 8.

Heme coordination of “as isolated” MpTsdBA (PDB code 5LO9). A, heme 1 is coordinated by His²⁵ and Met⁶⁵. B, heme 2 is coordinated by His¹²⁵ and Met¹⁶⁷. C, heme 3 is ligated to His²⁹¹ but not to Cys³³⁰. The distance of Sγ to the heme iron is 2.9 Å and thus not close enough for direct ligation. Thiosulfate covalently bound to Sγ of Cys³³⁰ is not shown here for clarity. Presence of thiosulfate is illustrated in detail in E and F. D, Heme 4 is ligated by His⁴⁰⁶ and Met⁴⁵⁰. E, heme 3 with Sγ of Cys³³⁰ covalently bound to thiosulfate, displayed in ball and stick, and polder map electron density contoured at 6σ level depicted as a black mesh. F, heme 3 in a similar view as in E but with positively charged residues surrounding the substrate cleft depicted as sticks. Scheme representation is shown in pale gray with heme moieties and coordinating amino acid residues shown as sticks; color code as in Fig. 7 with sulfur atoms in green.