FIGURE 3. HppE active sites displayed with 2F_o - F_c maps contoured from 1.0–1.5.

a, Apo-HppE active site with strands 5 and 6 labelled, and the -domain (red), -domain (blue) and carbon (grey), oxygen (red) and nitrogen (blue) positions shown. b, Fe(ii)-HppE active site. F_o - F_c electron density peaks contoured at 3 indicate the positions of water molecules (cyan) bound to Fe(ii) (brown). c, Monodentate substrate binding in form-1 S-HPP–Fe(ii)-HppE active site, coloured as in a, b with phosphorus in magenta. An omit map contoured at 6 shows an electron density peak (green) coincident with the phosphorus atom here and in d. d, Bidentate substrate binding in the form-2 S-HPP–Fe(ii)-HppE active site with domains coloured as in a and atoms coloured as in a–c. e, The Tris–Co(ii)-HppE active site, coloured as in a with cobalt shown as a magenta sphere. See Supplementary Fig. S1 for stereo images of S-HPP–Fe(ii)-HppE and Supplementary Fig. S3 for S-HPP–Co(ii)-HppE.