Figure 3

Calcium-coordination geometry at the N-terminal and C-terminal F-hand motifs of R. flavefaciens DocScaA. Panels A and B show a representation of RfDocScaA N- and C-terminal Ca²⁺-coordination regions, respectively. In both panels the amino-acid residues involved in the metal coordination are depicted as sticks, surrounded by a mesh representation of the Refmac5 maximum-likelihood σ_A–weighted 2F_o−F_c electron density map contoured at 1σ (0.46 electrons/A³). The labels show the RfDocScaA residue and coordination position numbers and also the atoms involved. Both calcium ions are depicted as purple spheres and are overlaid with an idealized geometry representation (green arrows), which is pentagonal bipyramidal for the N-terminal Ca²⁺ (Panel A) and tetrahedral for the C-terminal Ca²⁺ (Panel B). A single water molecule (Wat) completes the coordination sphere of the N-terminal Ca²⁺ ion (Panel A). The bidentate nature of the Asp-664 and Asp-712 coordination is highlighted with blue dashed lines (Panels A and B). The 12-residue insert at the C-terminal calcium coordination loop is colored in light green (Panel B). Panel C depicts the overlay of the C-terminal Ca²⁺ of RfDocScaA (purple) with the C-terminal Ca²⁺ of the group 4 dockerin of RfDocCttA (cyan), whose coordination is also disrupted by a 13-residue long insert (dark green), but maintains an octahedral geometry due to the contribution of 2 water molecules (Wat). The structure of RfDocScaA is colored tan, and the structure of RfDocCttA is colored blue.