FIGURE 4.

Refinement of different conformations of the acceptor saccharide. A, refinement of saccharide E as IdoA in the ⁴C₁ conformation (green). B–D, refinement of saccharide E as either GlcA in the ⁴C₁ conformation, IdoA ²S₀, or IdoA ¹C₄, respectively (yellow) superimposed with the IdoA ⁴C₁ structure (green). The simulated annealing omit F_o − F_c map contoured at 4.0σ is displayed in black. Positive F_o − F_c density from each respective refinement is shown in green (3.0σ), and negative density (3.0σ) is shown in red. In B the black dashed line represents a salt bridge (2.9 Å) between the IdoA and Arg-189. For GlcA this distance is 4.0 Å (red dashed line) and thus likely does not form a salt bridge.