FIGURE 4.

The active site of YeOGL. A, the octahedral metal binding site is displayed in wall-eyed stereo format with the weighted maximum likelihood (20)/σ_A (53) 2F_o − F_c map (cyan) contoured to 1.0 σ (0.43 e⁻/ų). The coordinated acetate ion (yellow sticks) is shown with an F_o − F_c omit map, produced from phases generated during refinements with the acetate molecule excluded from the model, contoured at 2.5 σ (0.18 e⁻/ų). This map demonstrates the high quality of unbiased electron density for this molecule. The three histidines (His²⁸⁷, His³⁵³, and His³⁵⁵) and glutamine (Gln³⁵⁰) are shown in stick representation, and the Mn²⁺ ion is modeled as a sphere. B, superimposition of catalytic substructures within the active site of YePL2A (yellow, Protein Data Bank entry 2V8J) and YeOGL (green). Four aligned regions are shown: metal coordination pocket (I), catalytic base (II), and potential residues involved in substrate stabilization (III and IV). Distances of the residues from the metal are labeled. C, cut-away surface representation of the active site with a GalA modeled in the +1 subsite. The GalA uronate group of the aglycone is superimposed with the coordinated acetate ion with the axial H5 pointing toward the candidate Brønstead base His²⁴² and O2 pointing toward the stabilizing Arg²¹⁷, and O4 is presented for polymerization.