FIG 4.

Resolution of the structure of FbiA in the presence of Fo, GDP, and DH-F₄₂₀-0 provides insight into its catalytic mechanism. (A) Fo and GDP in complex with Mol. B of FbiA in coordination with the catalytic Ca²⁺ ion. FbiA is shown as a sky blue cartoon, Fo and GDP as sticks, and Ca²⁺ as a sphere. (B) Stereoview of the catalytic center of the FbiA active site in complex with Fo and GDP, showing FbiA side chains involved in coordinating the catalytic metal ion and a coordinating H₂O molecule. Bond distances of <3.2 Å are shown as yellow dashed lines, and the distance between the terminal OH of Fo and P of the β-phosphate of GDP is highlighted in blue. (C) DH-F₄₂₀-0 in complex with FbiA, shown as in panel A. (D) Stereoview of the FbiA catalytic center with the reaction substrate EPPG model in place of GDP displayed in panel C, with the close proximity between the carboxylic acid group of EPPG and the terminal OH of Fo highlighted with a red dashed line. (E) Schematic showing the proposed catalytic mechanism for the formation of DH-F₄₂₀-0 by FbiA.