Figure 3. (A) Schematic illustration based on the crystal structure of Gi₁•GDP•AlF₄^-, the coordination sphere, and contact distances (Å) of AlF₄^- with active site residues, ß-phosphate and magnesium. Notice that, in contrast to the tetrahedral geometry of a ground state phosphate, AlF₄^- forms a square planar complex that is octahedrally coordinated to a ß-phosphate oxygen and to a putative water molecule as the trans-axial ligands. (B) Model of the active site of Gi₁ at the transition state in the phosphohydrolysis reaction. Notice the true trigonal bipyramidal geometry of the -phosphate at its transition state (derived from Coleman et al., 1994). Reprinted with permission from the journal Science and The American Association for the Advancement of Science.