Figure 2. (A) Structural similarities among AlF₄^-, BeF₃(OH₂)^-, and phosphate group. All three compounds exhibit tetrahedral geometry. BeF₃^- has an electron-deficient beryllium atom. Be completes an octet by accepting a pair of electrons from a water molecule. (B) The originally proposed -phosphate model for AlF₄^- and BeF₃^- activation of heterotrimeric G proteins. AlF₄^- and BeF₃^- bind with GDP and mimic the -phosphate of GTP. Notice that, to mimic the ground state of -phosphate, AlF₄^- has to lose one F^-. Compare this theoretical model with the revealed transitional geometry of AlF₄^- in Fig. 3A (derived from Bigay et al., 1987). Reprinted with permission from Oxford University Press and The European Molecular Biology Organization.