Figure 4. Schematic illustration based on the crystal structure of Ras•GDP•AlF₃•GAP. The important elements of catalysis and their interaction with AlF₃ (-phosphate) at the transition state are shown. Aluminum fluoride forms a trigonal bipyramidal complex. RasGAP stabilizes the transition state by supplying the critical arginine (Arg 789) in trans to neutralize the developing charges. In heterotrimeric G protein, such a critical arginine (Arg 178 in Gi₁) is supplied in cis from the Gi₁ itself (see Fig. 3A) (adapted from Scheffzek et al., 1997). Reprinted with permission from the journal Science and The American Association for the Advancement of Science.