text: In heterotrimeric G proteins, catalysis of GTP can be assisted by aluminum tetrafluoride () and RGS4. Heterotrimeric G proteins are larger three-part proteins serve in signal transduction of many pathways. The catalytic mechanism for GTP hydrolysis in heterotrimeric G proteins consists of an active state where catalysis is likely to occur and an inactive state where catalysis is unlikely. In the active state,  stabilizes the transition state and points the arginine finger residue towards GTP. This causes increased charge density on the beta phosphate of GTP and planarization of the gamma phosphate, which creates an opening and reduces steric hindrance for water to hydrolyze the phosphoanhydride beta-gamma bond. This is because the gamma phosphate's bond to the beta phosphate bends, exposing its connection and allowing the subsequent nucleophilic substitution reaction initiated by water. The complex formed with RGS4 assists in this process by creating strain on the bond between the gamma and beta phosphates and assisting in distributing more charge onto the beta phosphate.