GTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate (GTP). The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases. The hydrolysis of the γ phosphate of GTP into guanosine diphosphate (GDP) and Pi, inorganic phosphate, occurs by the SN2 mechanism (see nucleophilic substitution) via a pentavalent intermediate state and is dependent on the magnesium ion Mg²⁺.