Mechanism of GTP hydrolysis by G-protein α subunits

Hydrolysis of GTP by a variety of guanine nucleotide-binding proteins is a crucial step for regulation of these biological switches. Mutations that impair the GTPase activity of certain heterotrimeric signal-transducing G proteins or of p21(ras) cause tumors in man. A conserved glutamic residue in the α subunit of G proteins has been hypothesized to serve as a general base, thereby activating a water molecule for nucleophilic attack on GTP. The results of mutagenesis of this residue (Glu-207) in G(iα1) refute this hypothesis. Based on the structure of the complex of G(iα1) with GDP, Mg²⁺, and AlF₄/^-, which appears to resemble the transition state for GTP hydrolysis, we believe that Glu-204 of G(iα1), rather than Glu-207, supports catalysis of GTP hydrolysis by stabilization of the transition state.