Mechanisms of guanosine triphosphate (GTP) hydrolysis by members of the G protein alpha subunit-p21(ras) superfamily of guanosine triphosphatases have been studied extensively but have not been well understood. High-resolution x-ray structures of the GTP gamma S and GDP.AlF4- complexes formed by the G protein G(i alpha 1) demonstrate specific roles in transition-state stabilization for two highly conserved residues. Glutamine(204)(Gln(61) in p21(ras)) stabilizes and orients the hydrolytic water in the trigonal-bipyramidal transition state. Arginine 178 stabilizes the negative charge at the equatorial oxygen atoms of the pentacoordinate phosphate intermediate. Conserved only in the G(alpha) family, this residue may account for the higher hydrolytic rate of G(alpha) proteins relative to those of the p21(ras) family members. The fold of G(i alpha 1) differs from that of the homologous G(t alpha) subunit in the conformation of a helix-loop sequence located in the alpha-helical domain that is characteristic of these proteins; this site may participate in effector binding. The amino-terminal 33 residues are disordered in GTP gamma S-G(i alpha 1), suggesting a mechanism that may promote release of the beta gamma subunit complex when the a subunit is activated by GTP.