title: The K258R mutant of aspartate aminotransferase stabilizes the quinonoid intermediate.
abstract: Lys-258 of aspartate aminotransferase forms a Schiff base with pyridoxal phosphate and is responsible for catalysis of the 1,3-prototropic shift central to the transamination reaction sequence. Substitution of arginine for Lys-258 stabilizes the otherwise elusive quinonoid intermediate, as assessed by the long wavelength absorption bands observed in the reactions of this mutant with several amino acid substrates. The external aldimine intermediate is not detectable during reactions of this mutant with amino acids, although the inhibitor alpha-methylaspartate does slowly and stably form this species. These results suggest that external aldimine formation is one of the rate-determining steps of the reaction. The pyridoxamine-5'-phosphate-like enzyme form (330-nm absorption maximum) is unreactive toward keto acid substrates, and the coenzyme bound to this species is not dissociable from the protein.
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley 94720.
authors.forename: M D
authors.initials: MD
authors.lastname: Toney
authors.forename: J F
authors.initials: JF
authors.lastname: Kirsch
mesh_terms: D000595:Amino Acid Sequence; D001219:Aspartate Aminotransferases / Q000235:genetics / Q000378:metabolism*; D007651:Keto Acids / Q000378:metabolism; D007700:Kinetics; D008239:Lysine; D016297:Mutagenesis, Site-Directed; D011732:Pyridoxal Phosphate / Q000378:metabolism; D011809:Quinones / Q000378:metabolism; D012545:Schiff Bases; D013379:Substrate Specificity
chemical_list: D007651:Keto Acids; D011809:Quinones; D012545:Schiff Bases; D011732:Pyridoxal Phosphate; D001219:Aspartate Aminotransferases; D008239:Lysine