title: Reaction of aspartate aminotransferase with C5-dicarboxylic acids: comparison with the reaction with C4-dicarboxylic acids.
abstract: The reaction of Escherichia coli aspartate aminotransferase (AspAT) with glutamate and other C5-dicarboxylates was analyzed in order to compare its mechanism of action toward C5 substrates with that toward C4 substrates, which had been extensively characterized. The association of the amino-group protonated and unprotonated forms of glutamate (SH(+) and S, respectively) with the Schiff-base protonated and unprotonated forms of the enzyme (E(L)H(+) and E(L), respectively) yields at least three forms of the Michaelis complex, whereas in the case of aspartate, only two species of this complex exist, E(L).SH(+) and E(L)H(+).S. The reaction of AspAT with 2-methylglutamate can be explained only when we consider all the protonation states of the Michaelis complex. Based on the previous crystallographic studies [Miyahara et al. (1994) J. Biochem. 116, 1001-1012], we consider that glutamate binds to the open form of AspAT and takes an extended conformation in the Michaelis complex, with the alpha-amino group of glutamate oriented in the opposite direction to the Schiff base. This is in contrast to the Michaelis complex of aspartate, in which a strong interaction of the alpha-amino group of aspartate and the Schiff base excludes the presence of the species E(L)H(+).SH(+). It is concluded that AspAT recognizes the two types of dicarboxylates with different chain lengths by changing the gross conformation of the enzyme protein.
authors.affiliation: Department of Biochemistry, Osaka Medical College, 2-7 Daigakumachi, Takatsuki 569-8686.
authors.forename: Mohammad Mainul
authors.initials: MM
authors.lastname: Islam
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Hayashi
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Kagamiyama
mesh_terms: D001219:Aspartate Aminotransferases / Q000737:chemistry / Q000378:metabolism*; D001224:Aspartic Acid / Q000737:chemistry / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology; D018698:Glutamic Acid / Q000737:chemistry / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008958:Models, Molecular; D011485:Protein Binding; D013053:Spectrophotometry / Q000379:methods; D013379:Substrate Specificity; D013816:Thermodynamics
chemical_list: D001224:Aspartic Acid; D018698:Glutamic Acid; D001219:Aspartate Aminotransferases