title: Factors contributing to the inhibition of aspartate aminotransferase by dicarboxylic acids.
abstract: At pH 8.0 aspartate aminotransferase (L-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1) reacts with the modified substrate, erythro-beta-hydroxy-L-aspartate, to form a mixture of enzyme-substrate complexes absorbing at 492 nm. A variety of dicarboxylic acids were studied spectrophotometrically as competitive inhibitors of this reaction. All of the inhibitory dicarboxylic acids form a complex with the enzyme, absorbing at 362 nm. In addition, some of the dicarboxylic acids form a protonated complex absorbing at about 435 nm. This complex, which is the conjugate acid of that absorbing at 362 nm, is formed only by those dicarboxylic acids which can assume a configuration in which the two carboxyl groups are positioned as in maleic acid. Bulky substituents, such as aromatic rings or even methyl groups, prevent the formation of the protonated complex, presumably because of steric restrictions at the active site. Substitution of the central carbon atom of glutaric acid by heteroatoms of increasing charge density results in a progressive decrease in inhibitory effectiveness, at pH 8, primarily due to a loss of this pH-dependent stabilization of the enzyme-dicarboxylic acid complex. Acids with an aromatic ring are among the most potent dicarboxylic acid inhibitors of this enzyme in spite of the fact that they do not undergo the pH-dependent stabilization of their enzyme complexes. From these observations it was concluded that the affinity of aspartate aminotransferase for dicarboxylic acids is determined as much by the mechanism of binding as by the solvation and steric effects.
mesh_terms: D000818:Animals; D001219:Aspartate Aminotransferases / Q000037:antagonists & inhibitors*; D001665:Binding Sites; D003998:Dicarboxylic Acids / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008433:Mathematics; D008958:Models, Molecular; D008968:Molecular Conformation; D009206:Myocardium / Q000201:enzymology; D011485:Protein Binding; D013329:Structure-Activity Relationship; D013552:Swine
chemical_list: D003998:Dicarboxylic Acids; D001219:Aspartate Aminotransferases