title: Mechanism of racemization of amino acids by aspartate aminotransferase.
abstract: Aspartate aminotransferase (mitochondrial isoenzyme from chicken) has been found to racemize very slowly dicarboxylic amino acid substrates in the presence of their cognate oxo acids [Kochhar, S. & Christen, P. (1988) Eur. J. Biochem. 175, 433-438]. Tyrosine, phenylalanine and alanine are racemized at the same rate although they undergo the transamination reaction 3-5 orders of magnitude more slowly than the dicarboxylic substrates. Similarly, the truncated enzyme aspartate aminotransferase-(27/32-410) catalyzes the racemization at the same rate as the native enzyme, while its rate of transamination is decreased to 3% of that of the native enzyme. Apparently, the rate-limiting step in racemization is not immediately linked to the transamination cycle. Decreasing the water concentration in the reaction medium by adding methanol at 0 degrees C drastically reduces the rate of racemization without affecting the rate of transamination. On the basis of these and additional kinetic data and the model of the three-dimensional structure of the active site, we conclude that a water molecule is responsible for the protonation of C alpha of the coenzyme-substrate intermediate from the wrong side. The diffusion of the water molecule into the interior of the enzyme appears to be the rate-limiting step in aspartate-aminotransferase-catalyzed racemization.
authors.affiliation: Biochemisches Institut, Universität Zürich, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Kochhar
authors.forename: P
authors.initials: P
authors.lastname: Christen
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D000818:Animals; D001219:Aspartate Aminotransferases / Q000737:chemistry*; D002645:Chickens; D002851:Chromatography, High Pressure Liquid; D006863:Hydrogen-Ion Concentration; D000432:Methanol / Q000737:chemistry; D009206:Myocardium / Q000201:enzymology; D011487:Protein Conformation; D013237:Stereoisomerism
chemical_list: D000596:Amino Acids; D001219:Aspartate Aminotransferases; D000432:Methanol