pH-dependent decarboxylation of 2-amino-3-ketobutyrate, the unstable intermediate in the threonine dehydrogenase-initiated pathway for threonine utilization.
2-Amino-3-ketobutyrate can be readily formed enzymatically by the action of L-threonine dehydrogenase. A convenient assay for determining the half-life of this beta-keto acid is afforded by its rapid and quantitative conversion to glycine (+ acetyl CoA), as catalyzed by 2-amino-3-ketobutyrate CoA lyase. Using this system, we have found the half-life of 2-amino-3-ketobutyrate varies with pH from 8.6 minutes at pH 5.9 to 140 minutes at pH 11.1 yielding a theoretical titration curve that predicts a pKa value of 8.15 for the alpha-amino group of this intermediate. These data are considered relevant to discussions pertaining to a threonine dehydrogenase/2-amino-3-ketobutyrate CoA lyase enzyme complex in the threonine utilization pathway and to mechanistic aspects of the 5-aminolevulinate synthase-catalyzed reaction where 2-amino-3-ketoadipate is involved.
Marcus
J P
JP
Department of Biological Chemistry, University of Michigan, Ann Arbor 48109-0606.
Dekker
E E
EE
D000624:5-Aminolevulinate Synthetase / Q000378:metabolism; D000429:Alcohol Oxidoreductases / Q000378:metabolism*; D000596:Amino Acids / Q000378:metabolism*; D003653:Decarboxylation; D004926:Escherichia coli / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007651:Keto Acids / Q000378:metabolism*; D013912:Threonine / Q000378:metabolism*
D000596:Amino Acids; D007651:Keto Acids; D013912:Threonine; C079776:2-amino-3-ketobutyrate; D000429:Alcohol Oxidoreductases; C023159:L-threonine 3-dehydrogenase; D000624:5-Aminolevulinate Synthetase