The iron-sulfur-cluster-containing L-serine dehydratase from Peptostreptococcus asaccharolyticus. Stereochemistry of the deamination of L-threonine.
The stereochemistry of the deamination of L-threonine to 2-oxobutyrate, catalyzed by purified L-serine dehydratase of Peptostreptococcus asaccharolyticus, was elucidated. For this purpose the enzyme reaction was carried out with unlabelled L-threonine in 2H2O and in 3HOH, as well as with L-[3-3H]threonine in unlabelled water. Isotopically labelled 2-oxobutyrate thus formed was directly reduced in a coupled reaction with L- or D-lactate dehydrogenase and NADH. The (2R)- or (2S)-2-hydroxybutyrate species obtained were then subjected to configurational analyses of their labelled methylene group. The results from 1H-NMR spectroscopy and, after degradation of 2-hydroxybutyrate to propionate, the transcarboxylase assay consistently indicated that the deamination of L-threonine catalyzed by L-serine dehydratase of P. asaccharolyticus proceeds with inversion and retention in a 2:1 ratio. This partial racemization is the first ever to be observed for a reaction catalyzed by serine dehydratase, therefore confirming the distinction of the L-serine dehydratase of P. asaccharolyticus as an iron-sulfur protein from those dehydratases dependent on pyridoxal phosphate. For the latter enzymes exclusively, retention has been reported.
Hofmeister
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AE
Laboratorium für Mikrobiologie, Fachbereich Biologie, Philipps-Universität, Marburg, Federal Republic of Germany.
Berger
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S
Buckel
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W
D002250:Carbon Radioisotopes; D007506:Iron-Sulfur Proteins / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D012695:L-Serine Dehydratase / Q000302:isolation & purification / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D010462:Peptostreptococcus / Q000201:enzymology*; D011865:Radioisotope Dilution Technique; D013237:Stereoisomerism; D013912:Threonine / Q000378:metabolism*; D014316:Tritium
D002250:Carbon Radioisotopes; D007506:Iron-Sulfur Proteins; D014316:Tritium; D013912:Threonine; D012695:L-Serine Dehydratase