The iron‐sulfur‐cluster‐containing l‐serine dehydratase from Peptostreptococcus asaccharolyticus

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Abstract
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‐hydroxy‐butyrate 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.

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