Stereochemistry of the reaction of sheep liver threonine dehydratase. A nuclear magnetic resonance and optical rotatory dispersion study of its reaction pathway and products.

Products, substrates, and inhibitors of the threonine dehydratase from sheep liver (EC 4.2.1.16) have been investigated by proton nuclear magnetic resonance and optical rotation. The alpha-ketobutyrates produced from L-threonine and L-allothreonine in 2H2O have been shown to incorporate a single deuterium into the beta position. The dehydratase forms R-alpha-ketobutyrate-beta-d from L-threonine and L-allothreonine. The alpha protons of the substrates, threonine and allothreonine, do not exchange in the presence of the dehydratase. In the presence of dehydratase, the competitive inhibitors L-cysteine and L-alanine undergo alpha-proton exchange. Highly purified dehydratase has been used to determine kinetic parameters for the usbstrates L-threonine, L-allothreonine, L-serine, and L-chloroalanine. L-Chloroalanine, in addition to being a substrate, inhibits the dehydratase in a manner kinetically identical with that of L-serine.