Functional and Stereochemical Specificity at the fl Carbon Atom of Substrates in Threonine Dehydratase-catalyzed cw,#I Elimination Reactions*

GThreonine dehydratase catalyzes the nonoxidative conversion of /Y-substituted amino acids to cr-keto acids. In this manuscript the nature of the nucleophiles which are eliminated is established, new substrates are reported, and inhibition by j&substituted alanines investigated. It is now shown that threonine dehydratase will catalyze the q/3 elimination of -OH, -Cl, and -F as HzO, and HX from a large number of &substituted a-camino acids. Threonine dehydratase is found to be nonspecific for the erythro and thmo pairs of threonine, 3-chloro-a-aminobutyrate, 3-fluoro-a-aminobutyrate, 3-hydroxyleucine, and 3-hydroxynorvaline. There is, however, a catalytic preference shown for the three isomers by threonine dehydratase in all cases. Stereochemistry at the /3 carbon atom in substrates for threonine dehydratase influences the stereochemical course of the reaction as well as the catalytic efficiency. Substitution of the pro-R or pm-S proton of serine by alkyl and aryl groups has been shown to affect the binding and catalytic ability of threonine dehydratase for a number of substrates. By increasing the size of the alkyl groups at the /3 carbon atom of serine we can demonstrate a peak in binding efficiency for the methyl group, ie. threonine. Threonine dehydratase from sheep liver undergoes inactivation when catalyzing the ~9 elimination reaction of &substituted alanines. Previously it was observed that serine caused a time-dependent inactivation of threonine dehydratase, it is now established that 3-bromo-, 3-chloro-, and 3-fluoro-L-alanine will also serve as substrates and kcat inhibitors for threonine dehydratase. The kinetic data of the inactivation and protection afforded by nucleophiles have been studied and a systematic study has been made of competitive inhibitors, derived from alanine, of threonine dehydratase.