Mechanistic studies on threonine synthase

Abstract

Threonine synthase catalyses the conversion of (2S)-0-phosphohomoserine to (2S,3R)-threonine with the elimination of phosphate. A novel and efficient synthesis of (2S)-0-phosphohomoserine has been developed starting from (2S)- aspartic acid. Using this methodology the singly labelled isotopomers (2S,3S)- [3-²H1]-, (2S,3R)-[3-²H1]-, and (2S)-[2-²H1]-0-phosphohomoserine have also been synthesised from labelled (2S)-aspartic acid and used to probe the mechanism of the enzyme. Measurement of the kinetic deuterium isotope effects for the labelled substrates showed that both the Cα-H and Cβ-H proton removal steps in E. coli threonine synthase display primary deuterium isotope effects, and that cleavage of the C-3-(pro-S)-H bond is at least partially rate limiting. This confirms data already available for the yeast enzyme, which indicated that the 3-pro-S proton is removed in the course of the reaction. The kinetic isotope effect for the removal of the 3-pro-S proton indicates a high forward commitment for the elimination of phosphate. Threonine synthase from E. coli was also shown to be activated in the presence of S-adenosyl methionine. This activation has been documented for the plant enzyme, but this is the first time such an activation has been seen for the bacterial enzyme. Threonine synthase for the study was partially purified from E. coli K12 Tir8, using novel methodology.