Threonine synthetase

Scheme XX. Two possible steric arrangements of the PLP—amino acid Schiffs base in threonine synthetase.

Trapping of the aminoacrylate intermediate in the reactions catalyzed by cystathionine-y-synthase and y-cystathionase produced the same diastereomer of KEDB which was different from the one formed with bacterial L-threonine dehydratase. Unfortunately, this experiment has apparently not been done with threonine synthetase. 

Savageau, M. A. Steward, J. P. (1970). Repression of the threonine synthetase system in Escherichia coli. Arch. Biochem. Biophys. 137, 181-184. 

Threonine synthetase has been purified from N. crassa by Flavin [83] and catalyses the synthesis of l-threonine from O-phosphohomoserine and H2O via a y- replacement process. 

The enzyme that catalyzes the conversion of 0-homoserine phosphate to threonine, threonine synthetase, has been purified 500-fold from Neuro-apora by Flavin and Slaughter (107). The purification procedure utilized acetone and ammonium sulfate precipitation tmd chromatography on a DEAE-cellulose column. 

Threonine synthetase has been shown to require pjoidoxal phosphate (PLP) (107). 

In a study of the mechanism of the synthetase reaction Flavin and co-workers determined the incorporation of 0 (108) and of deuterium from the water (109) of the incubation medium into the products of the reaction. It was found that one atom of solvent oxygen was incorporated into threonine, and none into phosphate. This shows that the latter is removed by nonhydrolytic elimination, rather than hydrolysis. Incubation of 0-phos-phohomoserine with purified threonine synthetase in 100% DjO resulted in the incorporation of two atoms of D in the newly formed threonine, one in the a-position, the second in the -y-position. 

Fio. 4, Probable mechanism of threonine synthetase in formation of threonine. Modified from Flavin and Slaughter (f09). Experiments with HiO and DjO were performed independently. Pyridine substituents have been omitted from intermediates for convenience. 

Madison, J. T., and Thompson, J. F., 1976, Threonine synthetase from higher plants stimulation by S-adenosylmethionine and inhibition by cysteine, Biochem. Biophys. Res. Commun., 71 684. 

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