Threonine, synthesis

Figure 2.8 Catalysis by the protonated amine could explain the stereochemistry of threonine and 0/0-threonine synthesis by our catalyst 46 at low pH. (Reprinted from Ref. 47. Copyright 1994 American Chemical Society.)...

The final reaction in the biosynthesis of threonine involves a /8-y rearrangement and the loss of phosphate from O-phosphohomoserine (Fig. 2). Threonine synthases have been isolated from Lemna (Schnyder et al., 1975) radish, sugarbeet (Madison and Thompson, 1975), peas (Schnyder et al., 1975 Thoen et al., 1978b), and barley (Aames, 1978). None of these enzymes has been extensively characterized but a requirement for pyridoxyl-5 -phosphate was demonstrated after partial purification of the barley and pea enzymes. Unlike several other enzymes associated with threonine synthesis, the activity of threonine synthase was not stimulated by monovalent cations. However, all of the plant enzymes are strongly activated by 5-adeno-sylmethionine (Section III,B,5). 

In those plants in which it has been determined, O-phosphohomoserine is present in concentrations of less than 25 ixM (Datko et al., 1974b, 1977). Since the for O-phosphohomoserine in threonine synthesis (in the absence of AdoMet) is approximately 2 mM (Madison and Thompson, 1976 Thoen et al., 1978), and for cystathionine synthesis approximately 7 mM (Madison and Thompson, 1976), it follows that (unless compartmentation is very extreme) each of these enzymes will be operating below its for O-phosphohomoserine, and the two enzymes will therefore compete for the available supply of this substrate. 

The conversion of L-homoserine to L-threonine is performed by homoserine kinase and threonine synthase. Homoserine kinase phosphorylates L-homoserine to form L-homoserine-P which is then dephosphorylated by threonine synthase to produce L-threonine. Homoserine kinase is the third key enzyme of the pathway it controls carbon flux towards L-threonine synthesis at the branchpoint of L-homoserine. In both E. coli and C. glutamicum, homoserme kinase is encoded by the thrB gene and threonine synthase is by thrC (Follettie et al. 1988 Theze and Saint-Girons 1974). 

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