Threonine synthase

Aspartate y-decarboxylase Selenocysteine lyase NifS protein of nitrogenase Gamma elimination and replacement Cystathionine y-synthase Cystathionine y-lyase Threonine synthase... 

Write out an abbreviated reaction sequence for its conversion to L-threonine by the action of threonine synthase. 

Harde, C., Neff, K.-H., Nordhoff. li.. Gerbling, K.-P., Laber, B., and Pohlenz, H.-D., Syntheses of homoserine phosphate analogs as potential inhibitors of bacterial threonine synthase, Bioorg. Med. Chem. Lett.. 4, 273, 1994. 

Only two enzymes (threonine synthase (TS) and CGS) are known to catalyze the 7-replacement reaction, which is composed of two distinct half-reactions. The mechanism involves the elimination of the 7-leaving group, followed by a Michael addition, where water or cysteine reacts with the /3,7-unsaturated ketimine to form the final product, L-threonine and L-cystathionine, respectively. In the case of TS, the addition is on the /3-carbon. 

A more direct y replacement of the hydroxyl of homocysteine or 0-phosphohomoserine by a sulfide ion has also been reported for both Neurospora and green plants.Methylation of homocysteine to methionine (Fig. 24-13) has been considered previously, as has the conversion of homoserine to threonine by homoserine kinase and the PLP-dependent threonine synthase (p. 746, Fig. i4-7).254-255a standard PLP-requiring P elimination converts threonine to 2-oxobutyrate, a precursor to isoleucine (Fig. 24-13). ... 

CQGG13 6 Aspart-ate-semialdeliyde dehydrogenase COGG46G Homoserine dehydrogenase CQGGG83 Homoserine kinase CQGG 98 Threonine synthase... 

Biosynthesis Thr belongs biogenetically to the Asp group and is formed from Asp. The direct precursor is L- homoserine, which also forms Met via cystathionine and homocysteine. Homoserine is first converted to 0-phosphohomoserine by ATP under the action of homoserine kinase (EC 2.7.1.39) and then by threonine synthase (EC 4.2.99.2) to Thr. Thr is a component of glycoproteins. It frequently occurs in the free form, see also L-serine. 

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 most plants 0-phosphohomoserine rather than homoserine serves as the metabolic origin of the methionine branch of the aspartate pathway and as the direct precursor of threonine (Fig. 1). The synthesis of threonine from 0-phosphohomoserine is catalyzed by threonine synthase. This enzyme has been isolated from several plants and, in every case, enzyme activity was stimulated by a methionine derivative, 5-adenosylmethionine (Table VI). Enzyme activation is not as common as enzyme inhibition in regulating biosynthetic pathways. Nevertheless, the extent of activation of the plant methionine synthases can exceed 20-fold under some assay conditions. With the barley enzyme, only 60 ixM 5-adenosylmethionine was required for half-maximal activation (Aames, 1978). These characteristics are clearly indicative of a functional regulatory enzyme. Activation was considered to result from an increase in V iax rather than an increase in the affinity for the substrate, O-phosphohomoserine (Madison and Thompson, 1976). However, with the pea enzyme both V ,ax and are altered in the presence of the activator (Thoen et al., 1978a). The demonstration that both pea threonine... 

Cysteine inhibited sugar beet and radish threonine synthases. It was, therefore, proposed that O-phosphohomoserine would be diverted toward methionine synthesis as cysteine inhibited the enzyme (Madison and Thompson, 1976). Effective regulation could be achieved by the opposing effects of the methionine precursor, cysteine, and the methionine derivative, 5-adenosylmethionine. However, this can not be considered a universal regulatory pattern for plant threonine synthases since the barley enzyme was not inhibited by cysteine (Aarnes, 1978) and the effects of cysteine on the activity of the pea enzyme were questionable (Thoen et al., 1978b). 

The enzymes and control sites in the conversion of aspartate to 0-phosphohomoserine, and in the further metabolism of this branch-point intermediate to threonine and isoleucine have been covered in detail in the previous chapter. Here, those aspects of special relevance to methionine biosynthesis will be discussed. A proposed scheme for the control of methionine biosynthesis is presented in Fig. 6. In this scheme, the primary signal that overproduction of methionine has occurred in an increased concentration of AdoMet, which allosterically stimulates threonine synthase and thereby diverts 0-phosphohomoserine into the synthesis of threonine rather... 

In addition to the major elfectors (AdoMet, threonine, and lysine), cysteine and isoleucine may participate in the control of methionine biosynthesis, at least in some plants. Both isoleucine and cysteine would be expected to accumulate as a result of the diversion of O-phosphohomoserine toward threonine. Isoleucine is a potent competitive inhibitor of the homoserine kinase of pea seedlings (Thoenef aL, 1978), but not that of barley seedlings (Aarnes, 1976). Cysteine inhibits homoserine dehydrogenase (see Bryan, this volume. Chapter 11) and can inhibit the stimulation by AdoMet of some (Madison and Thompson, 1976) but not all (Aarnes, 1978 Thoen et al., 1978) preparations of threonine synthase. Any regulatory effect of cysteine may, however, be of short duration since the combined mechanisms described in Section II,D for regulation of cysteine biosynthesis would be expected to restore the normal concentration of this amino acid. Details of the control of methionine biosynthesis by the major effectors AdoMet, threonine, and lysine are presented below. 

Stimulation by AdoMet of threonine synthase activity by 4- to 20-fold, or more, has been observed with preparations from all plants examined (see Bryan, this volume. Chapter 11). 

The concentration of AdoMet is approximately 40 fiM in germinating pea seeds (Dodd and Cossins, 1968) and approximately 14-30 fiM in L. paucicostata (Table I and Fig. 5). These concentrations are within the range in which activity of threonine synthase is very sensitive to changes in AdoMet concentration (Madison and Thompson, 1976 Aarnes, 1978 Thoen et al., 1978). The concentration of AdoMet in L. paucicostata that had been grown in the presence of exogenous methionine (Fig. 5) was approximately 300 p,M. At this concentration, threonine synthase is almost maximally stimulated by AdoMet (Madison and Thompson, 1976 Thoen etal, 1978). 

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