Methionine threonine synthase

In mammals and in the majority of bacteria, cobalamin regulates DNA synthesis indirectly through its effect on a step in folate metabolism, catalyzing the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate via two methyl transfer reactions. This cytoplasmic reaction is catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyl-transferase), which requires methyl cobalamin (MeCbl) (253), one of the two known coenzyme forms of the complex, as its cofactor. 5 -Deoxyadenosyl cobalamin (AdoCbl) (254), the other coenzyme form of cobalamin, occurs within mitochondria. This compound is a cofactor for the enzyme methylmalonyl-CoA mutase, which is responsible for the conversion of T-methylmalonyl CoA to succinyl CoA. This reaction is involved in the metabolism of odd chain fatty acids via propionic acid, as well as amino acids isoleucine, methionine, threonine, and valine. 

Figure 7-10. Amino acids that can be converted to succinyl CoA. The amino acids methionine, threonine, isoleucine, and valine, which form succinyl CoA via methylmalonyl CoA, are all essential. The carbons of serine are converted to cysteine and do not form succinyl CoA by this pathway. A defect in cystathionine synthase (M) causes homocystinuria. SAM= S-adenosylmethionine PLP = pyridoxal phosphate.

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). ... 

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. 

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). 

Many plant proteins are of limited nutritional value because of their low content of methionine, which is one of the first essential amino acids to become inadequate in the human diet (Allaway and Thompson, 1966). In spite of the uncertainties of the regulatory scheme shown in Fig. 3, it provides a useful working model for efforts to increase the methionine content of plants. For example, the scheme predicts that an overproduction of methionine should result from inhibition of the conversion of methionine to AdoMet, or from inhibition of the allosteric stimulation of threonine synthase by AdoMet. These pro-... 

Phosphohomoserine serves as a precursor of both threonine and methionine in higher plants, and regulation of its utilization in both branches of the pathway would be expected. This appears to occur, in part, by 5-adenosylmethio-nine activation of threonine synthase (5). Results obtained with partially purified Lemna threonine synthase (Giovanelli et al, 1984) indicate that the enzyme is essentially inactive in the absence of -adenosylmethionine, which cooperatively activates the enzyme at concentrations of less than 100 /iM. Conceptually, methionine could be synthesized and converted to S-adenosylmethionine prior to enzyme activation and the synthesis of threonine. Both orthophosphate and AMP inhibit Lemna threonine synthase in vitro, but the physiological significance of these effects is uncertain (Giovanelli et al, 1986). 

Phosphohomoserine is a substrate for both threonine synthase and cystathionine y-synthase. Thus, although threonine synthase is not involved in the synthesis of either methionine or phosphohomoserine the properties of this enzyme are relevant to methionine synthesis as it competes with cystathionine y-synthase for the same substrate. Moreover, as discussed in the ensuing section, the activity of threonine synthase and the synthesis of phosphohomoserine are regulated by products of the methionine biosynthetic pathway. 5-Aden-osylmethionine is an extremely potent positive effector of threonine synthase, virtually serving as an absolute requirement for enzyme activity (Aames, 1978 Giovanelli et a/., 1984 Madison and Thompson, 1976 Thoen eta/., 1978). In the presence of SAM, Giovanelli et al. (1984) found that threonine synthase had an extremely high affinity for phosphohomoserine (A = 2.2-6.9 nM). 

Fig. 10. Schematic representation of the split biosynthetic pathway of L-lysine in wildtype Corynebacterium glutamicum including the branch point of aspartate semialdehyde distribution. The metabolites derived from the aldehyde via the synthase activity are D,L-di-aminopimelate and L-lysine, whereas that resulting from dehydrogenase activity are L-threo-nine, L-methionine, and L-isoleucine. The activity of the dehydrogenase is inhibited at elevated L-threonine concentrations and its synthesis is repressed by L-methionine. Accumulating intracellular lysine causes feedback inhibition of aspartate kinase and activates lysE transcription...

Aspartate 4-semialdehyde, seen, for example, in Scheme 12.13, which provided a pathway for the biosynthesis of the essential amino acid methionine (Met, M) and in Scheme 12.14, which holds a representation of the biosynthesis of threonine (Thr, T), is also a place to begin to describe a pathway to lysine (Lys, K). As shown in Scheme 12.19, aspartate 4-semialdehyde undergoes an aldol-type reaction with pyruvate (CHsCOCO ") in the presence of dihydropicoUnate synthase (EC 4.2.1.52) to produce a series of intermediates that, it is presumed, lead to (5)-23-dihydropyridine-2,6-dicarboxylate. Then, dihydrodipicolinate reductase (EC 1.3.1.26) working with NADPH produces the tetrahydropyridine, (S)-2,3,4,5-tetrahydropyridine-2,6-dicarboxylate.This heterocycle, in the presence of glutamate (Glu, E) and water, is capable of transamination directly to 2-oxoglutarate and (2S, 6S)-2,3-diaminopimelate in the presence of LL-diaminopimelate aminotransferase (EC 2.6.1.83), while the latter, in the presence of the pyridoxal dependent racemase... 

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