Homoserine kinase and

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). ... 

Starting from the building block of L-aspartate, the biosynthesis of L-threonine comprises five successive reactions sequencially catalyzed by aspartate kinase, aspartyl semialdehyde dehydrogenase, homoserine dehydrogenase, homoserine kinase and threonine synthase. 

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). 

Fig. 5. Regulation of the enzymes of methionine biosynthesis and related pathways. Enzymes catalyzing the synthesis of methionine and 5 -adenosylmethionine (SAM) from cysteine are (1) cystathionine y-synthase, (2) j9-cystathionase, (3) methionine synthase, and (4) SAM synthetase. Enzymes associated with the synthesis and metabolism of phospbohomoserine which are relevant to the regulation of methionine synthesis are (5) aspartate kinase, (6) homoserine kinase, and (7)...

Azevedo, R.A. Smith, R.J. Lea, P.J. Aspartate kinase regulation in maize Evidence for co-purification of threonine-sensitive aspartate kinase and homoserine dehydrogenase. Phytochemistry, 31, 3731-3734 (1992)... 

As mentioned earlier, L-threonine production can be enhanced by engineering the export or uptake system. An efficient L-threonine producer strain of E. coli KY10935, which was derived from the wild-type strain by multiple rounds of random mutation and selection, was able to produce 100 g L-1 L-threonine after 77 h cultivation [53]. In this strain, the two key enzymes in the L-threonine biosynthesis (homoserine dehydrogenase and homoserine kinase) were identified to be still inhibited by much lower intracellular concentrations of L-threonine than... 

Shames, S.L. Wedler, RC. Homoserine kinase of Escherichia coli kinetic mechanism and inhibition by L-aspartate semialdehyde. Arch. Biochem. Biophys., 235, 359-370 (1984)... 

Theze, J. Kleidman, L. Saint Girons, L Homoserine kinase from Escherichia coli K-12 properties, inhibition by L-threonine, and regulation of biosynthesis. J. Bacteriol., 118, 577-581 (1974)... 

Riesmeier, J. Klonus, A.-K. Pohlenz, H.-D. Purification to homogeneity and characterization of homoserine kinase from wheat germ. Phytochemistry, 32, 581-584 (1993)... 

Ramos, C. Delgado, M.A. Calderon, I.L. Inhibition by different amino acids of the aspartate kinase and the homoserine kinase of the yeast Sac-charomyces cerevisiae. FEBS Lett., 278, 123-126 (1991)... 

Mannhaupt, G. Pohlenz, H.D. Seefluth, A.K. Pilz, U. Feldmann, H. Yeast homoserine kinase. Characteristics of the corresponding gene, THRl, and the purified enzyme, and evolutionary relationships with other enzymes of threonine metabolism. Eur. J. Biochem., 191, 115-122 (1990)... 

Huo, X. Viola, R.E. Substrate specificity and identification of functional groups of homoserine kinase from Escherichia coli. Biochemistry, 35, 16180-16185 (1996)... 

Lee, M. Leustek, T. Identification of the gene encoding homoserine kinase from Arabidopsis thaliana and characterization of the recombinant enzyme derived from the gene. Arch. Biochem. Biophys., 372, 135-142 (1999)... 

Zhou, T. Daugherty, M. Grishin, N.V Osterman, A.L. Zhang, H. Structure and mechanism of homoserine kinase prototype for the GHMP kinase superfamily. Structure Fold Des., 8, 1247-1257 (2000)... 

Krishna, S.S. Zhou, T. Daugherty, M. Osterman, A. Zhang, H. Structural basis for the catalysis and substrate specificity of homoserine kinase. Biochemistry, 40, 10810-10818 (2001)... 

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. 

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. 

Lysine plus threonine severely inhibits growth of maize in a synergistic manner. Growth inhibition could result from combined effects of lysine on aspartate kinase and threonine on homoserine dehydrogenase, resulting in starvation for methionine. Growth inhibition by lysine -i- threonine can be overcome by supplying methionine. Bryan [1980) Miflin [1977). 

Morinage Y, Takagi H, Ishida M, Miwa K, Sato T, Nakamori S, et al. Threonine production by coexistence of cloned genes coding homoserine dehydrogenase and homoserine kinase in Brevibacterium lactofermentum. Agric Biol Chem 1987 51 93-100. 

The biosynthesis of threonine (Thr,T) as shown in Scheme 12.14 also begins with aspartate semialdehyde, which is reduced, as before (homoserine dehydrogenase, EC 1.1.1.3), to homoserine (Scheme 12.13). Now, however, for threonine (Thr, T), the phosphorylating enzyme homoserine kinase (EC 2.7.1.39) effects the transfer of phosphate to the primary hydroxyl of homoserine from ATP, forming ADP and 4-phosphohomoserine. The latter, with threonine synthase (EC 4.2.3.1) and using pyridoxal as cofactor, generates threonine (Thr,T). 

Fig. 14.1 The biosynthesis pathway of L-threonine. The pathway consists of centeral metabolic pathways and the threonine terminal pathways. The centeral metabolic pathways involve glycolysis, phosphate pentose pathway, TCA cycle and anaplerotic pathways. The threonine terminal pathway consists of five enzymetic steps. The first, third, and fourth reactions are catalyzed by the three key enzymes aspartate kinase, homoserine dehydrogenase, tmd homoserine kinase, respectively. There are four competing pathways that affect the biosynthesis of L-threonine, leading to formation of L-lysine, L-metMonine, L-isoleucdne, and glycine...

The inhibition of E. coli homoserine kinase is complicated. It is inhibited by the substrates, L-homoserine at a concentration above 1 mM and ATP above 3 mM in a hypothetical preferred order manner, by L-threonine in a competitive manner, and by L-lysine in a non-competitive manner (Chassagnole et al. 2001). 

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