Homoserine dehydrogenase reaction

Aspartate kinase [EC 2.T.2.4], also known as asparto-kinase, catalyzes the reaction of aspartate with ATP to produce 4-phosphoaspartate and ADP. The enzyme isolated from E. coli is a multifunctional protein, also exhibiting the ability to catalyze the reaction of homoserine with NAD(P) to produce aspartate 4-semialdehyde and NAD(P)H (that is, the activity of homoserine dehydrogenase, EC 1.1.1.3). 

ATP -I- L-aspartate = ADP -I- 4-phospho-L-aspartate (The enzyme from E. coli is a multifunctional protein, which also catalyses the reaction of EC 1.1.1.3 homoserine dehydrogenase)... 

In E. coli there are three aspartokinases that catalyze the conversion of aspartate to p-aspartyl phosphate. All three catalyze the same reaction, but they have very different regulatory properties, as is indicated in Fig. 24-13. Each enzyme is responsive to a different set of end products. The same is true for the two aspartate semialdehyde reductases which catalyze the third step. Both repression of transcription and feedback inhibition of the enzymes are involved. Two of the aspartokinases of E. coli are parts of bifimctional enzymes, which also contain the homoserine dehydrogenases that are needed to reduce aspartate semialdehyde in the third step. These aspar-tokinase-homoserine dehydrogenases 1 and 11 (Fig. 

The next two reactions in the pathway are catalyzed by aspartate semialdehyde dehydrogenase and homoserine dehydrogenase, respectively (Fig. 2). Both enzymes were initially detected in extracts of pea seedlings (Sasa-oka and Inagaki, 1960 Sasaoka, 1961) and subsequently characterized as B stereospecific with respect to hydride transfer from the dihydropyridine ring of NADPH (Davies el al., 1972). Aspartate semialdehyde dehydrogenase has also been detected in extracts of maize shoots, roots, developing kernels,... 

Fig. 1. Bio thetic pathways for the essential aspartate-family amino adds. The numbers represent enzymes catalyzing the reaction 1, aspartate kinase 2, homoserine dehydrogenase 3, homoserine kinase 4, threonine thase 5, threonine dehydrogenase 6, acetolactate thase 7, dihydrodipicolinate thase 8, diaminopimelate decarboiylase.

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...

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 forward and reverse reactions were verified by showing a stoichiometric relation between decrease of 3-aspartyl phosphate and TPNH and the increase of aspartyl semialdehyde in the forward reaction and an equivalent formation of /3-aspartyl phosphate and TPN when the components of the reverse reaction were incubated. Participation of phosphate in the reverse reaction was demonstrated by the dependence of the equilibrium level of TPNH on the phosphate concentration. The product of aspartyl phosphate reduction was established to be L-aspartyl /3-semialdehyde by its subsequent reaction with DPNH and homoserine dehydrogenase to yield homoserine. /3-Aspartyl phosphate formed by aspartyl semi-aldehyde oxidation was identified by its reaction with ADP in the (8-aspartokinase system. 

Reduction of L-aspartyl /3-semialdehyde to L-homoserine is catalyzed by homoserine dehydrogenase (Fig. 3, reaction 3). Both DPNH and TPNH can serve as hydrogen donors. However, DPNH is about three times as effective as TPNH. This dehydrogenase has also been purified from yeast extracts about a hundredfold by heat treatment, precipitation at pH 4.6 with ammonium sulfate, followed by chromatography on a calcium phosphate gel column. 

So, the biosynthesis of methionine (Met, M), the first of the essential amino adds to be considered (Scheme 12.13), begins by the conversion of aspartate (Asp, D) to aspartate semialdehyde in the same way glutamate (Glu, E) was converted to glutamate semialdehyde (vide supra. Scheme 12.6). Phosphorylation on the terminal carboxylate of aspartate (Asp, D) by ATP in the presence of aspartate kinase (EC 2.7.2.4) and subsequent reduction of the aspart-4 yl phosphate by NADPH in the presence of aspartate semialdehyde dehydrogenase (EC 1.2.1.11) yields the aspartate semialdehyde. The aspartate semialdehyde is further reduced to homoserine (homoserine oxoreductase, EC 1.1.1.3) and the latter is succinylated by succinyl-CoA with the liberation of coenzyme A (CoA-SH) in the presence of homoserine O-succinyl-transferase (EC 2.3.1.46). Then, reaction with cysteine (Cys, C) in the presence of cystathionine y-synthase (EC 2.5.1.48) produces cystathionine and succinate. In the presence of the pyridoxal phosphate protein cystathionine P-lyase (EC 4.4.1.8), both ammonia and pyruvate are lost from cystathionine and homocysteine is produced. Finally, methylation on sulfur to generate methionine (Met, M) occurs by the donation of the methyl from 5-methyltetrahydrofolate in the presence of methonine synthase (EC 2.1.1.13). 

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