Homoserine phosphate

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. 

The final enqnnic steps in the conversion of homoserine to threonine were established by Watanabe et al. (104-106), who showed that at least two sepsutible enzyme fractions sue required. These invest tors separated homoserine kinase from a second enzyme and confirmed the formation of 0-homoserine phosphate. Watanabe and co-workers also determined the conversion of 0-homoserine phosphate to threonine. 

The enzyme that catalyzes the conversion of 0-homoserine phosphate to threonine, threonine synthetase, has been purified 500-fold from Neuro-apora by Flavin and Slaughter (107). The purification procedure utilized acetone and ammonium sulfate precipitation tmd chromatography on a DEAE-cellulose column. 

FIGURE 10.10 The reaction of tridated sodium borohydride with the aspartyl phosphate at the active site of Na, K -ATPase. Acid hydrolysis of the enzyme following phosphorylation and sodium borohydride treatment yields a tripeptide containing serine, homoserine (derived from the aspartyl-phosphate), and lysine as shown. The site of phosphorylation is Asp" in the large cytoplasmic domain of the ATPase. 

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.1] (also referred to as homoserine deaminase, homoserine dehydratase, y-cystathionase, cystine desulfhy-drase, cysteine desulfhydrase, and cystathionase) catalyzes the hydrolysis of cystathionine to produce cysteine, ammonia, and a-ketobutanoate (or, 2-oxobutanoate). 

This pyridoxal-phosphate-dependent enzyme [EC 4.2.99.9], also known as cystathionine y-synthase, catalyzes the reaction of O-succinyl-L-homoserine with L-cysteine to produce cystathionine and succinate. The enzyme can also use hydrogen sulfide and methanethiol as substrates, producing homocysteine and methionine, respectively. In the absence of a thiol, the enzyme can also catalyze a /3,y-elimination reaction to form 2-oxobu-tanoate, succinate, and ammonia. 

Canaline is a potent inhibitor of all seven pyridoxal phosphate-containing enzymes studied by Rahiala et (27) but it lacks adverse effects on three ornithine-utiTTzing enzymes lacking a Bg cofactor. Finally, in jack bean, Canavalia ensiformis, ornithine carbamoyl transferase can form 0-ureido-L-homoserine from canaline and carbamoyl phosphate as it does citrulline from ornithine and carbamoyl phosphate. Nevertheless, neither compound inhibited formation of the reaction products (31). 

FIGURE 3.4 The common pathway of the aspartate-derived amino acids in Corynebacteria. The mnemonic of the genes involved are shown in parentheses below the enzymes responsible for each step. Dotted lines indicate multiple enzymatic steps, and 16 is L-aspartic acid, 17 is L-aspartyl phosphate, 18 is L-aspartate semialdehyde, 19 is L-lysine, 20 is L-homoserine, 21 is L-isoleucine, 22 is L-threonine, and 23 is L-methionine. 

Figure 1. Transcriptionally regulated network of exopolysaccharide galactoglucan biosynthesis in the Gram-negative soil bacterium S. meliloti that is affected by extracellular phosphate concentration and bacterial population density (AHL, N-acyl homoserine lactone HL, homoserine lactone).

Aspartyl-phosphate is an intermediate in the conversion of aspartate to homoserine (see here) in the pathway leading to biosynthesis of threonine, isoleucine, and methionine. 

Mosf of the chemistry has been considered already. The reduction of aspartate via p-aspartyl phosphate and aspartate p-semialdehyde ° is a standard one. Conversion to methionine can occur in two ways. In E. coU homoserine is succinylated with succinyl-CoA. The y-succinyl group is then replaced by the cysteine molecule in a PLP-dependent y-replacement reaction (Fig. 24-13). The product cystathionine (Eq. 14-33) undergoes elimination to form homocysteine. A similar pathway via 0-phospho-homoserine occurs in chloroplasts of green plants. ... 

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 amino acid L-threonine 128 is synthesized in nature from L-homoserine 104 via the 3-phosphate 126. Fuganti studied the stereochemistry of this... 

The biosynthesis of threonine from aspartate involves formation of the four metabolic intermediates illustrated in Fig. 2. The /3-carboxyl group of aspartate is first activated by formation of an acylphosphate, and this reaction is followed by two reductive steps resulting in the synthesis of homoserine. After formation of a C4 phosphate ester of homoserine, threonine is synthesized by a complex rearrangement which entails formation of the terminal methyl group and transfer of the oxygen atom from C-4 to form a hydroxyl group at C-3. 

In plants and microorganisms, l-T. is biosynthesized from phosphohomoserine by a y-elimination of phosphate followed by P-replacement with an OH-group. Hiis total reaction is catalysed by the pytidoxal phosphate enzyme, l-T. synthase (EC 4.2.99.2). Hie phosphohomoserine is derived from aspartate via as-partyl phosphate, aspartate semialdehyde and homoserine. 

The carbon skeleton of homocysteine is derived from the corresponding hydroxy amino acid, homoserine. The hydroxyl of homoserine is acylated with either a succinyl (bacteria) or acetyl (yeast, fungi, plants) group derived from the corresponding coenzyme A derivative. The O-acyl substituent is then displaced by the thiol group of cysteine producing a mixed thioether, cystathionine. This in turn undergoes a pyridoxal phosphate dependent -elimination to homocysteine, pyruvate and... 

These reactions which lead to homocysteine formation in some creatures and its utilization in others are undoubtedly representative of a general thiol group transfer mechanism. The initial condensation of the donor thiol, most commonly cysteine, with some suitably reactive receptor generates a thioether. The differences in the requirement for O-acylation when starting from serine and homoserine may refiect two completely different mechanisms for this thiol substitution reaction. In the case of serine, the removal of the hydroxyl as hydroxide and the stabilization of an electrophilic centre on the side-chain carbon can be achieved through the pyridoxal phosphate-amino acid adduct. A similar example is in the carbon-carbon condensation between serine and imidazole in tryptophan... 

L-Threonine is synthesized from L-aspartic acid via L-aspartic acid-4-phosphate and L-aspartic-jS-semialdehyde (Fig. 203). The latter is a precursor of L-lysine (D 18) as well as of L-homoserine (D 12). L-Threonine is formed from L-homoserine by synthesis of the 4-phosphate and shift to the hydroxyl group by a pyridoxal-phosphate-dependent enzyme. 

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