Homoserine biosynthesis

In pea seedlings during germination there is a massive synthesis of homoserine, an intermediate in threonine and methionine synthesis (see Bryan, this volume. Chapter 11). There is little doubt that homoserine is derived from aspartate in pea chloroplasts (Lea et aL, 1979a) but whether this is the sole route of homoserine biosynthesis in pea seedlings is still in doubt (Mitchell and Bidwell, 1970 Bauer ef a/., 1977b). 

TABLE 1. Influence of leaf age on homoserine biosynthesis in isolated intaa chloroplasts. 

Scheme 2 shows the biosynthesis ofN-(3-oxooctanoyl)-L-homoserine lactone by Tral protein from Agrobacterium using 3-oxooctanoyl-ACP, derived from fatty acid metabolism, as a substrate [29, 33]. Recently, the first crystal structure of a LuxI protein homologue [34] has provided new insights into the function of AHL synthases which will aid the design of novel inhibitors of AHL biosynthesis. 

When the biosynthetic pathways given above are examined, it is apparent that several intermediates are indeed nonprotein ct-amino acids. Ornithine, homoserine, homocysteine, and ct-e-diaminopimelic acid are a few examples. This shows that some nonprotein amino acids originate as intermediates during the biosynthesis of... 

Homocysteine3 and homoserine are among the important a-amino acids that are not constituents of proteins. These substances are precursors in the biosynthesis of methionine. 

Many additional examples of the elucidation of prostereoisomerism in biochemical reactions could be given, for example the elegant elucidation by Comforth and coworkers 111, n8,137) of the biosynthesis of squalene, which was recognized by the Nobel prize in chemistry in 1975, or the recent studies of the enzymatic decarboxylation of tyrosine 138) and histidine 139) and of the condensation of homoserine with cysteine to give lanthionine 140), but the examples already provided should illustrate the principles and techniques involved in such studies. 

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... 

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).

Figure 1 P. aeruginosa QS system. Its mechanism, (a) Biosynthesis of acyl-homoserine lactone (AHL). Abbreviations SAM, 5-adenosyl methionine ACP, acyl carrier protein, (b) General chemical structure of AHL molecules, generally called autoinducer-1 (AI-1). (c) Chemical structure of V. fischeri AI-1. (d) Chemical structure of P. aeruginosa 3-oxo-C y-HSL and (e) C4-HSL. (f) Pseuodomonas quinolone signal, PQS.

Homocysteine, homoserine, ornithine, and citrulline are intermediates in the biosynthesis of certain other amino acids. 

Cystathionine y-synthase (CGS) is a rather unique PLP-enzyme that catalyzes a transsulfuration reaction important in microbial methionine biosynthesis. It is the only known enzyme whose function is the catalysis of a PLP-dependent replacement reaction at the y-carbon of the amino acid substrate the succinyl moiety of O-succinyl-L-homoserine is replaced by i-Cys to give the thioether linkage of L,/.-cystathionine (scheme II). In the absence of L-Cys, the enzyme catalyzes a net y-elimination reaction from OSHS (scheme II). Because both reactions require the elimination of succinate, the catalytic pathways must diverge from a common reaction intermediate. It was originally hypothesized that a vinylglycine quinonoidal intermediate (structure 11)... 

The pathway of biosynthesis of L-lysine and L-threonine in Corynebacterium glutamicum is shown in Fig. 1. The first step, the formation of phosphoaspartate from aspartate, is catalyzed by aspertokinase and this enzyme is susceptible to the concerted feedback inhibition by L-lysine and L-threonine. The auxotrophic mutant of homoserine (or threonine plus methionine), lacking homoserine dehydrogenase, was constructed and found to produce L-lysine in the culture medium. Second, the mutants which show the threonine or methionine sensitive phenotype caused by the mutation on homoserine dehydrogenase (low activity) was also found to produce appreciable amounts of L-lysine in the culture medium. Furthermore, a lysine analogue (S-aminoethylcysteine) resistant mutant was obtained as an L-lysine producer and in this strain aspartokinase was insensitive to the feedback inhibition. 

Fig. 1. Regulation of lysine biosynthesis. ASA, aspartate-/3-semialdehyde DDP, dihydro-dipicolinate DAP, a, -diaminopimelate Hse, homoserine...

CGS catalyzes the 7-replacement reaction of an activated form of L-homoserine with L-cysteine, leading to cystathionine. 0-Succinyl-L-homoserine (l-OSHS), 0-acetyl-L-homoserine (OAHS), and 0-phospho-L-homoserine (OPHS) are substrates for CGS ftom bacteria, fungi, and plants, respectively. The plant enzyme is also able to convert the microbial substrates, albeit at much higher values. This reaction is the first step in the transsulfuration pathway that converts L-Cys into L-homocysteine, the immediate precursor of L-methionine. The 0-activated L-homoserine substrate is situated at a metabolic branch point between L-Met and L-Thr biosynthesis, and which substrate is used by CGS depends on the species. In analogy with TS, CGS is tightly regulated by SAM concentration in plants. ... 

Aspartate is involved in the control point of pyrimidine biosynthesis (Reaction 1 below), in transamination reactions (Reaction 2 below), interconversions with asparagine (reactions 3 and 4), in the metabolic pathway leading to AMP (reaction 5 below), in the urea cycle (reactions 2 and 8 below), IMP de novo biosynthesis, and is a precursor to homoserine, threonine, isoleucine, and methionine (reaction 7 below). It is also involved in the malate aspartate shuttle. 

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. 

Figure 21.6 Biosynthesis of methionine from homoserine, as it occurs in plants and bacteria. 

Aspartate has many fates, too. For example, its nitrogen is used in the biosynthesis of arginine and urea. Similar reactions are involved in purine nucleotide synthesis. The entire aspartate molecule is used in pyrimidine nucleotide biosynthesis. In plants and bacteria, aspartate is a precursor to three other amino acids (i.e., methionine,threonine, and isoleucine) via its conversion to homoserine (see here). Homoserine then leads in separate pathways to methionine and threonine. Threonine, in turn, can be converted to isoleucine. In bacteria, aspartic / -semialdehyde is a precursor to lysine. 

Thoen, A. Rognes, S.E. Aarnes, H. Biosynthesis of threonine from homoserine in pea seedlings I. Homoserine kinase. Plant Sci. Lett., 13, 103-112 (1978)... 

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)... 

Miyajima, R. Shiio, I. Regulation of aspartate family amino acid biosynthesis in Brevibacterium flavum. V. Properties of homoserine kinase. J. Bio-chem., 71, 219-226 (1972)... 

Strain D-60 was then used to select mutants resistant to the threonine analog, S-hydroxynorvaline, following nitrosoguanidine mutagenesis (j, ). Three kinds of resistant strain were saved for subsequent use. One of these (HNr31) had an Incomplete but undefined block in methionine biosynthesis, which may have accounted for the small amount of threonine that was accumulated from homoserine being funneled into the threonine pathway. 

Nocardicins.—Nocardicin A has the unusual structure (167), part of which is a /3-lactam ring. Its biosynthesis has been studied and it has been observed that two molecules of L-tyrosine (with proven loss of the carboxy-group), L-homoserine, and L-serine account for the carbon skeleton of (167). It has been suggested that the tyrosine is utilized by way of L-p-hydroxy-phenylglycine (168). 

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