Aspartate semialdehyde reductases

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. 

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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