Human Biochemistry, Volume 1 - Murray R. 1993

Metabolism of Proteins and Amino Acids
Biosynthesis of Amino Acids
Biosynthesis of Essential Amino Acids

Below is a Brief Overview of The Biosynthesis of Essential Amino Acids in Bacteria, starting from glutamate, aspartate, or other amphibolic metabolites. These reactions do not occur in mammalian Tissues.

Arginine

Arginine—an essential dietary amino acid for humans during growth—can be synthesized by rats, though only in amounts insufficient to support normal growth. In microorganisms, arginine is biosynthesized from glutamate via N-acetylated intermediates. One of these is N-acetylglutamate-y-semialdehyde, which also serves as a precursor for Proline in bacteria. In humans and animals, however, proline is formed directly from glutamate.

Aspartate serves as the precursor for a family of amino acids that includes Lysine, Methionine, Threonine, and isoleucine (see Fig. 10.6). The regulation of these pathways in bacteria was discussed in Chapter 10.

Methionine and Threonine

Following The conversion of aspartate-β-semialdehyde to homoserine, the biosynthetic pathways for methionine and threonine diverge. The interconversions of homoserine and methionine are discussed in Chapter 31.

Lysine

Bacteria synthesize lysine from aspartate-β-semialdehyde via its Condensation with Pyruvate. The resulting dihydropicolinate also plays a specific role in spore formation in certain spore-forming bacteria, while another intermediate, diaminopimelate, is involved in Introduction/37.html">Bacterial Cell wall synthesis.

The biosynthesis of lysine in Yeast begins with a-ketoglutarate and acetyl-CoA, proceeding through a series of reactions analogous to those of The Citric Acid Cycle, catalyzed by Enzymes with slightly different substrate specificities.

Leucine, Valine, and Isoleucine

Although leucine, valine, and isoleucine are essential amino acids for humans and other higher animals, mammalian tissues contain transaminases that catalyze the reversible interconversion of all Three amino acids and their corresponding a-keto acids (see Chapter 31). This explains why the corresponding keto acids can effectively replace these amino acids in the diet.

Histidine

Histidine, much like arginine, can be classified as semi-essential. Adult humans and mature rats can maintain nitrogen balance for some time even in the absence of dietary histidine. However, a growing Organism requires histidine. It is quite likely that long-term studies would reveal a dietary requirement for histidine in adults as well.

The biosynthetic process begins with 5-phosphoribosyl-1-pyrophosphate (PRPP), which condenses with ATP to yield N'-(5-phosphoribosyl)-ATP. This reaction is thus analogous to the initial step of Purine Biosynthesis.

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