Nutritionally Nonessential Amino Acids Have Short Biosynthetic Pathways

The enzymes glutamate dehydrogenase, glutamine syn-thetase, and aminotransferases occupy central positions in amino acid biosynthesis. The combined effect of those three enzymes is to transform ammonium ion into the a-amino nitrogen of various amino acids.

Glutamate and Glutamine. Reductive amination of a-ketoglutarate is catalyzed by glutamate dehydrogenase (Figure 28-1). Amination of glutamate to glutamine is catalyzed by glutamine synthetase (Figure 28-2).

Alanine. Transamination of pyruvate forms alanine (Figure 28-3).

Aspartate and Asparagine. Transamination of oxaloacetate forms aspartate. The conversion of aspartate

Table 28-1. Amino acid requirements of humans.

Nutritionally Essential

Arginine1

Histidine

Isoleucine

Leucine

Lysine

Methionine

Phenylalanine

Threonine

Tryptophan

Valine

Nutritionally Nonessential

Alanine

Asparagine

Aspartate

Cysteine

Glutamate

Glutamine

Glycine

Hydroxyproline2

Hydroxylysine2

Proline

Serine

Tyrosine

'"Nutritionally semiessential." Synthesized at rates inadequate to support growth of children.

2Not necessary for protein synthesis but formed during post-translational processing of collagen.

a-Ketoglutarate

NH3+

OO L-Glutamate

Figure 28-1. The glutamate dehydrogenase reaction.

Figure 28-1. The glutamate dehydrogenase reaction.

NH3+

Pyruvate

Alanine

Glu or Asp a-Ketoglutarate or oxaloacetate

Figure 28-3. Formation of alanine by transamination of pyruvate. The amino donor may be glutamate or aspartate. The other product thus is a-ketoglutarate or oxaloacetate.

to asparagine is catalyzed by asparagine synthetase (Figure 28-4), which resembles glutamine synthetase (Figure 28-2) except that glutamine, not ammonium ion, provides the nitrogen. Bacterial asparagine synthetases can, however, also use ammonium ion. Coupled hydrolysis of PP; to P; by pyrophosphatase ensures that the reaction is strongly favored.

Serine. Oxidation of the a-hydroxyl group of the glycolytic intermediate 3-phosphoglycerate converts it to an oxo acid, whose subsequent transamination and dephosphorylation leads to serine (Figure 28-5).

Glycine. Glycine aminotransferases can catalyze the synthesis of glycine from glyoxylate and glutamate or alanine. Unlike most aminotransferase reactions, these strongly favor glycine synthesis. Additional important mammalian routes for glycine formation are from choline (Figure 28-6) and from serine (Figure 28-7).

Proline. Proline is formed from glutamate by reversal of the reactions of proline catabolism (Figure 28-8).

Cysteine. Cysteine, while not nutritionally essential, is formed from methionine, which is nutritionally essential. Following conversion of methionine to ho-

L-Glutamate

L-Glutamine

Figure 28-2. The glutamine synthetase reaction.

Figure 28-2. The glutamine synthetase reaction.

L-Aspartate

L-Asparagine

Gln-

Mg-ATP

Figure 28-4. The asparagine synthetase reaction. Note similarities to and differences from the glutamine synthetase reaction (Figure 28-2).

®-O O D-3-Phosphoglycerate

NADH

Phosphohydroxy pyruvate a-AA

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