Biosynthesis of Amino Acids

As with amino acid catabolic pathways, a detailed coverage of the biosynthetic pathways for all 20 amino acids would take far too much space. We ll therefore look only at several representative schemes. 

Seven of the eleven nonessential amino acids are synthesized either from pyruvate or from the citric acid cycle intermediates oxaloacetate and a-keto-glutarate. Alanine is biosynthesized hy transamination of pyruvate, aspartate from oxaloacetate, and glutamate from a-ketoglutarate. The mechanisms of 

FIGURE 20.11 Biosynthesis ofthe 20 protein amino acids. Essential amino acids (red) are synthesized in plants and bacteria and must be obtained in our diet. Humans can synthesize only the nonessential amino acids (blue). 

FIGURE 20.12 Biosynthetic pathways for asparagine and glutamine by amide formation from aspartate and glutamate, respectively. As explained in the text, the mechanisms of the acyl substitution reactions are given in abbreviated form without showing the tetrahedral intermediates. 

Proline also is synthesized from glutamate 5-semialdehyde hy nonenzy-matic formation of a cyclic imine followed by enzymatic reduction of the C=N bond with NADH in a nucleophilic addition reaction. 

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As an offshoot of studies of genes concerned with the biosynthesis of amino acids, a range of E. coli (see, e.g., Yanofsky, 1971) and Salmonella typhimurium strains (see, e.g., Ames, 1971) with relatively well-defined mutations in known genes... 

Biosynthesis of Amino Acids with Aliphatic Side Chains 7... 

Nature utilizes the shikimate pathway for the biosynthesis of amino acids with aryl side chains. These nonprotein amino acids are often synthesized through intermediates found in the shikimate pathway. In many cases, L-a-amino acids are functionalized at different sites to yield nonprotein amino acids. These modifications include oxidation, hydroxylation, halogenation, methylation, and thiolation. In addition to these modifications, nature also utilizes modified biosynthetic pathways to produce compounds that are structurally more complex. When analyzing the structures of these nonprotein amino acids, one can generally identify the structural similarities to one of the L-a-amino acids with aromatic side chains. 

Pyruvate and TCA cycle intermediates serve as precursors for the biosynthesis of amino acids (see Chapter 9). 

Biosynthesis of Amino Acids, Nucleotides, and Related Molecules 843... 

Biosynthesis of amino acids, nucleotides, and biological amines... 

BIOSYNTHESIS OF AMINO ACIDS, NUCLEOTIDES, AND RELATED MOLECULES... 

Several Classes of Reactions Play Special Roles in the Biosynthesis of Amino Acids and Nucleotides... 

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