Amino Acid Biosynthesis

Abstract

Amino acids are classically considered as the building blocks for the synthesis of proteins. Besides this, some of them play a major role in other functions, such as regulation of protein turnover and signal transduction, transport of nitrogen and carbon across the organs, or neurotransmission. The unique characteristics of amino acids are the presence of a free amino group in the α‐carbon and a free carboxyl group. The amino acids differ from each other with respect to their side‐chains. The amino acids are classified into subgroups according to their similarity in carbon skeleton, substituent groups or a common metabolic pathway. The biosynthesis of amino acids involves several biochemical pathways in which amino acids are assembled from other precursors. The biosynthesis of amino acids is distinct from that involving lipids or carbohydrates because it includes the use of nitrogen.

Key Concepts:

  • Amino acids are classically considered as the building blocks from which proteins are synthesised. Besides this, some of them play a major role in the regulation of protein turnover and signal transduction, transport of nitrogen and carbon across the organs, and neurotransmission.

  • The biosynthesis of amino acids involves several biochemical pathways in which amino acids are assembled from other precursors. The biosynthesis of amino acids is distinct from that involving lipids or carbohydrates because it includes the use of nitrogen.

  • The fixation of nitrogen is a process that converts atmospheric nitrogen to a form that can be used biologically.

  • The pathways for the synthesis of essential amino acids are present only in microorganisms and plants.

  • Nine of 12 nonessential amino acids are synthesised from amphibolic intermediates, whereas three amino acids (tyrosine, cysteine and hydroxylysine) derive from essential amino acids. Amino acid transaminases, glutamate dehydrogenase and glutamine synthetase play a central role in the synthesis of nonessential amino acids.

  • The major pathway by which ammonia is incorporated into amino acids is through the reductive amination of α‐ketoglutarate to glutamate. Ammonia is highly toxic for animals. Glutamine is a nontoxic carrier of ammonia.

  • In the human adult as much as 200–250 g of proteins are degraded daily, and their constituent amino acids are in large part reutilised in protein synthesis.

  • Amino acid deficiency states can result if any of the essential amino acid is present in inadequate amounts or omitted from the diet.

  • Alanine, glutamate and glutamine are crucial links between energy and protein metabolism. Moreover, glutamine and alanine biosynthesis in the peripheral tissues (muscle) provides a means for the transport of carbon to the liver for gluconeogenesis and nitrogen for ureagenesis.

  • There are several nonprotein functions of amino acids, such as the biosynthesis of purines and pyrimidines, which involve more nonessential than essential amino acids.

Keywords: proteins; nutrition; nitrogen; carbon; ammonia

Figure 1.

The nitrogen cycle.

Figure 2.

Outline of pathways of synthesis of essential amino acids.

Figure 3.

The hydroxylation of phenylalanine to tyrosine. From Scriver et al..

Figure 4.

The reaction of glutamate dehydrogenase.

Figure 5.

The reactions of glutaminase and glutamine synthetase.

Figure 6.

Outline of pathways of synthesis of nonessential amino acids. Modified from Munro HM (ed.) (1969) Evolution of protein metabolism in mammals. In: Mammalian Protein Metabolism, vol. 3, pp. 133–182. New York: Academic Press.

Figure 7.

Pathways of homocysteine–cysteine metabolism. THF, tetrahydrofolate; B12, vitamin B12 and B6, vitamin B6.

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Further Reading

Deferrari G, Garibotto G, Robaudo C et al. (1994) Renal ammoniagenesis and interorgan flow of glutamine in chronic metabolic acidosis. Contributions to Nephrology 110: 144–149.

Garibotto G, Valli A, Anderstam B et al. (2009) The kidney is a major site of S‐adenosylhomocysteine disposal in humans. Kidney International 76: 293–296.

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Rose WC (1957) The amino acid requirements of adult man. Nutrition Abstracts and Reviews 27: 631–647.

Tessari P and Garibotto G (2000) Inter‐organ amino acid exchange. Current Opinion in Clinical Nutrition and Metabolic Care 3: 51–57.

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Deferrari, Giacomo, Mannucci, Irene, and Garibotto, Giacomo(Sep 2010) Amino Acid Biosynthesis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000628.pub2]