Biosynthesis of amino acids

A very limited number of microorganisms are able to utilise molecular nitrogen from the atmosphere by incorporating it initially into ammonia and subsequently into organic compounds (see Chapter 7). Most organisms, however, need to have their nitrogen supplied as nitrate, nitrite, or ammonia itself. Ammonia can be incorporated into organic nitrogen compounds in several ways, including glutamate formation from a-ketoglutarate (see TCA cycle, above):

a-ketoglutarate + NH4+ + NADPH* + H+ glutamate + NADP+ + H2O (*Some species utilise NADH as their electron donor)

The amino group can subsequently be transferred from the glutamate to make other amino acids by transamination reactions involving other keto acids:

e.g. glutamate + oxaloacetate —> aspartate + a-ketoglutarate

Glutamate plays a central role in the biosynthesis of other amino acids, as it usually donates the primary amino group of each:

COO-

H-C-NH2

COOH Glutamate

COOH Pyruvic acid

COO-

COOH Alanine

COOH a-Ketoglutarate

According to the precursor molecule from which they derive, amino acids can be placed into six 'families' (Figure 6.37). The precursors are intermediates in metabolic pathways we have already encountered in this chapter, such as glycolysis or the TCA cycle. When

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Ribose 5-PO.

Pyruvate

3-Phospho-glycerate

Phosphoenol-pyruvate

Erythose 4-PO4

Chorismate

Alanine Valine Leucine Glycine

Cysteine

Phenylalanine

Tyrosine

Tryptophan

Pyruvate family

Serine family

Aromatic family

Ribose 5-PO.

-> Histidinol

Histidine

Histidine family a-Keto-glutarate

Oxalo-acetate

-> Glutamate

-> Aspartate

Glutamine

Proline

Arginine

Lysine Threonine Aspartate Asparagine

Glutamate family

-> Isoleucine Aspartate family

Figure 6.37 Amino acid biosynthesis. The carbon skeleton of amino acids is obtained from a limited number of precursor molecules, mainly intermediates in glycolysis or the TCA cycle. The amino group originally derives from inorganic sources, but can then be transferred from one organic molecule to another amino acids are broken down, they are likewise broken down into a handful of metabolic intermediates, which then feed into the TCA cycle.

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